Receiver

0001The present disclosure relates to the field of receivers, and in particular, although not exclusively, receivers for televisions that are subjected to relatively strong interference signals.

0002In the A74 standard for television reception, the sensitivity of the receiver is specified in the presence of strong interference signals. For example a DTV receiver must be able to receive a channel of −68 dBm when an interference signal 57 dB higher is at a frequency that is 36 MHz from the wanted channel. The TV spectrum is from 40 MHz to 1 GHz.

0003Selectivity in front of the receiver is of great interest in order to satisfy specifications such as A74 because in this case the receiver is protected against strong interference signals. Indeed, if the wanted channel is weak and if strong interference signals are present close to the wanted channel, then the linearity of a receiver that processes both the wanted channel and the interference signals must be high to avoid SNR degradation. Now if a selective filter is placed in front of the receiver then the level of the interference signals is attenuated, and the linearity specification of the receiver is relaxed for the same SNR performance.

0004The listing or discussion of a prior-published document or any background in the specification should not necessarily be taken as an acknowledgement that the document or background is part of the state of the art or is common general knowledge.

0005According to a first aspect of the invention, there is provided a receiver comprising: <ul id="ul0001" list-style="none"><li id="ul0001-0001" num="0000"><ul id="ul0002" list-style="none"><li id="ul0002-0001" num="0006">an amplifier having an input and an output, wherein the input of the amplifier is configured to receive a signal; and</li><li id="ul0002-0002" num="0007">a feedback path between the output and the input of the amplifier, wherein the feedback path includes a filter and a buffer amplifier in series, wherein the input of the buffer amplifier is connected to the output of the amplifier, the output of the buffer amplifier is connected to the input of the filter, the output of the filter is connected to the input of the amplifier, and wherein the filter is configured to pass signals having a desired frequency.</li></ul></li></ul>

0008The filter is input-referred and due to the Miller effect it performs as if it were connected directly to the input of the amplifier, thereby filtering out unwanted signals before they are processed by the amplifier. This can be considered as advantageous as the amplifier is not required to process strong unwanted signals across the full frequency spectrum, and therefore can provide better sensitivity for weak signals having the desired frequency. In addition, the filter does not have to be placed in series at the input of the amplifier and therefore can use components that have a lower quality than may be necessary in the prior art.

0009Use of the buffer amplifier can enable the gain of the amplifier to be independent from the impedance of the filter, as the buffer amplifier can adapt the impedance between the output of the amplifier and the filter.

0010The input of the amplifier may be configured to receive the signal either directly or indirectly from an antenna, and in some examples the input of the amplifier may be connectable either directly or indirectly to an antenna.

0011The amplifier may be a low noise amplifier.

0012It will be appreciated that any of the connections disclosed herein may be direct or indirect connections. For example, two components may be indirectly connected via intermediary components whilst still providing embodiments of the invention.

0013The buffer amplifier may be a class AB amplifier, and may be a unity gain amplifier.

0014Any one or more of the filters disclosed herein may or may not be a tuneable filter.

0015The filter may be a band pass filter, and this may be a convenient component for enabling the signals with the desired frequency to pass and filtering out any interference signals.

0016The amplifier may be a variable gain amplifier. In this way, operational parameters of the receiver can be adjusted in order to improve the performance of the receiver.

0017The receiver may further comprise a detector configured to detect the output of the amplifier. The detector may control the gain of the variable gain amplifier in accordance with the output of the amplifier in order to avoid distortion of the output. In this way, the amplifier can be operated with a large gain value, but without degrading the quality of the output signal.

0018The receiver may further comprise a controller configured to adjust the gain of the variable gain amplifier in order to change the input impedance of the receiver. The controller may be configured to adjust the input impedance such that it is brought into conformity with the impedance of the antenna to which the receiver is connected in use. Improving the impedance matching of the receiver can improve the performance of the receiver.

0019The feedback path may include a variable resistor. The receiver may also include a controller configured to adjust the value of the variable resistor in order to change the input impedance of the receiver such that it is brought into conformity with the impedance of the antenna to which the receiver is connected in use. This is an alternative way of improving the impedance matching of the receiver, and hence improving the performance of the receiver.

0020The receiver may comprise a plurality of feedback paths in parallel between the output and the input of the amplifier. One or more of the feedback paths may be configured to be included or excluded from the receiver in accordance with the frequency of the desired signals. In this way, a feedback path that provides improved performance at specific frequencies when compared with the other feedback paths can be used, thereby improving the performance of the receiver.

0021One or more of the plurality of feedback paths may comprise a switch that is operable to include or exclude the feedback path. The switch may be a MOS switch, a controllable buffer or any other suitable switch.

0022One or more of the feedback paths may comprise a controllable buffer. The gain of the controllable buffers may be configured to be set to zero in order to exclude the components in the associated feedback path. The gain of the controllable buffers may be configured to be set to one in order to provide a unity gain buffer and include the components in the associated feedback path. The gain of the controllable buffers may be configured to be set to any non-zero value in order to include the components in the associated feedback path.

0023The plurality of feedback paths may comprise filters that are configured to pass signals of different frequencies. In this way, one or more filters may be used in order to provide improved performance for signals with certain desired frequencies.

0024The receiver may further comprise a first, second and third feedback path in parallel with each other. The first feedback path may comprise a buffer amplifier and a variable capacitor. The second feedback path may comprise a controllable buffer and a first inductor. The third feedback path may comprise a controllable buffer and a second inductor. The two controllable buffers may be operable in accordance with the frequency of the desired signals. In this way, an acceptable quality factor can be maintained for different desired frequencies by utilising different component values in the feedback path of the amplifier. It will be appreciated that there they may be more than three parallel feedback paths so that different components such as capacitors, inductors and resistors, having different values, can be selectively included or excluded in the feedback path.

0025The receiver may further comprise a variable resistive attenuator connected to the input of the amplifier and a detector, wherein the detector is configured to sense the output signal of the amplifier and adjust the value of the resistive attenuator so that the output signal is not significantly distorted, and this may include preventing the amplitude of the output signal from exceeding a take over point (TOP). The value of the resistive attenuator may be adjusted such that the amplitude of the output signal is substantially similar to the take over point in order to protect the amplifier against clipping.

0026The term substantially similar may mean that two values differ by less than 0.001%, 0.01%, 0.1%, 0.2%, 0.5%, 1%, 2%, 5%, 10%, or 20%, for example.

0027The input of the amplifier may be connected to the input of a low noise amplifier. The amplifier may be a “single input to single output” LNA. The amplifier may be a “single input to differential outputs” LNA. The amplifier may be a “differential inputs to differential outputs” LNA.

0028There may be provided an integrated circuit comprising any receiver or circuit disclosed herein.

0029There may be provided a television set including any receiver, circuit or integrated circuit disclosed herein.

0030There may be provided a computer program, which when run on a computer, causes the computer to configure any apparatus, including a receiver, controller, integrated circuit, television set, or device disclosed herein. The computer program may be a software implementation, and the computer may be considered as any appropriate hardware, including a digital signal processor, a microcontroller, and an implementation in read only memory (ROM), erasable programmable read only memory (EPROM) or electronically erasable programmable read only memory (EEPROM), as non-limiting examples. The software may be an assembly program.

0031The computer program may be provided on a computer readable medium such as a disc or a memory device, or may be embodied as a transient signal. Such a transient signal may be a network download, including an internet download.

A description is now given, by way of example only, with reference to the accompanying drawings, in which:

<figref idref="DRAWINGS">FIG. 1</figref> shows a prior art CAN tuner;

<figref idref="DRAWINGS">FIG. 2</figref> shows part of a prior art silicon tuner;

<figref idref="DRAWINGS">FIG. 3</figref> illustrates schematically the performance of prior art silicon tuners;

<figref idref="DRAWINGS">FIG. 4</figref> shows a receiver according to an embodiment of the invention;

<figref idref="DRAWINGS">FIG. 5</figref> shows a receiver according to another embodiment of the invention;

<figref idref="DRAWINGS">FIG. 6</figref> illustrates schematically the performance of a receiver according to an embodiment of the invention;

<figref idref="DRAWINGS">FIG. 7</figref> illustrates graphically a comparison between the performance of a prior art silicon tuner and a receiver according to an embodiment of the invention;

<figref idref="DRAWINGS">FIG. 8</figref> illustrates schematically a definition of gains for a low noise amplifier;

<figref idref="DRAWINGS">FIG. 9</figref> shows the equivalent circuit of a tuneable filter that can be used with an embodiment of the invention;

<figref idref="DRAWINGS">FIG. 10</figref> illustrates schematically the performance of a receiver according to an embodiment of the invention;

<figref idref="DRAWINGS">FIG. 11</figref> illustrates a matching network in a CAN tuner;

<figref idref="DRAWINGS">FIG. 12</figref> shows in-band input matching in a CAN tuner;

<figref idref="DRAWINGS">FIG. 13</figref> illustrates a receiver according to an embodiment of the invention;

<figref idref="DRAWINGS">FIG. 14</figref> illustrates schematically the performance of a receiver according to an embodiment of the invention;

<figref idref="DRAWINGS">FIG. 15</figref> illustrates graphically the performance of a receiver according to an embodiment of the invention;

<figref idref="DRAWINGS">FIG. 16</figref> illustrates graphically the output current versus output voltage of a buffer that can be used with an embodiment of the invention;

<figref idref="DRAWINGS">FIG. 17</figref> illustrates graphically the transient current and voltage, both with and without large interference signals, of a buffer that can be used with an embodiment of the invention;

<figref idref="DRAWINGS">FIG. 18</figref> illustrates schematically a buffer that can be used with embodiments of the invention;

<figref idref="DRAWINGS">FIG. 19</figref> illustrates graphically how quality factor changes with frequency for tuneable filters having different inductance values;

<figref idref="DRAWINGS">FIG. 20</figref> illustrates a receiver according to an embodiment of the invention;

<figref idref="DRAWINGS">FIG. 21</figref> illustrates a programmable capacitor bank that can be used with embodiments of the invention;

<figref idref="DRAWINGS">FIG. 22</figref> illustrates graphically the distortion of parasitic junction capacitances in MOS switches;

<figref idref="DRAWINGS">FIG. 23</figref> illustrates a receiver according to an alternative embodiment of the present invention;

<figref idref="DRAWINGS">FIG. 24</figref> illustrates a programmable capacitor bank that can be used with embodiments of the invention;

<figref idref="DRAWINGS">FIG. 25</figref> illustrates schematically a receiver according to another embodiment of the invention;

<figref idref="DRAWINGS">FIG. 26</figref> illustrates graphically how the composite voltage gain of a receiver according to an embodiment of the invention can vary;

<figref idref="DRAWINGS">FIG. 27</figref> illustrates a receiver according to an embodiment of the invention;

<figref idref="DRAWINGS">FIG. 28</figref> illustrates graphically how the composite voltage gain of a filter can be compensated by a variation of the LNA voltage gain;

<figref idref="DRAWINGS">FIG. 29</figref> illustrates a receiver according to an embodiment of the invention;

<figref idref="DRAWINGS">FIGS. 30</figref><i>a </i>to <b>30</b><i>d </i>illustrate schematically receivers according to embodiments of the invention;

<figref idref="DRAWINGS">FIG. 31</figref> illustrates a receiver according to an embodiment of the invention;

<figref idref="DRAWINGS">FIG. 32</figref> illustrates graphically the performance of a receiver according to an embodiment of the invention; and

<figref idref="DRAWINGS">FIG. 33</figref> illustrates graphically the performance of a receiver according to an embodiment of the invention.

0066One or more embodiments described herein relate to an amplifier for receiving an input signal, which in one example is from an antenna. The receiver has a feedback path between the input and the output of the amplifier, wherein a tuneable filter is provided in series with a buffer in the feedback path. The tuneable filter is tuneable so as to pass signals of a desired frequency. Due to the Miller effect, the tuneable filter is input-referred and acts as if it were connected directly to the input of the amplifier, thereby filtering out unwanted signals before they are processed by the amplifier.

0067In this way, the tuneable filter does not have to be placed in series at the input of the amplifier, nor is the amplifier required to process signals across the full frequency spectrum. As described in more detail below, this can provide advantages over the prior art.

0068Two main types of TV tuners are known: CAN tuners and silicon tuners.

0069<figref idref="DRAWINGS">FIG. 1</figref> shows a classical CAN tuner <b>100</b> (so-called because they are housed in metal enclosures to minimize RF interference and crosstalk). The tuner <b>100</b> of <figref idref="DRAWINGS">FIG. 1</figref> has a tuneable filter <b>102</b> placed between an antenna <b>104</b> and a low noise amplifier (LNA) <b>106</b>. The tuneable filter <b>102</b> requires very high quality factor lumped inductor and capacitor components in order to operate correctly and is expensive because it is made of air-wound coils aligned by hand. In addition, the filter <b>102</b> is costly to adapt to various regions and standards.

0070Known CAN tuners are being replaced by silicon tuners. In silicon tuners the antenna filter <b>102</b> arrangement of <figref idref="DRAWINGS">FIG. 1</figref> is not feasible because the integrated components that are used in the silicon tuner do not have a sufficiently high quality factor. Even with capacitor banks, the quality factor is deteriorated and the tuner noise figure (NF) is higher than 5 dB. This degrades the overall performance of the receiver as silicon tuners typically have an NF that is inferior to 4 dB.

0071<figref idref="DRAWINGS">FIG. 2</figref> shows part of the silicon tuner <b>200</b> of the NXP integrated circuit TDA18273. Any problems in relation to a reduced quality factor with silicon tuners are often overcome by connecting the selective filter <b>202</b> just after the LNA <b>206</b> as shown in <figref idref="DRAWINGS">FIG. 2</figref>.

0072The noise figure of the silicon tuner of <figref idref="DRAWINGS">FIG. 2</figref> is fixed by the noise figure of the LNA <b>206</b>, as opposed to the noise figure of the selective filter <b>202</b>. The disadvantage of this solution is that the LNA <b>206</b> is connected to the antenna <b>204</b>, and therefore must handle the whole input spectrum. Thus the linearity of the LNA <b>206</b> must be very high in case of a weak wanted channel with strong adjacent channels as long term evolution (LTE). Otherwise it will not be possible to properly process the received wanted signal.

0073<figref idref="DRAWINGS">FIG. 3</figref> illustrates schematically operation of existing silicon tuners. In known silicon tuners, the NF is increased in the presence of interference signals because the gain of the LNA <b>302</b> is decreased to avoid saturation of the interference signals at the LNA output. The wanted signals (w) are shown with reference <b>304</b> in <figref idref="DRAWINGS">FIG. 3</figref> and the unwanted interference signals (unw) are shown with reference <b>306</b>. It can be seen in <figref idref="DRAWINGS">FIG. 3</figref> that the unwanted signals <b>306</b> are a lot stronger than the wanted signals <b>304</b>.

0074With known submicron technologies the supply voltage can be reduced to only a few volts (1.2V in CMOS 65 nm), and with such a low supply voltage the maximum achievable linearity is also small. In particular, the maximum output voltage swing can be limited and cannot be higher than the supply voltage. To avoid clipping of the LNA <b>302</b> output signal, a detector <b>308</b> can sense the LNA <b>302</b> output signal and then adjust the gain of the LNA <b>302</b> so that the output signal is kept below a pre-defined amplitude. This is illustrated schematically in <figref idref="DRAWINGS">FIG. 3</figref> as the LNA Take Over Point or LNA TOP <b>310</b>. In this case the LNA <b>302</b> is protected against clipping at its output, but the NF is high even if the interference signal <b>306</b> is far away (in terms of frequency) from the wanted channel <b>304</b>. This, in turn, only allows the filter <b>314</b> to pass a weak wanted signal as the strength of the wanted signal has been kept low due to the low gain value of the amplifier <b>302</b> that is necessary to prevent LNA TOP <b>310</b> from being exceeded. The frequency response of the filter <b>314</b> is shown with reference <b>312</b> in <figref idref="DRAWINGS">FIG. 3</figref>, and the filtered signals at the output of the receiver are shown schematically with reference <b>316</b> in <figref idref="DRAWINGS">FIG. 3</figref>.

0075Embodiments of the present invention can provide a solution that integrates a tracking filter in front of a receiver/amplifier without significant signal to noise (SNR) degradation of the receiver.

0076<figref idref="DRAWINGS">FIG. 4</figref> shows a receiver <b>400</b> according to an embodiment of the invention. The receiver <b>400</b> has an amplifier <b>406</b> having an input <b>408</b> and an output <b>410</b>. The amplifier <b>406</b> may or may not be the low noise amplifier (LNA) of the receiver as will be appreciated from the description of the embodiments of the invention that follows, particularly the embodiments that are illustrated as <figref idref="DRAWINGS">FIGS. 30</figref><i>a </i>to <b>30</b><i>d. </i>

0077The input <b>408</b> of the amplifier <b>406</b> receives a signal, which in this example is from an antenna <b>404</b>. In other examples, the amplifier <b>406</b> can receive a signal from another amplifier, such as a low noise amplifier (LNA), any component in a receive chain, or any component that can provide a signal that is suitable for processing by the receiver <b>400</b>. A feedback path <b>412</b> is provided between the output <b>410</b> and the input <b>408</b> of the amplifier <b>406</b>, and includes a buffer <b>414</b> and a filter <b>402</b> in series. The input of the buffer <b>414</b> is connected to the output <b>410</b> of the amplifier <b>406</b>, and the output of the buffer <b>406</b> is connected to an input of the filter <b>402</b>. The output of the filter <b>402</b> is connected to the input <b>408</b> of the amplifier <b>406</b>.

0078The filter <b>402</b> can pass signals having a desired frequency. Typically, the filter <b>402</b> is a band-pass filter wherein the desired signals relate to the pass-band of the filter <b>402</b> and correspond to a channel that a user wishes to receive.

0079The buffer <b>414</b> provides for impedance adaptation between the output <b>410</b> of the amplifier <b>406</b> and the filter <b>402</b>. In this way the gain of the amplifier <b>410</b> can be made independent from the impedance of the filter <b>402</b>. As the buffer <b>414</b> has the filter <b>402</b> as its load, the buffer <b>414</b> can absorb the power of the interference signals so that it is not passed on to the filter <b>402</b>. In this way, the power in the amplifier <b>406</b> can fix the noise figure of the receiver.

0080In the configuration of <figref idref="DRAWINGS">FIG. 4</figref>, the filter <b>402</b> acts as if it was connected directly to the antenna <b>404</b> due to the Miller effect. Also due to the Miller effect, the quality factor of the amplifier <b>406</b> is hardly impacted by the direct connection to the low impedance antenna (which may be 75 ohms in television systems). This embodiment of the invention can filter out interference signals before they are processed by the amplifier <b>406</b>, thereby enabling the amplifier <b>406</b> to maintain a required sensitivity to properly process a desired signal that is weaker than any interference signals.

0081<figref idref="DRAWINGS">FIG. 5</figref> illustrates a receiver <b>500</b> according to another embodiment of the invention. In this embodiment the amplifier <b>506</b> is a low noise amplifier (LNA) and the filter is a tuneable filter <b>502</b>. The receiver <b>500</b> includes a buffer amplifier <b>514</b> in the feedback path <b>512</b> between the output <b>510</b> of the LNA <b>506</b> and the tuneable filter <b>502</b>. The buffer amplifier <b>514</b> and tuneable filter <b>502</b> are provided in series in the feedback path <b>512</b> between the output <b>510</b> and the input <b>508</b> of the LNA <b>506</b>.

0082The tuneable filter <b>502</b> is a high quality factor selective filter that is associated with the LNA <b>506</b> of the receiver <b>500</b> to form an active tracking filter with high quality factor. Also, as this tuneable filter <b>502</b> is associated with the LNA <b>506</b>, the overall noise figure is kept low (for example, less than 4 dB). The centre frequency of the tracking filter <b>502</b> is programmable to be at the same frequency as the wanted channel, and can be programmable over a wideband spectrum.

0083<figref idref="DRAWINGS">FIG. 6</figref> illustrates schematically the performance of a receiver according to an embodiment of the invention in a way that is similar to the performance of a prior art receiver shown as <figref idref="DRAWINGS">FIG. 3</figref>. The receivers of <figref idref="DRAWINGS">FIGS. 3 and 6</figref> are shown with the same LNA TOP levels <b>310</b>; <b>610</b>.

0084As shown in <figref idref="DRAWINGS">FIG. 6</figref>, the gain of the LNA <b>602</b> can be kept at a higher value than is possible with <figref idref="DRAWINGS">FIG. 3</figref> because the filtering is effectively performed at the input to the LNA <b>602</b>. It can be seen that the amplitude of the wanted signals <b>616</b> in <figref idref="DRAWINGS">FIG. 6</figref> is greater than the amplitude of the wanted signals “w” <b>316</b> in <figref idref="DRAWINGS">FIG. 3</figref>. Therefore, the gain-reducing effect on the amplifier <b>602</b> caused by the unwanted signals <b>606</b> is diminished. The filtering is shown schematically with reference <b>612</b> in <figref idref="DRAWINGS">FIG. 6</figref>. The NF can also be kept low, especially if the unwanted interference signals <b>606</b> are far away from the wanted channel <b>604</b>.

0085Thus embodiments of the present invention can achieve acceptable sensitivity if the interference signal <b>606</b> is far enough away (in terms of frequency) from the wanted channel <b>604</b> such that it is filtered out before being applied to the input to the amplifier <b>602</b>. This means that the impact on the sensitivity of the receiver by the presence of this interference signal is reduced when compared with the prior art.

0086<figref idref="DRAWINGS">FIG. 7</figref> shows graphically the effect of a −10 dBm blocker/interference signal as the distance in frequency from the wanted signal is varied. Line <b>702</b> shows the performance of a prior art tuner and line <b>704</b> shows the performance of a tuner/receiver according to an embodiment of the invention. It can be seen that the equivalent NF in decibels of the embodiment of the invention reduces significantly as the distance between the interference signal and wanted signal is increased. The term “equivalent NF” on the vertical axis of the graph of <figref idref="DRAWINGS">FIG. 7</figref> indicates that both noise and distortion are considered.

Composite Voltage Gain

0087<figref idref="DRAWINGS">FIG. 8</figref> illustrates schematically an LNA <b>802</b> in order to discuss the “composite voltage gain” of the LNA <b>802</b>. The composite voltage gain (Gvc) is a term that is well known in the art of radio frequency (RF) signal processing and is given, generically, as:

0000<maths id="MATH-US-00001" num="00001"><math overflow="scroll"><mrow><mi>Gvc</mi><mo>=</mo><mfrac><msub><mi>V</mi><mi>out</mi></msub><mi>E</mi></mfrac></mrow></math></maths>

0088Where 2E plus the voltage dropped across resistor Rs is equal to the input voltage, V_in, of the LNA <b>802</b>. The corresponding output voltage is V_out.

0089The composite voltage gain of the LNA <b>802</b> is:

0000<maths id="MATH-US-00002" num="00002"><math overflow="scroll"><mrow><mrow><mi>Gvc_selective</mi><mo></mo><mi>_LNA</mi></mrow><mo>=</mo><mrow><mn>2</mn><mo></mo><mstyle><mspace width="0.3em" height="0.3ex" /></mstyle><mo></mo><mi>Gv</mi><mo></mo><mstyle><mspace width="0.3em" height="0.3ex" /></mstyle><mo></mo><mfrac><mi>Zfilter</mi><mrow><mi>Zfilter</mi><mo>+</mo><mrow><mrow><mo>(</mo><mrow><mn>1</mn><mo>+</mo><mi>Gv</mi></mrow><mo>)</mo></mrow><mo></mo><mi>Rs</mi></mrow></mrow></mfrac></mrow></mrow></math></maths>

0090Where: <ul id="ul0003" list-style="none"><li id="ul0003-0001" num="0000"><ul id="ul0004" list-style="none"><li id="ul0004-0001" num="0091">Zfilter is the impedance of the tuneable filter in the feedback path (not shown in <figref idref="DRAWINGS">FIG. 8</figref>). The tuneable filter may be an LC tank circuit, and</li><li id="ul0004-0002" num="0092">Gv is the voltage gain of the LNA.</li></ul></li></ul>

0093<figref idref="DRAWINGS">FIG. 9</figref> shows the equivalent circuit of a tuneable filter <b>902</b> that can be used with an embodiment of the invention. The equivalent circuit is an LC tank circuit.

0094The impedance of the tank circuit is:

0000<maths id="MATH-US-00003" num="00003"><math overflow="scroll"><mrow><mi>Zfilter</mi><mo>=</mo><mfrac><mrow><mi>j</mi><mo></mo><mstyle><mspace width="0.3em" height="0.3ex" /></mstyle><mo></mo><mi>L</mi><mo></mo><mstyle><mspace width="0.3em" height="0.3ex" /></mstyle><mo></mo><mi>ω</mi></mrow><mrow><mn>1</mn><mo>+</mo><mrow><mfrac><mi>L</mi><mi>Rp</mi></mfrac><mo></mo><mi>j</mi><mo></mo><mstyle><mspace width="0.3em" height="0.3ex" /></mstyle><mo></mo><mi>ω</mi></mrow><mo>-</mo><mrow><mi>LC</mi><mo></mo><mstyle><mspace width="0.3em" height="0.3ex" /></mstyle><mo></mo><msup><mi>ω</mi><mn>2</mn></msup></mrow></mrow></mfrac></mrow></math></maths>

0095At the resonant frequency the impedance of the tank circuit is Rp. Thus at the resonant frequency of the tank circuit, the composite voltage gain of the present invention is:

0000<maths id="MATH-US-00004" num="00004"><math overflow="scroll"><mrow><mrow><mi>Gvc_selective</mi><mo></mo><mi>_LNA</mi><mo></mo><mi>_resonnance</mi></mrow><mo>=</mo><mrow><mn>2</mn><mo></mo><mstyle><mspace width="0.3em" height="0.3ex" /></mstyle><mo></mo><mi>Gv</mi><mo></mo><mstyle><mspace width="0.3em" height="0.3ex" /></mstyle><mo></mo><mfrac><mi>Rp</mi><mrow><mi>Rp</mi><mo>+</mo><mrow><mrow><mo>(</mo><mrow><mn>1</mn><mo>+</mo><mi>Gv</mi></mrow><mo>)</mo></mrow><mo></mo><mi>Rs</mi></mrow></mrow></mfrac></mrow></mrow></math></maths>

0096At the resonant frequency the composite voltage gain of the selective LNA is equal to the voltage gain of the LNA if the input matching is perfect.

Input Impedance

0097The input impedance of the LNA of an embodiment of the invention due to the Miller effect is:

0000<maths id="MATH-US-00005" num="00005"><math overflow="scroll"><mrow><mi>Zin</mi><mo>=</mo><mfrac><mi>Zfilter</mi><mrow><mn>1</mn><mo>+</mo><mi>Gv</mi></mrow></mfrac></mrow></math></maths>

0098At the resonant frequency, we have:

0000<maths id="MATH-US-00006" num="00006"><math overflow="scroll"><mrow><mi>Zin_resonnance</mi><mo>=</mo><mfrac><mi>Rp</mi><mrow><mn>1</mn><mo>+</mo><mi>Gv</mi></mrow></mfrac></mrow></math></maths>

0099Providing impedance matching at the wanted frequency, which is the resonant frequency of the tank circuit, can give particularly good performance. In order to have Zin_resonnance close to Rs, then Rp or Gv can be tuned (as described below). Outside the bandwidth of interest, Zin can be unmatched as shown in <figref idref="DRAWINGS">FIG. 10</figref>, which illustrates the input matching can be narrowband.

Matching Network

0100In CAN tuners the selective filter <b>1104</b> can be made of an LC tank circuit, and a matching network is required to adapt the impedance between the source (Rs) and the tank circuit. This matching network is often a basic inductance <b>1102</b> as shown in <figref idref="DRAWINGS">FIG. 11</figref>, the performance of which is shown in <figref idref="DRAWINGS">FIG. 12</figref> as a Smith chart. <figref idref="DRAWINGS">FIG. 12</figref> shows in-band input matching in a CAN tuner.

0101Embodiments of the present invention may not require a matching network with the LNA as will be described with reference to the schematic diagram of <figref idref="DRAWINGS">FIG. 13</figref>. <figref idref="DRAWINGS">FIG. 13</figref> illustrates a receiver <b>1300</b> according to an embodiment of the invention. The required input matching can be performed by: (i) adjusting the gain (Gv) of the LNA <b>1302</b>; and/or (ii) adjusting a variable resistance Rf <b>1304</b> that is in parallel with the tank circuit <b>1306</b>. It will be appreciated from the above equations that the input impedance of the receiver <b>1300</b> at the resonant frequency is affected by the values of Gv and Rf. Such functionality is shown in the Smith chart of <figref idref="DRAWINGS">FIG. 14</figref>.

0102The embodiment of <figref idref="DRAWINGS">FIG. 13</figref> illustrates a controller <b>1308</b> that is used to adjust the gain (Gv) of the LNA <b>1302</b> and/or adjust the variable resistance Rf <b>1304</b>. The controller <b>1308</b> may have access to a look up table (LUT) or database to determine which values should be applied for the gain (Gv) and/or variable resistance Rf in accordance with the frequency to which the receiver is tuned.

Quality Factor

0103As shown in <figref idref="DRAWINGS">FIG. 11</figref>, a matching network is necessary in CAN tuners to adapt the impedance between the source and the tank circuit. This is necessary for the input matching and also to provide a good quality factor of the selective filter. Indeed, if the tank circuit is directly connected to Rs, then the quality factor would be:

0000<maths id="MATH-US-00007" num="00007"><math overflow="scroll"><mrow><mrow><mfrac><mi>RsRp</mi><mrow><mi>Rs</mi><mo>+</mo><mi>Rp</mi></mrow></mfrac><mo></mo><msqrt><mfrac><mi>C</mi><mi>L</mi></mfrac></msqrt></mrow><mo>,</mo></mrow></math></maths>

0000which leads to a low quality factor with typical LC values.

0104In contrast, the selective filter used in embodiments of the invention can be directly connected to the source with a limited degradation of the quality factor of the tank. Indeed the quality factor of the receiver <b>500</b> according to the embodiment of the invention of <figref idref="DRAWINGS">FIG. 5</figref> is:

0000<maths id="MATH-US-00008" num="00008"><math overflow="scroll"><mrow><mrow><mi>Q_selective</mi><mo></mo><mi>_LNA</mi></mrow><mo>=</mo><mrow><mfrac><mi>RsRp</mi><mrow><mi>Rs</mi><mo>+</mo><mfrac><mi>Rp</mi><mrow><mo>(</mo><mrow><mn>1</mn><mo>+</mo><mi>Gv</mi></mrow><mo>)</mo></mrow></mfrac></mrow></mfrac><mo></mo><msqrt><mfrac><mi>C</mi><mi>L</mi></mfrac></msqrt></mrow></mrow></math></maths>

0105Thus, if perfect input matching is achieved then the quality factor of the selective LNA is:

0000<maths id="MATH-US-00009" num="00009"><math overflow="scroll"><mrow><mrow><mi>Q_selective</mi><mo></mo><mi>_LNA</mi><mo></mo><mi>_matched</mi></mrow><mo>=</mo><mrow><mfrac><mi>Rp</mi><mn>2</mn></mfrac><mo></mo><msqrt><mfrac><mi>C</mi><mi>L</mi></mfrac></msqrt></mrow></mrow></math></maths>

0106<figref idref="DRAWINGS">FIG. 15</figref> illustrates graphically the performance of an embodiment of the invention that is tuned to receive a signal at 40 MHz. The receiver has source impedance Rs=75 ohms, voltage gain of the LNA Gv=10 (20 dB), value of the inductance of the selective filter L=110 nH, quality factor of the selective filter Q=15, and input power Pin=−5 dBm.

0107The available/composite voltage gain of the LNA is “Gav” and is shown with reference <b>1502</b> in <figref idref="DRAWINGS">FIG. 15</figref>. The matching impact (S<b>11</b> (dB)) is shown with reference <b>1504</b>, where a value of zero represents no matching, and a large negative value represents good matching. The impedance of the filter (Zfilter) is shown with reference <b>1506</b> in <figref idref="DRAWINGS">FIG. 15</figref>.

The Role of the Buffer

0108<figref idref="DRAWINGS">FIG. 16</figref> illustrates graphically the performance of the same embodiment that is illustrated with reference to <figref idref="DRAWINGS">FIG. 15</figref>. <figref idref="DRAWINGS">FIG. 16</figref> shows the output voltage (Vout) with reference <b>1602</b> and the current consumption of the buffer (Ibuffer) with reference <b>1604</b>. The current consumption <b>1604</b> is determined by the input power of a far-away interference signal, and has a maximum value of about 4.1 mA in <figref idref="DRAWINGS">FIG. 16</figref>.

0109<figref idref="DRAWINGS">FIG. 17</figref> illustrates a transient simulation of the output current of the same embodiment that is illustrated with reference to <figref idref="DRAWINGS">FIG. 15</figref>. The wanted frequency is 40 MHz, and the interference frequency is 76 MHz (N+6). The wanted and interference input powers are set to −5 dBm.

0110<figref idref="DRAWINGS">FIG. 17</figref> shows two graphs <b>1702</b> and <b>1704</b> that illustrate the buffer output current and voltage when the receiver receives the wanted signal without interference, and two graphs <b>1706</b> and <b>1708</b> that illustrate the buffer output current and voltage when the receiver receives the wanted signal along with interference.

0111It can be seen from graphs <b>1704</b> and <b>1708</b> that the output voltage of the buffer has only a small increase when the interference signal is present. Similarly, the output buffer current is 2.7 mAp with no interference (graph <b>1702</b>), and has a maximum value of 6.8 mAp with interference (graph <b>1708</b>). Therefore, it will be appreciated that the necessary construction of the buffer, and the output current and voltage, does not vary significantly in the presence of interference.

0112If an interference signal is present, then the power required in the buffer <b>514</b> increases. In an embodiment of the invention the buffer <b>514</b> is a class AB buffer. The power in the class AB buffer will adapt to the presence or absence of an interference signal, thereby acting as a power on demand buffer. An example or realization of such a class AB buffer is depicted as <figref idref="DRAWINGS">FIG. 18</figref>.

0113The buffer <b>1800</b> of <figref idref="DRAWINGS">FIG. 18</figref> is a unity voltage gain buffer with an “enable” input <b>1802</b> that can be used in a selective LNA according to an embodiment of the invention in order to enable or disable the buffer <b>1800</b>. Such an example is illustrated as <figref idref="DRAWINGS">FIG. 20</figref> and is described below. This unity voltage gain buffer <b>1800</b> consists of a super emitter follower composed of transistors Q<b>1</b> and Q<b>2</b>. The buffer <b>1800</b> can also adapt the impedance between the LNA output and the selective filter so that the voltage gain of the LNA is not impacted by the impedance of the selective filter, as described in more detail below.

0114The biasing of the super emitter follower is controlled by fixing the DC collector current of Q<b>1</b> and Q<b>2</b> by voltage source V<b>2</b>. The base voltage of Q<b>1</b> is fixed by voltage source V<b>1</b>.

Small Signal Behaviour of the Buffer of FIG. 18

0115When a voltage is applied to the input <b>1804</b> at the base of Q<b>1</b>, it is converted into a collector current by Q<b>1</b>. This current is flowing into C<b>2</b> and is amplified by Q<b>2</b>. The output current <b>1806</b> of the buffer <b>1800</b> is composed of the collector current of Q<b>2</b>, and of the emitter current of Q<b>1</b> which is about β times smaller than the collector current of Q<b>2</b> (β is the current gain of Q<b>2</b>). As the collector current of Q<b>1</b> is small compared to a classical emitter follower, the base-emitter voltage of Q<b>1</b> is also small, leading to a highly linear unity voltage gain buffer.

Large Signal Behaviour of the Buffer of FIG. 18

0116The C<b>2</b> feedback loop allows class AB operation with this buffer <b>1800</b>. In effect, when a large negative alternance is present at the buffer input “in”, then a positive alternance is present on the base of Q<b>2</b>. This positive alternance can have a voltage level, leading to an output current <b>1806</b> of the buffer that is higher than the DC current of the buffer <b>1800</b>. Now when a large positive alternance is present at the buffer input “in” <b>1804</b>, Q<b>2</b> is turned OFF. The collector voltage of Q<b>1</b> has a negative alternance increasing the current flowing into R<b>2</b>. Finally the output current <b>1806</b> of the buffer <b>1800</b> is higher than the DC current of the buffer <b>1800</b> for both positive and negative alternance of the input voltage. This is true while the base-collector voltage of Q<b>1</b> stays negative. If not, Q<b>1</b> enters saturation and the output signal <b>1806</b> of the buffer is no more linear.

Enable Command for the Buffer of FIG. 18

0117A control voltage signal “en” <b>1802</b> is used to switch ON or OFF the buffer of <figref idref="DRAWINGS">FIG. 18</figref>. This control voltage is connected to the gate of M<b>4</b> and the inverse control voltage/en is connected to the gate of M<b>1</b>, M<b>2</b>, M<b>3</b>. When the control voltage <b>1802</b> is “1” the base of Q<b>1</b> and the base of Q<b>2</b> are shunted to the ground so that no current is flowing into the buffer <b>1800</b>. In this case the output impedance of the buffer is high and the buffer <b>1800</b> is switched off.

0118One or more embodiments disclosed below can benefit from the option of being able to switch off the buffer.

Centre Frequency Tunability

0119Embodiments of the present invention are intended to be used in applications with a broadband input spectrum, for example in silicon tuners for TV reception of signals having a frequency from 40 MHz up to 1 GHz. To cover such a large frequency range it can be advantageous to have different bands for the selective filter. In effect with an integrated capacitor bank it may not be possible to maintain a good quality factor for a selective filter having a fixed inductance over a wide frequency range.

0120<figref idref="DRAWINGS">FIG. 19</figref> shows how the quality factor changes with frequency for selective/tuneable filters having different inductance values. It can be seen that for each inductance value there is a limited range of frequencies that can be used whilst maintaining an acceptable quality factor, and that several inductors are necessary to cover the whole TV band with a good quality factor.

0121According to an embodiment of the invention, multiple selective feedback paths can be provided between the output of the amplifier and the input of the amplifier such that components can be included in, or excluded from, the feedback path in order to best suit a frequency of wanted signals that are to be received.

0122A receiver <b>2000</b> according to a further embodiment of the invention is shown as <figref idref="DRAWINGS">FIG. 20</figref>. The receiver <b>2000</b> provides an implementation for using switchable inductances in order to be able to provide a good quality factor over a range of signal frequencies.

0123The receiver <b>2000</b> has an LNA <b>2002</b>, a first feedback path including a unity gain buffer <b>2004</b> and a tuneable filter <b>2006</b> in series, and a second feedback path including a unity gain buffer <b>2008</b> and a tuneable filter <b>2010</b> in series. Each unity gain buffer <b>2004</b>, <b>2008</b> can be switched on or off in order to selectively include or exclude the associated tuneable filter <b>2006</b>, <b>2010</b> in the feedback path, and may be referred to as controllable buffers. The buffers <b>2004</b>, <b>2008</b> can be switched on and off using an enable signal as described above in relation to <figref idref="DRAWINGS">FIG. 18</figref>. In this example, only one buffer <b>2004</b>, <b>2008</b> is switched on at a time, corresponding to the inductance for the desired band as shown in <figref idref="DRAWINGS">FIG. 19</figref> for example.

0124The capacitors in the tuneable filters <b>2006</b>, <b>2010</b> of <figref idref="DRAWINGS">FIG. 20</figref> are programmable/tuneable in order to tune the receiver <b>2000</b> to a desired centre frequency. The centre frequency of the in use filter <b>2006</b>, <b>2010</b> is located at or near the frequency of the wanted channel.

0125<figref idref="DRAWINGS">FIG. 21</figref> illustrates a programmable capacitor bank with MOS switches <b>2100</b> that is one example implementation of a variable capacitor that can be used with embodiments of the invention.

0126The circuit of <figref idref="DRAWINGS">FIG. 21</figref> includes a first capacitor <b>2102</b>. In parallel with the first capacitor <b>2102</b> is a second capacitor <b>2104</b> and a MOS switch <b>2106</b>. Also in parallel with the first capacitor <b>2102</b> is a third capacitor <b>2108</b> and a MOS switch <b>2110</b>. The MOS switches <b>2106</b>, <b>2110</b> are operable to selectively include or exclude the associated capacitors <b>2104</b>, <b>2108</b> in parallel with the first capacitor <b>2102</b> thereby adjusting the overall capacitance of the circuit of <figref idref="DRAWINGS">FIG. 21</figref>. A required granularity and calibration range can define the size and the number of switches/additional capacitors that are required.

0127The MOS switches <b>2106</b>, <b>2110</b> can be connected to the input of an LNA to limit the voltage swing on the switches. This connection can limit linearity degradation due to the switches. On the other hand, the connection of the switches to the LNA input can increase the parasitic capacitance on this node, which can degrade the input matching, the NF, and the gain at high frequencies.

0128MOS switches that are off can also increase the distortion due to parasitic junction capacitances between drain and bulk, and between source and bulk. The junction capacitance is a non linear function of the drain/bulk voltage:

0000<maths id="MATH-US-00010" num="00010"><math overflow="scroll"><mrow><mi>Cj</mi><mo>=</mo><mfrac><msub><mi>C</mi><mn>0</mn></msub><msup><mrow><mo>(</mo><mrow><mn>1</mn><mo>-</mo><mfrac><msub><mi>V</mi><mi>bg</mi></msub><mi>φ</mi></mfrac></mrow><mo>)</mo></mrow><mi>n</mi></msup></mfrac></mrow></math></maths>

0129Where C₀, φ and n are technology dependent (n=0.2 . . . 0.5).

0130As depicted in <figref idref="DRAWINGS">FIG. 22</figref>, the distortion of parasitic junction capacitances in the MOS switches can be reduced by reverse biasing the bulk/drain diode.

0131If the MOS switches <b>2106</b>, <b>2110</b> of <figref idref="DRAWINGS">FIG. 21</figref> are made smaller to reduce parasitic capacitance then the ON resistance of the switches will increase, thereby leading to a decrease of the quality factor of the filter and additional distortion.

0132<figref idref="DRAWINGS">FIG. 23</figref> illustrates a receiver <b>2300</b> according to an alternative embodiment of the present invention. The receiver <b>2300</b> includes an LNA <b>2302</b> with three parallel branches in its feedback path. The first branch of the feedback path has a unity gain buffer <b>2304</b> and a variable capacitor <b>2306</b> in series. In this example, the buffer <b>2304</b> is always on so that the variable capacitor <b>2306</b> is always included in the feedback path of the LNA <b>2302</b>.

0133The second branch of the feedback path includes a unity gain buffer <b>2308</b> and a first inductor <b>2310</b> in series. The unity gain buffer <b>2308</b> in the second branch can be switched on or off to effectively include or exclude the first inductor <b>2310</b> as part of a tuneable filter along with the capacitor <b>2306</b>. The third branch of the feedback path includes a unity gain buffer <b>2312</b> and a second inductor <b>2314</b> in series. The unity gain buffer <b>2312</b> in the third branch can also be switched on and off in the same way as the unity gain buffer <b>2308</b> in the second branch.

0134In some embodiments, only one of the unity gain buffers <b>2308</b>, <b>2312</b> in the second and third branches can be switched on at any one time. The second inductor <b>2314</b> can have a different inductance value to the first inductor <b>2310</b> so that different branches of the feedback path can be used in accordance with a desired frequency that is to be received.

0135The example of <figref idref="DRAWINGS">FIG. 23</figref> can be advantageous as the parasitic capacitance at the antenna side of the receiver can be reduced as only one capacitor bank is driven by a dedicated buffer <b>2304</b>. In this case the output impedance of the buffer will degrade the quality factor of the LC tank.

0136If the parasitic capacitance of the MOS switches at the antenna node is still a problem with the implementation of <figref idref="DRAWINGS">FIG. 23</figref>, then the MOS switches could be replaced entirely or partially by buffers, and such an example is depicted as <figref idref="DRAWINGS">FIG. 24</figref>. This may be at the cost of additional power consumption.

0137<figref idref="DRAWINGS">FIG. 24</figref> illustrates a programmable capacitor bank that is one example implementation of a variable capacitor that can be used with embodiments of the invention. <figref idref="DRAWINGS">FIG. 24</figref> illustrates a circuit that is similar to that of <figref idref="DRAWINGS">FIG. 21</figref>, except that the MOS switches of <figref idref="DRAWINGS">FIG. 21</figref> have been replaced with controllable unity gain buffers <b>2402</b>, <b>2404</b>.

Centre Frequency Calibration

0138<figref idref="DRAWINGS">FIG. 25</figref> illustrates schematically a receiver <b>2500</b> according to another embodiment of the invention. A calibration of the centre frequency of the receiver <b>2500</b> can be performed by injecting a calibration tone <b>2504</b> to the input of the receiver <b>2500</b> as depicted in <figref idref="DRAWINGS">FIG. 25</figref>.

0139An amplifier <b>2502</b> is used to inject the calibration tone <b>2504</b> tone, and should have an output impedance close to 75 ohms to match with the input of the receiver <b>2500</b>. It may be necessary to add a series switch <b>2506</b> between the antenna and the receiver <b>2500</b> to avoid emission of the calibration signal. This series switch <b>2506</b> may be necessary in cable applications where it can be forbidden to inject a signal on the cable. Also the power of the calibration tone signal <b>2504</b> should be sufficiently low to avoid perturbations on the cable.

0140The calibration tone <b>2504</b> is injected at the input to the receiver <b>2500</b>. The output signal of the receiver <b>2500</b> is down converted by a mixer <b>2508</b> with the proper local oscillator (LO) signal. The output intermediate frequency (IF) signal of the mixer is low pass filtered by filter <b>2510</b>. A power detector <b>2512</b> senses the output signal of the low filter <b>2510</b> and a digital state machine <b>2514</b> is used to adjust the value of the capacitor <b>2516</b> in the feedback path of the receiver <b>2500</b> until the centre frequency of the selective filter that is provided by the capacitor <b>2516</b> and inductor <b>2518</b> is as close as possible to the calibration tone.

Gain Variation

0141Changing the value of the capacitor of the selective filter in the feedback path of the amplifier may change the composite voltage gain of the receiver. In some embodiments it may be desirable to keep the composite voltage gain constant, in which case the intrinsic voltage gain of the LNA can be changed to a certain extent. Such a variation in the intrinsic voltage gain of the LNA may be at the cost of a decreased quality factor.

0142<figref idref="DRAWINGS">FIG. 26</figref> shows graphically how the composite voltage gain of a receiver according to an embodiment of the invention can vary when the centre frequency is tuned to different values.

0143<figref idref="DRAWINGS">FIG. 27</figref> illustrates a receiver <b>2700</b> according to an embodiment of the invention whereby the intrinsic voltage gain of the LNA <b>2702</b> can be changed in accordance with the value of the variable capacitor <b>2704</b> of the selective filter such that the composite voltage gain of the selective LNA is kept substantially constant.

0144<figref idref="DRAWINGS">FIG. 27</figref> illustrates a controller <b>2706</b> that can provide an output signal for controlling the intrinsic voltage gain of the LNA <b>2702</b>. The controller <b>2706</b> may have access to a look up table (LUT) or database to determine which values should be applied for the intrinsic voltage gain of the LNA <b>2702</b> in accordance with the frequency to which the receiver is tuned.

0145<figref idref="DRAWINGS">FIG. 28</figref> illustrates graphically how the composite voltage gain of the selective filter can be compensated by a variation of the LNA voltage gain to a certain extent. Rs=75 ohms in this example. For low values of Rp, the composite voltage gain cannot be overtaken even with higher LNA voltage gain.

0146If a receiver according to an embodiment of the present invention must have variable gain then the voltage gain of the LNA can be changed. As this can degrade the quality factor, a switchable matching network can be incorporated into the feedback path of the LNA. This matching network can be an integrated switched capacitor bank.

0147In some examples, it can become difficult to make the receiver stable for the whole gain settings by switching the voltage gain of the LNA. In this case, the gain variation can be performed with a resistive attenuator in front of the receiver. <figref idref="DRAWINGS">FIG. 29</figref> illustrates schematically such a resistive attenuator <b>2902</b> at the input to the receiver <b>2904</b>. As the attenuator <b>2902</b> is made of resistances, then the input matching is more and more broadband as the series resistor is increased. The receiver <b>2904</b> includes a detector <b>2906</b> that can sense the LNA output signal and then adjust the value of the resistive attenuator <b>2902</b> so that the output signal amplitude does not exceed a take over point (TOP) but is substantially similar to the TOP in order to protect the LNA against clipping. In this way, significant distortion at the output of the amplifier can be reduced or avoided. <figref idref="DRAWINGS">FIGS. 30</figref><i>a </i>to <b>30</b><i>d </i>illustrate schematically receivers according to embodiments of the invention. <figref idref="DRAWINGS">FIG. 30</figref><i>a </i>illustrates an LNA with a single input to a single output. FIG. <b>30</b><i>b </i>illustrates an LNA with a single input to differential outputs. <figref idref="DRAWINGS">FIG. 30</figref><i>c </i>illustrates an LNA with differential inputs to differential outputs.

0148<figref idref="DRAWINGS">FIG. 30</figref><i>d </i>is slightly different as the “amplifier” that is disclosed herein as having a tuneable filter in its feedback path is not the LNA. <figref idref="DRAWINGS">FIG. 30</figref><i>d </i>illustrates an embodiment of the invention having an LNA <b>3006</b> that receives an input signal from an antenna (not shown) and provides the output signal. Also connected to the input of the LNA <b>3006</b> is an amplifier <b>3002</b>. The output of the amplifier <b>3002</b> is connected to its input using a feedback path comprising a tuneable filter <b>3004</b>. The feedback path can also include any of the features disclosed in other embodiments of the invention.

0149The example of <figref idref="DRAWINGS">FIG. 30</figref><i>d </i>provides the same advantages as the other embodiments of the invention as the Miller effect of the tuneable filter serves to filter out the interference signals before they are processed by the LNA <b>3006</b>.

0150In some examples, the presence of a filter in the feedback of the LNA may create instability. An example of a receiver according to an embodiment of the invention whereby the stability is improved is provided as <figref idref="DRAWINGS">FIG. 31</figref>. The receiver is a single input to a differential output configuration.

0151<figref idref="DRAWINGS">FIG. 31</figref> shows the components of the LNA within box <b>3102</b>, the components of the buffer in box <b>3104</b> and the components of the tuneable filter in the box <b>3106</b>.

0152In the implementation of <figref idref="DRAWINGS">FIG. 31</figref> the LNA <b>3102</b> is a basic differential pair. The feedback loop contains only two active stages, and only one gain stage. The loop gain is defined by the differential pair and is about 20 dB in DC. In this configuration the stability is ensured even if the LC tank is tuned as can be seen from the graphical results of <figref idref="DRAWINGS">FIGS. 32 and 33</figref>.

0153<figref idref="DRAWINGS">FIG. 32</figref> illustrates the gain and S<b>11</b> versus LC tank center frequency for the implementation of <figref idref="DRAWINGS">FIG. 31</figref>.

0154<figref idref="DRAWINGS">FIG. 33</figref> illustrates the loop gain and phase for the implementation of <figref idref="DRAWINGS">FIG. 31</figref>, showing that this implementation is stable with more than 85° phase margin.

0155Embodiments of the invention can be used in, or with, silicon tuners for television reception, for example in LCD televisions. Embodiments can be used in any receiver where it is advantageous to improve immunity to strong interference signals without impacting other performances like noise figure, such as: GPS, terrestrial television and cellular, Wifi, Bluetooth, cordless, and satellite communications, as non-limiting examples.

0156Embodiments of the invention can include one or more of the following: <ul id="ul0005" list-style="none"><li id="ul0005-0001" num="0000"><ul id="ul0006" list-style="none"><li id="ul0006-0001" num="0157">A tracking filter, which may be a narrowband tracking filter;</li><li id="ul0006-0002" num="0158">A Broadband low noise amplifier (LNA);</li><li id="ul0006-0003" num="0159">A selective filter;</li><li id="ul0006-0004" num="0160">A selective filter connected between the input and the output of an LNA;</li><li id="ul0006-0005" num="0161">A centre frequency of a selective filter that is programmable to track the frequency of a wanted channel;</li><li id="ul0006-0006" num="0162">The interference signals are filtered at an LNA input;</li><li id="ul0006-0007" num="0163">LNA input and output can be single or differential; and</li><li id="ul0006-0008" num="0164">If the gain of an LNA is variable, the core amplifier gain can be changed. If this causes a stability issue then the gain of the LNA can be varied by connecting an attenuator in front of the LNA.</li></ul></li></ul>

0165An embodiment of the invention can include an LNA, a feedback amplifier, and a selective filter. The LNA and the feedback amplifier can be broadband and have a high input impedance. The selective filter can be an LC tank circuit, and the feedback amplifier can be a unity voltage gain buffer.

0166Immunity to close-in-channel interference signals can be of great importance in TV tuners for terrestrial reception. Also immunity to strong out of band interference signals can be important due to new LTE standard for mobile communications. A low noise amplifier/receiver according to embodiments of the present invention can have selectivity to filter out interference signals. The receiver of an embodiment of the present invention is made of a core amplifier and of a band pass filter. The band pass filter is connected between the input and the output of the core amplifier. Due to the Miller effect, this filtering network is input-referred and acts as if it was connected directly to the antenna. This configuration can be very effective to filter out interference signals directly at the antenna. As the band pass filter is tunable, the centre frequency is adaptive to the wanted channel. Embodiments disclosed herein can improve immunity against LTE interference signals. It can also be a key feature to enable CAN tuners to be replaced by silicon tuners in future televisions.