Current controlled power amplifier

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention is generally related to power amplifiers, and, more specifically, to a system and method for regulating the power produced by a power amplifier.

Discussion of the Background

In certain communication systems, for example, the Global System for Mobile Communication (GSM), the output power of a transmitter within a communication device (e.g., a mobile phone) has to be well controlled due to the specifications of the communication system. In transmitters used in GSM and GSM like systems, the output power of the transmitter is controlled by employing a control circuit that produces a control voltage for biasing a power amplifier of the transmitter. For example, a high control voltage produces higher output power and a low control voltage produces lower output power. In this way, a control voltage that biases the power amplifier can be used to control the output power.

An example of such a power amplifier is known from <patcit id="pcit0001" dnum="US5497125A"><text>US 5 497 125</text></patcit> in which a control circuit generates a control voltage for biasing the power amplifier by comparing a sense voltage correlated to the current drawn by the power amplifier to a set voltage. A similar circuit is disclosed in <patcit id="pcit0002" dnum="EP772292A"><text>EP 772 292</text></patcit>. In this case the control voltage is generated by a differential amplifier as a function of a voltage difference between a feed-back voltage indicative of the RF transmission power and a reference voltage.

In <patcit id="pcit0003" dnum="US4158180A"><text>US 4 158 180</text></patcit> a voltage indicative of the current drawn by a power amplifier is compared with a reference voltage indicative of the temperature of the amplifier circuit in an operational amplifier. The output of the operational amplifier is to the base of a transistor to control the current to a driver circuit for the power amplifier.

However, a problem with using a control voltage to bias the power amplifier is that there is not a pure linear relationship between the control voltage and the output power. In other words, there is not a constant gain. <figref idref="f0001">FIG. 1</figref> is a graph that illustrates the relationship between the control voltage and the current consumed by the power amplifier (also referred to as the "collector current"). The collector current is proportional to the output power. Therefore, <figref idref="f0001">FIG. 1</figref> illustrates indirectly the relationship between the control voltage and the output power. As seen in <figref idref="f0001">FIG. 1</figref>, there is a low gain when the control voltage is below V_low, a high gain when the control voltage is between V_low, and V_high, and a low gain when the control voltage is greater than V_high. Thus, a constant gain is not achieved. Specifically, when the control voltage is either low or high, the gain is essentially zero, and when the control voltage is in the middle region (i.e., between low and high), the gain can vary by a factor greater than 10.

It is desirable that there be a constant gain between the control signal and collector current.

SUMMARY OF THE INVENTION

The present invention which is defined in the claims overcomes the disadvantages described above by using a control current, rather than a control voltage, to bias the power amplifier. As shown in <figref idref="f0001">FIG. 2</figref>, the relationship between the control current and the collector current is much more linear than the relationship between the control voltage and the collector current. Thus; using a control current to bias the power amplifier a more constant gain is achieved.

Advantageously, a feed forward technique is used in conjunction with the control current. The feed forward technique is most useful in the situations where it is desired to operate the power amplifier at high output power levels.

Further features and advantages of the present invention, as well as the structure and operation of various embodiments of the present invention, are described in detail below with reference to the accompanying drawings.

It should be emphasized that the term "comprises/comprising" when used in this specification is taken to specify the presence of stated features, integers, steps, or components but does not preclude the presence or addition of one or more other features, integers, steps, components or groups thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated herein and form part of the specification, illustrate various embodiments of the present invention and, together with the description, further serve to explain the principles of the invention and to enable a person skilled in the pertinent art to make and use the invention. In the drawings, like reference numbers indicate identical or functionally similar elements. Additionally, the left-most digit(s) of a reference number identifies the drawing in which the reference number first appears. <ul id="ul0001" list-style="none" compact="compact"><li><figref idref="f0001">FIG. 1</figref> is a graph that illustrates the relationship between the control voltage and the collector current.</li><li><figref idref="f0001">FIG. 2</figref> is a graph that illustrates the relationship between the control current and the collector current.</li><li><figref idref="f0001">FIG. 3</figref> is a functional block diagram of one embodiment of a circuit.</li><li><figref idref="f0002">FIG. 4</figref> illustrates an example of a practical application.</li><li><figref idref="f0002">FIG. 5</figref> is a block diagram that illustrates a feed forward circuit used in conjunction with the present invention.</li><li><figref idref="f0003">FIG. 6</figref> is a circuit diagram illustrating a circuit for implementing the application as it shown in <figref idref="f0002">FIG. 4</figref></li><li><figref idref="f0004">FIG. 7</figref> is a circuit diagram illustrating the circuit shown in <figref idref="f0003">FIG. 6</figref> with a feed forward circuit, according to one embodiment.</li></ul>

DETAILED DESCRIPTION OF THE EMBODIMENTS

<figref idref="f0001">FIG. 3</figref> is a functional block diagram of one embodiment of a circuit 300. Circuit 300 includes a power amplifier 302 having an input 304 for receiving an input signal, an output 306 for connecting to a load 312, a control input 308 for receiving a control current (I_cont) 309, and a power input 310 for coupling power amplifier 302 to a power source (V_bat) 314.

Advantageously, circuit 300 also includes a controllable current source 319 that produces control current 309. Control current 309 is used to control the amount of power delivered to load 312 (also referred to as the "output power"). As shown in <figref idref="f0001">FIG 2</figref>, there is a linear (or nearly linear) relationship between control current 309 -and a collector current 318, which is proportional to the output power. Therefore, by changing the amount of control current 309, one is able to change the collector current 318 by a proportional amount, and, thereby, change the output power by a proportional amount. For example, if the output power should be increased, current source 319 is set to increases the amount of control current 309, which causes the output power to increase. Similarly, if the output power should be decreased, current source 319 is set to decrease the amount of control current 309, which causes the output power to decrease.

<figref idref="f0002">FIG. 4</figref> illustrates an example practical application. As shown in <figref idref="f0002">FIG. 4</figref>, a current sensor 402 is used to provide a reading of collector current 318. More specifically, current sensor 402 outputs a signal, such as a voltage (V_sense), that corresponds to the amount of current 318 flowing into power amplifier 302 (i.e., the "collector current"). This signal is provided as an input to a comparator 404, which is coupled to and controls current source 319. Comparator 404 compares the signal to a reference signal and adjusts the magnitude of control current 309 as a function of the difference between the signal and the reference signal by sending a control signal 405 to current source 319. In this way, comparator 404 and current source 319 can be used to regulate the output power as a function of the reference voltage.

For example, if it is desired to keep the output power at a constant level, the reference signal is kept constant and comparator 404 is configured to increase control current 309 if the signal from current sensor 402 is less than the reference signal and to decrease control current 309 if the signal from current sensor 402 is greater than the reference signal. In this way, comparator 404 and current source 319 keep the collector current 318 constant, and, thereby regulates the output power.

<figref idref="f0002">FIG. 5</figref> is a block diagram that illustrates a feed forward circuit 502 used in conjunction with the present invention. Feed forward circuit 502 functions to feed a portion of the control current 309 to an adder 504 that adds the output of feed forward circuit 502 to the output of current sensor 402. The output of adder is provided to an input of comparator 404. The reference signal is provided to the other input of comparator 404. Thus, comparator 404 will compare a signal equivalent to Icol + Icont/X to the reference signal. This embodiment of the invention is useful in the situation where a high output power is desired.

<figref idref="f0003">FIG. 6</figref> is a circuit diagram illustrating a circuit 600 for implementing the application as it shown in <figref idref="f0002">FIG. 4</figref>. One skilled in the art understands that circuit 600 is not the only way to implement the high level circuit shown in <figref idref="f0002">FIG. 4</figref>. As shown in <figref idref="f0003">FIG. 6</figref>, power amplifier 302 is implemented with two bi-polar junction transistors 601 and 602, current sensor 402 is implemented with a resistor 604, and comparator 404 and current source 319 are implemented using an operational amplifier (op-amp) 606. The power amplifier 302 shown in <figref idref="f0003">FIG. 6</figref> functions to receive an input signal and produce an output signal that is an amplified version of the input signal. Op-amp 606 functions to output a current 309 that is used to control the output power of power amplifier 302.

<figref idref="f0004">FIG. 7</figref> is a circuit diagram illustrating circuit 600 with a feed forward circuit 502 and an adder 504, according to one embodiment. One skilled in the art will appreciate that the specific feed forward circuit 502 that is shown in <figref idref="f0004">FIG. 7</figref> is not the only way to implement a feed forward circuit and that the invention is not limited to the specific circuit shown in <figref idref="f0004">FIG. 7</figref> or any other specific circuit. As shown in <figref idref="f0004">FIG. 7</figref>, feed forward circuit 502 includes a bi-polar junction transistor 710, a DC current source 712, and a first resistor 714, and adder 504 is implemented with a resistor. DC current source 712 is connected between ground and the base of transistor 710. First resistor 714 is connected between the base of transistor 710 and node N1. A first input 721 of adder 504 is connected to the collector of transistor 710 and a second input 722 of adder 504 is connected to node N2. The output 723 of adder 504 is connected to an input of op-amp 606. Lastly, the emitter of transistor 710 is connected to ground.

As described above, feed forward circuit 502 functions to feed a portion of the control current 309 to adder 504, which adds the output of feed forward circuit 502 to the output of current sensor 402 (in this case, resistor 604) to produce an output which is provided to an input of op-amp 606.

While various embodiments of the present invention have been described above, it should be understood that they have been presented by way of example, and not limitation. It will be understood by those skilled in the art that various changes in form and detail may be made therein without departing from the scope of the invention as defined by the following claims. Thus the breadth and scope of the present invention should not be limited by any of the above-described exemplary embodiments, but should be defined only in accordance with the following claims.