A system and method for minimizing dissipation in rf power amplifiers

<u>BACKGROUND OF THE INVENTION</u>

0001The design of radio frequency ("RF") power amplifiers is typically accomplished by taking into account the expected operating environment of the amplifier and the desired output characteristics of the amplifier, such as the output load line impedance. There are usually interrelated yet contradictory requirements that must be taken into account in setting the output load line impedance and trade-offs must be made that affect the performance of the amplifier. Some of the factors to be considered in setting the output load line impedance are output peak and average signal requirements, distortion limits, efficiency requirements, and the available power supply voltage. The trade-off problem is made more acute when the amplifier is to be designed for high peak to average signal conditions, such as those found in Trellis Coded 8-Level Vestigial Side Band ("8VSB") and/or Coded Orthogonal Frequency Division Multiplexing ("COFDM") television transmitters.

0002For instance, one example of the contradictory requirements and trade-offs that need to be made involves the distortion limit of an amplifier. Typically, a designer strives to keep distortion to a minimum and amplifier efficiency to a maximum. The distortion limit is set by the maximum linear amplifier output power of the amplifier. The maximum linear output is a function of the amplifier output load line impedance and the voltage supplied to the amplifier by a power supply, e.g., the drain source power supply. A lower output load line impedance and a higher amplifier supply voltage typically result in a higher maximum linear output (i.e., a higher distortion limit), all other factors being relatively constant. A higher distortion limit allows for more amplifier head room before the negative effects of distortion, such as non-linear amplification and a lack of spectral containment, need to be considered. However, the efficiency of the amplifier depends on the output signal level in relation to the maximum linear output such that amplifier efficiency increases as the output signal level increases up to and beyond the maximum linear output power of the amplifier. Therefore, maximum amplifier efficiency is attained in the distortion operating region.

0003As mentioned above, 8VSB and COFDM signals have high peak to average power ratios. An 8VSB signal has a peak to average power ratio in the range of 5 to 8 dB, typically around 6.5 dB. A COFDM signal has a peak to average power ratio in the range of 7 to 11 dB, typically around 9 dB. For either an 8VSB or COFDM signal, in order to have acceptable spectral containment of the output signal, signal peaks may only exceed the distortion limit, i.e., push the amplifier into its non-linear region, by a limited amount. As a result, the output signal average power is well below the amplifier's maximum linear power capability therefore resulting in a low amplifier efficiency.

0004One drawback of a low amplifier efficiency is that the amplifier must dissipate the excess power as heat. Reliable amplifier design depends on, among other things, maintaining reasonable limits on transistor junction temperatures. Low amplifier efficiencies require cooling systems with a higher heat transfer capacity, and therefore higher cost, in order to prevent overheating and subsequent failure of the amplifier.

0005Where an amplifier is designed to operate over a range of frequencies ("wide band") the load line impedance will change due to the natural behavior of the amplifier's output network as a function of the frequency of the input signal to the amplifier. Typical prior art wide band amplifiers are designed based on a minimum average output power capability and are operated at the same average output power capability at all frequencies.

0006However, a wide band amplifier designed for a minimum average output power capability over all frequencies in its range will have a higher capability, at most frequencies, to operate at an average output power that is greater than the minimum average output power capability. This is due to the fact that designing for a minimum average power capability means designing an output network whose load line is never higher (as input signal frequency varies) than the impedance dictated by the drain supply voltage for that power level. Since it is impossible to eliminate variations in the impedance transformation of the network, the impedance the network presents at other frequencies will always be lower than the design target impedance at the design minimum average output power capability. The reduced efficiency will define the worst case efficiency and hence the worst case excess power dissipation requirement.

0007The present invention takes advantage of the fact that it is possible to operate a wide band power amplifier at an applied supply voltage other than the operating voltage for a given design condition, such as the minimum target power condition described above. For example, if a wide band amplifier is operated at the same average output power at two frequencies and the load line impedance is higher for one of the two frequencies, the amplifier will operate at a higher efficiency where the load line impedance is higher. This is due to the fact that the amplifier makes more effective use of the applied voltage swing allowed by the applied power supply voltage, e.g., V_cc where the impedance is higher. In the case where the load line impedance is lower for the same average output power, the amplifier is being supplied with more supply voltage than required by the amplifier resulting in an increase in excess power and a decrease in the efficiency of the amplifier. In order to decrease the excess power and thereby increase the efficiency of the amplifier, the amplifier supply voltage can be decreased.

0008Therefore, once the design of the amplifier and the electrical circuit to which the amplifier is connected is known, the behavior of the load line can be characterized as a function of one or more of the operating conditions of the amplifier and/or electrical circuit, specifically the frequency of the input signal to the amplifier. Other operating conditions that are related in a known manner to a change in the input signal frequency may be sensed. Therefore, the effects of the changing load line due to the changing operating conditions can be compensated for by adjusting the voltage supplied to the amplifier by the amplifier power supply as a function of one or more of the sensed operating conditions. Some of the operating conditions that may change, and therefore indicate a change in the input signal frequency, and can be sensed as affecting the load line impedance are the frequency of the input signal to the amplifier, the impedance of the electrical circuit to which the amplifier is connected, the efficiency of the amplifier, the modulation of the input signal to the amplifier, the ratio of peak power to average power of the input signal to the amplifier, and the ratio of peak voltage to average voltage of the input signal to the amplifier. It is to be understood that other operating conditions of the amplifier or the electrical circuit to which the amplifier is connected may also affect the load line impedance of the amplifier and are contemplated by the present invention.

0009The present invention overcomes the limitations of the prior art by enabling the voltage supplied to the amplifier by the amplifier power supply to be adjusted either dynamically as a function of one or more of the operating conditions described above or manually by an operator either in response to or independently of one or more of the operating conditions. For example, the operator may be informed as to the input signal frequency by sources not based on a measurement of the input signal frequency, such as when the operator is told by another (e.g., the system user) that the input signal frequency either is a certain value or will be changed to a certain value. This last option does not form part of the invention.

0010Accordingly, it is an object of the present invention to obviate many of the above problems in the prior art and to provide a novel amplifier system and method for dynamically adjusting the power supplied to an amplifier according to the claims.

0011These and many other objects and advantages of the present invention will be readily apparent to one skilled in the art to which the invention pertains from a perusal of the claims, the appended drawings, and the following detailed description of the preferred embodiments.

0012Document <patcit id="pcit0001" dnum="EP0458071A"><text>EP-A-0 458 071</text></patcit> discloses an adaptive power amplifier system for use with VSWR loads. The system includes an adaptive power supply, which includes a dynamic power feedback system and a power source. The dynamic feedback system monitors the power drawn by the amplifier. When a high dissipation is sensed, the adaptive power supply dynamically adapts the amount of power supplied to the amplifier. The amount of power supplied to the amplifier is determined by the control circuits. The control circuits, in response to the feedback voltage and current feedback generate control signals such that voltage of the output of the power stages follow predetermined voltage and current characteristics.

0013Document <patcit id="pcit0002" dnum="US5251331A"><text>US-A-5 251 331</text></patcit> discloses a dual mode power amplifier, used in a hybrid analog/digital radio telephone. The mode can be digital or analogue and is detected by the mode detection line. Depending on the mode, detected by the mode selection line, the voltage converter is either switched on or off, thus providing two different predetermined voltages to the amplifier based on the sensed modulation of the signal.

0014Document <patcit id="pcit0003" dnum="EP1120904A"><text>EP-A-1 120 904</text></patcit> discloses the power amplifier system which can reconfigure the amplifier based on the upstream signal information, such as the measured peak power, the measured average power, the number of users, the type of carriers, the number of carriers and/or average power per carrier. Processing circuitry obtains or receives the information about the signal and provides the control signal in order to control the peak power capability of the amplifier.

0015Document <patcit id="pcit0004" dnum="US2001014613A1"><text>US 2001/014613 A1</text></patcit> discloses a communication system having a base station and a communication device to communicate with the base station. The communication device includes, among other components, an RF amplifier and power supply. Power supply can control RF output in dependence on the control input. The communications device also includes a table means containing a listing of possible supply control input values, related to possible RF output power values. The table selection input is coupled to the control input of the power supply.

0016Document <patcit id="pcit0005" dnum="US4547746A"><text>US-A-4 547 746</text></patcit> discloses a power amplifier, constructed to change its operating conditions to match the load as presented at the output of the amplifier. The output of the RF transistor power amplifier is coupled through a directional coupler to a load and the forward power. Reflected power coupled through a comparator circuit to provide a difference which represents the true load power. The input current of the RF power amplifier is sensed by the differential amplifier to provide a second signal which is compared with the true load power in an analogue divider to produce a control voltage. The control voltage is coupled to control variable voltage supply, driving the supply voltage of the power amplifier.

Summary of the Invention

0017The present invention provides a method of dynamically controlling the amplification linear range applied by an amplifier in an electrical circuit having an input terminal for receiving an input signal comprising the steps of claim 1. These steps include providing an electrical circuit having an input terminal for receiving an input signal and providing an amplifier in said electrical circuit; amplifying the input signal with the amplification linear range set as a function of an operating condition of the electrical circuit; applying a variable voltage to said amplifier from a variable power supply to thereby control the amplification linear range applied by said amplifier to said input signal; sensing said operating condition of the electrical circuit; populating a lookup table by (i) collecting data on the performance of a test amplifier circuit representative of the design of said amplifier in said electrical circuit when operating said test amplifier during data collection at various supply voltages and frequencies, while maintaining the required spectral containment; and by (ii) collecting data on the performance of said amplifier from said actual operating electrical circuit; determining from said lookup table the desired variable voltage applied to said amplifier as a function of said operating condition; and controlling the level of said variable voltage applied to said amplifier as a function of said operating condition, thereby dynamically controlling the amplification linear range of said amplifier.

<u>BRIEF DESCRIPTION OF THE DRAWINGS</u>

0018<ul id="ul0001" list-style="none" compact="compact"><li><figref idref="f0001">Figure 1</figref> is a simplified schematic circuit diagram of a first embodiment of the present invention illustrating a single amplifier stage.</li><li><figref idref="f0001">Figure 2</figref> is a depiction of a lookup table illustrating the indexing of the table by operating conditions so as to extract a control signal(s).</li><li><figref idref="f0002">Figure 3</figref> is a simplified schematic circuit diagram of a second embodiment of the present invention illustrating a series cascade of amplifier stages powered by a single variable power supply.</li><li><figref idref="f0003">Figure 4</figref> is a simplified schematic circuit diagram of a third embodiment of the present invention illustrating a series cascade of amplifier stages where each amplifier stage is powered by a separate variable power supply.</li><li><figref idref="f0004">Figure 5</figref> is a simplified schematic circuit diagram of a fourth embodiment of the present invention illustrating a parallel cascade of amplifier stages where select sensing signal connections and power supply connections are not shown for clarity.</li></ul>

<u>DESCRIPTION OF PREFERRED EMBODIMENTS</u>

0019Present known amplifier arrangements, especially for television transmitters for signals with large peak to average power ratios such as 8VSB and COFDM signals, do not take advantage of the fact that the efficiency of the amplifier, or amplifier network, can be improved by dynamically controlling the amplifier power control voltage as a function of one or more operating condition, as defined above, while maintaining acceptable levels of distortion and spectral containment.

0020The present invention overcomes the limitations of the prior art by enabling the voltage supplied to the amplifier, or amplifier network, by the amplifier power supply to be adjusted either dynamically as a function of one or more of the operating conditions described above. The amplifier network may be a cascaded series of amplifiers or a parallel cascade of amplifiers. The amplifiers in a network of amplifiers may be supplied with power either from independent variable power supplies or from a common variable power supply.

0021In one embodiment of the present invention, the variable power supply to an amplifier in an electrical circuit is dynamically controlled through the use of a lookup table responsive to one or more of the operating conditions of the electrical circuit. The lookup table is indexed by one or more of the operating conditions and the amount of amplification to be applied to an input signal to the amplifier is determined. Another embodiment of the invention comprises a television transmitter circuit including an RF power amplifier circuit capable of amplifying a variable frequency COFDM or 8VSB input signal where the amount of amplification applied to the input signal is dynamically controlled through the use of a lookup table as a function of the frequency of the input signal.

0022With reference to the drawings, like numerals represent like components throughout the several drawings.

0023With reference now to the embodiment of the present invention shown in <figref idref="f0001">Figure 1</figref>, the amplifier 10 receives an input signal 11 and outputs a signal to an electrical circuit 18 depicted simply as a block in <figref idref="f0001">Figure 1</figref> for clarity purposes. It is to be understood that the electrical circuit 18 can be any electrical circuit and preferentially represents an electrical circuit that would be in a television transmitter downstream of the power amplification stage. The electrical circuit 18 may contain amplifier stages in addition to the amplifier 10 depicted in <figref idref="f0001">Figure 1</figref>. In one embodiment, not shown for clarity, the electrical circuit 18 may include the amplifier 10. The variable power supply 12 supplies a voltage to the amplifier 10 in response to the control signal 13 from the system controller 14.

0024The system controller 14 preferentially includes sensing means for sensing the operating conditions, as defined above, via sensors 17A, 17B, and 17C adapted to sense the operating conditions, of the input signal 11, the amplifier 10, and the electrical circuit 18. Sensors 17A, 17B, and 17C transmit sensing signal 15A, 15B, and 15C, respectively, to the sensing means in the system controller 14. The sensing means may also include a lookup table so that a control signal 13 can be sent to the variable power supply 12 so as to dynamically adjust the voltage being supplied by the variable power supply to the amplifier 10 as a function of the operating conditions sensed.

0025The sensing means is comprised of known prior art components and circuitry for operating as described herein. The sensing means includes the sensors 17A and 17B for sensing the operating conditions of the circuit shown in <figref idref="f0001">Figure 1</figref>. The sensing means includes a lookup table for determining the amount of amplification to be applied to the input signal 11 by the amplifier 10 as a function of the sensed operating conditions and for sending the control signal 13 to the variable power supply 12 thereby controlling the voltage level supplied by the variable power supply to the amplifier in response to one or more of the sensed operating conditions.

0026The user interface 16 may be used to interface with the system controller 14 so that human interaction with the circuit depicted in <figref idref="f0001">Figure 1</figref> is possible. The user interface 16 may be used to manually control the voltage supplied by the variable power supply 12 to the amplifier 10 either in response to one or more of the operating conditions or altogether independent of the operating conditions, as discussed above.

0027In one mode of operation, the circuit shown in <figref idref="f0001">Figure 1</figref> the input signal 11 is applied to an input terminal of the amplifier 10. The input signal 11 may be an 8VSB or COFDM signal as described herein. The sensors 17A, 17B, and 17C sense the operating conditions as described herein. The system controller 14, which comprises sensing means including a lookup table, as later described herein, accesses the lookup table to determine the characteristics of the control signal 13 based on one or more of the operating conditions. The system controller 14 sends a control signal 13 to the variable power supply 12, which is responsive to the particular characteristics of the control signal 13, such as voltage or current level, frequency, or phase, so as to dynamically control the voltage of the power supplied to the amplifier 10 as a function of one or more of the sensed operating condition or conditions.

0028With reference now to <figref idref="f0001">Figure 2</figref>, a lookup table 20 is depicted illustrating the indexing of the table by the operating conditions 22 so as to extract a control signal 13 for controlling the variable power supply 12 in any of <figref idref="f0001">Figures 1</figref>, <figref idref="f0002">3</figref>, <figref idref="f0003">4</figref>, or <figref idref="f0004">5</figref>.

0029The lookup table 20 is populated by collecting data on the performance of a test amplifier representative of the design of the amplifier 10 in response to input signals generated by an 8VSB or COFDM signal source where the test amplifier is connected to an instrumented test load. Similarly, the lookup table 20 is populated by collecting data on the performance of a test amplifier/test network system representative of the design of the amplifier 10 and the electrical circuit 18 in response to input signals of any modulation and of any frequency within the frequency range of the amplifier 10. The signal source may generate input signals such as the amplifier 11 may receive during the operation of the circuit in any of <figref idref="f0001">Figure 1</figref>, <figref idref="f0002">3</figref>, <figref idref="f0003">4</figref>, or <figref idref="f0004">5</figref>. The test amplifier may be operated under a nominal supply voltage at a number of input signal frequencies in the expected operating range of the circuit in any of <figref idref="f0001">Figure 1</figref>, <figref idref="f0002">3</figref>, <figref idref="f0003">4</figref>, or <figref idref="f0004">5</figref> while maintaining the required spectral containment, average output power, and amplifier efficiency and/or power dissipation. The test amplifier may then be operated at supply voltages above and below the nominal supply voltage. The results of these tests may be used to populate the lookup table 20 in <figref idref="f0001">Figure 2</figref>. The lookup table is populated in conjunction with the description above, by collecting data from the actual operating circuit shown in <figref idref="f0001">Figure 1</figref> by known techniques.

0030With reference now to <figref idref="f0002">Figure 3</figref>, where like numerals represent like components in <figref idref="f0001">Figure 1</figref>, which operate in a like manner, a second embodiment of the invention is shown where the amplifiers 10-1 through 10-N are arranged in a cascaded series. A plurality of amplifiers is contemplated in the cascaded series shown, such as two amplifiers, three amplifiers, etc. In this embodiment, the variable power supply 12 is shown as supplying power to each of the cascaded amplifiers. The system controller 14, which preferentially comprises sensing means including a lookup table as described above, receives the sensing signals 15D and 15C. The sensing signal 15D may originate from one or more of the sensors 17N. Only one of the sensors 17N is labeled in <figref idref="f0002">Figure 3</figref> for clarity, but it is to be understood that the sensing signal 15D may originate from any of the sensors 17N shown immediately preceding or following one of the cascaded amplifiers 10-1 through 10-N. Sensing signal 15C may originate from the sensor 17C sensing the operating conditions of the electrical circuit 18 as described above in the description of <figref idref="f0001">Figure 1</figref>.

0031In one embodiment of the present invention, an electrical circuit, such as the electrical circuit 18 shown in the Figures, may be included after each or some of the amplifiers 10-1 through 10-N. Where there is more than one electrical circuit, the electrical circuits may all be the same, some of the electrical circuits may be the same as some other electrical circuits, or all of the electrical circuits may be different.

0032In one mode of operation, the circuit shown in <figref idref="f0002">Figure 3</figref> senses one or more of the operating conditions via one or more of sensors 17N and/or 17C and a sensing signal 15D and/or 15C, respectively, may be sent to the system controller 14. The system controller comprises a sensing means, as described above, including a lookup table. The lookup table determines the characteristics of the control signal 13 as a function of one or more of the operating conditions. The control signal 13 is sent from the system controller 14 to the variable power supply 12 so as to dynamically control the voltage of the power supplied to the amplifier 10 as a function of one or more of the sensed operating condition or conditions.

0033In the particular embodiment shown in <figref idref="f0002">Figure 3</figref>, the amplifiers 10-1 through 10-N are dynamically adjusted in concert as the variable power supply 12 is adjusted in response to one or more of the sensed operating conditions.

0034With reference now to <figref idref="f0003">Figure 4</figref>, where like numerals represent like components in <figref idref="f0001">Figures 1</figref> and <figref idref="f0002">3</figref>, which operate in a like manner, a third embodiment of the invention is shown where the amplifiers 10-1 through 10-N are arranged in a cascaded series. A plurality of amplifiers is contemplated in the cascaded series shown, such as two amplifiers, three amplifiers, etc. In this embodiment, a one of variable power supply 12-1 through 12-N is associated with each one of the amplifiers 10-1 through 10-N, respectively, as shown. The system controller 14, which preferentially comprises sensing means including at least one lookup table as described above, receives the sensing signals 15D and 15C. As described above in the description of <figref idref="f0002">Figure 3</figref>, the sensing signal 15D may originate from one or more of the sensors 17N. Only one of the sensors 17N is labeled in <figref idref="f0003">Figure 4</figref> for clarity, but it is to be understood that the sensing signal 15D may originate from any of the sensors 17N shown immediately preceding or following one of the cascaded amplifiers 10-1 through 10-N. Sensing signal 15C may originate from the sensor 17C sensing the operating conditions of the electrical circuit 18 as described above in the description of <figref idref="f0002">Figure 3</figref>.

0035As with the circuit in <figref idref="f0002">Figure 3</figref> above, an electrical circuit, such as the electrical circuit 18, may be placed after some or all of the amplifiers 10-1 through 10-N.

0036The system controller 14 may send one or more of the control signals 13-1 through 13-N to the variable power supplies 12-1 through 12-N, respectively, so as to control the amplifiers 10-1 through 10-N in response to one or more of the sensed operating conditions. The system controller 14 includes sensing means, as described above, which may include one or more lookup tables and may include one lookup table for each of the variable power supply/amplifier combinations. The system controller 14 may be capable of controlling each of the variable power supplies 12-1 through 12-N separately, all together as one unit, or as multiple groups of varying numbers of amplifiers.

0037In one mode of operation, the circuit shown in <figref idref="f0003">Figure 4</figref> senses one or more of the operating conditions via one or more of sensors 17N and/or 17C and a sensing signal 15D and/or 15C, respectively, may be sent to the system controller 14. The system controller comprises a sensing means, as described above, including at least one lookup table. Any one of the lookup tables may determine the characteristics of a control signal 13-1 through 13-N as a function of one or more of the operating conditions. The control signals 13-1 through 13-N may be sent from the system controller 14 to one or more of the variable power supplies 12-1 through 12-N so as to dynamically control the voltage of the power supplied to one or more of the amplifiers 10-1 through 10-N as a function of one or more of the sensed operating condition or conditions.

0038With reference now to <figref idref="f0004">Figure 5</figref>, where like numerals represent like components in <figref idref="f0001">Figures 1</figref>, <figref idref="f0002">3</figref>, and <figref idref="f0003">4</figref> which operate in a like manner, a fourth embodiment of the invention is shown where the amplifiers 10-1 through 10-N are arranged in a parallel cascade. A plurality of amplifiers is contemplated in the parallel cascade shown, such as two amplifiers, three amplifiers, etc. In this embodiment, the variable power supply 12 is shown as supplying power to each of the parallel cascade of the amplifiers 10-1 through 10-N as shown. It is to be understood that the present invention also contemplates an individual variable power supply for each of the amplifiers 10-1 through 10-N, similar to the arrangement shown in the embodiment in <figref idref="f0003">Figure 4</figref>. The embodiment with a plurality of variable power supply/amplifier pairs in a parallel cascade is not shown but may be inferred from this description.

0039The system controller 14, which preferentially comprises sensing means including at least one lookup table as described above, receives the sensing signals 15E and 15C. The sensing signal 15E is analogous to the sensing signal 15D as described above in the description of <figref idref="f0002">Figure 3</figref>. The sensing signal 15E may originate from one or more of the sensors 17N. Only two of the sensors 17N are connected to the system controller 14 for clarity and only one of the sensors 17N is labeled in <figref idref="f0004">Figure 5</figref> for clarity, but it is to be understood that the sensing signal 15E may originate from any of the sensors 17N shown immediately preceding or following one of the parallel cascade amplifiers 10-1 through 10-N. Sensing signal 15C may originate from the sensor 17C sensing the operating conditions of the electrical circuit 18 as described above in the description of <figref idref="f0002">Figure 3</figref>.

0040The electrical circuit 18 may preferentially include a combiner (not shown for clarity) to combine the output signals from the amplifiers 10-1 through 10-N. The electrical circuit 18 may also include one or more of the amplifiers 10-1 through 10-N.

0041The system controller 14 may send the control signal 13 to the variable power supply 12 so as to control the amplifiers 10-1 through 10-N in response to one or more of the sensed operating conditions. The system controller 14 includes sensing means, as described above, including a lookup table, the operation of which is as described above.

0042In one mode of operation, the circuit shown in <figref idref="f0004">Figure 5</figref> senses one or more of the operating conditions via one or more of sensors 17N and/or 17C and a sensing signal 15E and/or 15C, respectively, may be sent to the system controller 14. The system controller comprises a sensing means, as described above, including a lookup table. The lookup tables may determine the characteristics of the control signal 13 as a function of one or more of the operating conditions. The control signal 13 may be sent from the system controller 14 to the variable power supply 12 so as to dynamically control the voltage of the power supplied to one or more of the amplifiers 10-1 through 10-N as a function of one or more of the sensed operating condition or conditions.

0043As mentioned above, the present invention also contemplates an embodiment comprising a parallel cascade arrangement of amplifier/variable power supply combinations. The operation of this embodiment may be inferred from the description provided above for the embodiments shown in <figref idref="f0001">Figures 1</figref>, <figref idref="f0002">3</figref>, <figref idref="f0003">4</figref>, and <figref idref="f0004">5</figref>.

0044While preferred embodiments of the present invention have been described, it is to be understood that the embodiments described are illustrative only and that the scope of the invention is to be defined solely by the appended claims.