Compensation circuit of power amplifier and associated compensation method

A compensation circuit of a power amplifier includes a varactor, a voltage sensor and a control circuit. The varactor is coupled to an input terminal of the power amplifier. The voltage sensor is arranged for detecting an amplitude of an input signal of the power amplifier to generate a detecting result. The control circuit is coupled to the varactor and the voltage sensor, and is arranged for controlling a bias voltage of the varactor to adjust a capacitance of the varactor according to the detecting result.

BACKGROUND

A linearity of a power amplifier is largely affected by an input capacitance of the power amplifier, and a phase of an output signal of the power amplifier may be changed due to the input capacitance when an amplitude of an input signal increases, where this phenomenon is called AM-PM distortion. To make the power amplifier have a stable phase of the output signal, how to design a compensation circuit to improve the linearity of the power amplifier is an importance topic.

SUMMARY

It is therefore an objective of the present invention to provide a compensation circuit of a power amplifier, where the compensation circuit can dynamically adjust the input capacitance of the power amplifier by referring to the amplitude of the input signal. By using the embodiments of the present invention, the AM-PM distortion and the linearity of the power amplifier can be largely improved, and the overall circuit has higher efficiency.

According to one embodiment of the present invention, a compensation circuit of an amplifier includes a varactor, a voltage sensor and a control circuit. The varactor is coupled to an input terminal of the amplifier. The voltage sensor is arranged for detecting an amplitude of an input signal of the amplifier to generate a detecting result. The control circuit is coupled to the varactor and the voltage sensor, and is arranged for controlling a bias voltage of the varactor to adjust a capacitance of the varactor according to the detecting result.

According to another embodiment of the present invention, a method for compensating an amplifier comprises: providing a varactor coupled to an input terminal of the amplifier, wherein a capacitance of the varactor serves as part of input capacitance of the amplifier; detecting an amplitude of an input signal of the amplifier to generate a detecting result; and controlling a bias voltage of the first varactor to adjust the capacitance of the varactor according to the detecting result.

According to another embodiment of the present invention, a circuit comprises a power amplifier and a compensation circuit, where the compensation circuit of a power amplifier comprises a varactor, a voltage sensor and a control circuit. The varactor is coupled to an input terminal of the power amplifier. The voltage sensor is arranged for detecting an amplitude of an input signal of the power amplifier to generate a detecting result. The control circuit is coupled to the varactor and the voltage sensor, and is arranged for controlling a bias voltage of the varactor to adjust a capacitance of the varactor according to the detecting result.

DETAILED DESCRIPTION

FIG. 1is a diagram illustrating a power amplifier system100according to one embodiment of the present invention. As shown inFIG. 1, the power amplifier system100comprises a compensation circuit110and a power amplifier120, where the compensation circuit110comprises a voltage sensor112, an amplifier114, a control circuit116, two varactors VC1and VC2, and two capacitors C; and the power amplifier120comprises transistors M1-M4, a capacitor C1and an inductor L1. In this embodiment, the power amplifier system110is arranged to receive a differential input signal Vp and Vn to generate a differential output signal Vout_p and Vout_n, and the compensation circuit110is arranged to compensate an input capacitance of the power amplifier120.

Regarding the operations of the compensation circuit110, the voltage sensor112receives the differential input signal Vp and Vn via the capacitors C to detect an amplitude of the differential input signal Vp and Vn to generate a detecting result, where the capacitors C are used to block DC voltages. In this embodiment, the detecting result is a voltage signal that represents the amplitude of the differential input signal Vp and Vn, and the detecting result is inputted to the control circuit116via the amplifier114for adjusting a voltage level of the detecting result. Then, the control circuit116controls/adjusts the capacitance of the varactors VC1and VC2to compensate the input capacitance of the power amplifier120according to the detecting result. In one embodiment, the input terminals of the power amplifier have fixed DC voltages (i.e. the upper node of the varactor VC1or the lower node of the varactor VC2has a fixed DC voltage), and the control circuit116can adjust the capacitance of the varactors VC1and VC2by setting the appropriate bias voltages to the lower node of the varactor VC1and the upper node of the varactor VC2.

In this embodiment, the control circuit116is represented by a switch, and the switch is turned on or off according to the detecting result. For example, when the detecting result indicates that the differential input signal Vp and Vn has large power/amplitude, the switch is turned on to connect the varactor VC1to the varactor VC2as shown inFIG. 2, and the control circuit116can set the bias voltage Vb to the connection node of the varactors VC1and VC2to control/adjust the capacitance of the varactors VC1and VC2. When the detecting result indicates that the differential input signal Vp and Vn has lower power/amplitude and it is not required to compensate the input capacitance of the power amplifier120, the switch is turned off, and the lower node of the varactor VC1and the upper node of the varactor VC2are at floating state. In addition, in one embodiment, when the detecting result indicates that the amplitude of the differential input signal Vp and Vn increase, the control circuit116controls the bias voltage of the varactors VC1and VC2to increase the capacitance of the varactors VC1and VC2; and when the detecting result indicates that the amplitude of the differential input signal Vp and Vn decrease, the control circuit116controls the bias voltage of the varactors VC1and VC2to decrease the capacitance of the varactors VC1and VC2.

By using the aforementioned compensation method, the input capacitance of the power amplifier120can be dynamically compensated according to the amplitude of the input differential signal Vp and Vn, therefore, the phase of the differential output signal Vout_p and Vout_n will be more stable even if the amplitude of the input differential signal Vp and Vn has frequent changes, and the AM-PM distortion can be effectively improved.

FIG. 3is a diagram illustrating the voltage sensor112according to one embodiment of the present invention. As shown inFIG. 3, the voltage sensor112comprises three P-type metal-oxide semiconductors (PMOS) MP1-MP3, three NMOSs MN1-MN3, two current source I1-I2, three resistors R1-R3and two capacitors C2-C3. The embodiment shown inFIG. 3is a peak detector that compares the differential input signal Vp and Vn with a reference voltage to detect a swing amplitude or a peak value of the differential input signal Vp and Vn, and a voltage level of the detecting result is used to represent the swing amplitude or the peak value of the differential input signal Vp and Vn.

FIG. 4is a diagram illustrating a power amplifier system400according to another embodiment of the present invention. As shown inFIG. 4, the power amplifier system100comprises a compensation circuit410and a power amplifier420, where the compensation circuit410comprises two voltage sensors412and432, two amplifiers414and434, two control circuits416and436, two varactors VC1and VC2, a transformer419, and four capacitors C; and the power amplifier120comprises transistors M1-M4, a capacitor C1and an inductor L1. In this embodiment, the power amplifier system410is arranged to receive a differential input signal Vp and Vn via a transformer419to generate a differential output signal Vout_p and Vout_n, and the compensation circuit410is arranged to compensate an input capacitance of the power amplifier420.

Regarding the compensation circuit410, the voltage sensor412, the amplifier414, the control circuit416and the varactors VC1and VC2can be viewed as a phase-modulation (PM) compensation loop to make the phase of the differential input signal Vp and Vn more stable, and the voltage sensor432, the amplifier434, the control circuit436and the transformer419can be viewed as a amplitude-modulation (AM) compensation loop to make the power amplifier420work with higher linearity and efficiency. Firstly, regarding the operations of the PM compensation loop, the voltage sensor412receives the differential input signal Vp and Vn via the capacitors C to detect an amplitude of the differential input signal Vp and Vn to generate a detecting result, where the capacitors C are used to block DC voltages. In this embodiment, the detecting result is a voltage signal that represents the amplitude of the differential input signal Vp and Vn, and the detecting result is inputted to the control circuit416via the amplifier414for adjusting a voltage level of the detecting result. Then, the control circuit416controls/adjusts the capacitance of the varactors VC1and VC2to compensate the input capacitance of the power amplifier420according to the detecting result.

In this embodiment, the control circuit416is represented by a switch, and the switch is turned on or off according to the detecting result. For example, when the detecting result indicates that the differential input signal Vp and Vn has large power/amplitude, the switch is turned on to connect the varactor VC1to the varactor VC2(e.g. the embodiment shown inFIG. 2), and the control circuit416can set an appropriate bias voltage to the connection node of the varactors VC1and VC2to control/adjust the capacitance of the varactors VC1and VC2. When the detecting result indicates that the differential input signal Vp and Vn has lower power/amplitude and it is not required to compensate the input capacitance of the power amplifier420, the switch is turned off, and the lower node of the varactor VC1and the upper node of the varactor VC2are at floating state. In addition, in one embodiment, when the detecting result indicates that the amplitude of the differential input signal Vp and Vn increase, the control circuit416controls the bias voltage of the varactors VC1and VC2to increase the capacitance of the varactors VC1and VC2; and when the detecting result indicates that the amplitude of the differential input signal Vp and Vn decrease, the control circuit416controls the bias voltage of the varactors VC1and VC2to decrease the capacitance of the varactors VC1and VC2.

By using the aforementioned PM compensation method, the input capacitance of the power amplifier420can be dynamically compensated according to the amplitude of the input differential signal Vp and Vn, therefore, the phase of the differential output signal Vout_p and Vout_n will be more stable even if the amplitude of the input differential signal Vp and Vn has frequent changes, and the AM-PM distortion can be effectively improved.

Regarding the operations of the AM compensation loop, the voltage sensor432receives the differential input signal Vp and Vn via the capacitors C to detect an amplitude of the differential input signal Vp and Vn to generate a detecting result, where the capacitors C are used to block DC voltages. In this embodiment, the detecting result is a voltage signal that represents the amplitude of the differential input signal Vp and Vn, and the detecting result is inputted to the control circuit436via the amplifier434for adjusting a voltage level of the detecting result. Then, a resistor R4and a capacitor C4within the control circuit436serve as a voltage divider to provide a bias voltage to a center tap of the transformer419according to the detecting result.

In this embodiment, when the amplitude of the differential input signal Vp and Vn increase, the control circuit436applies the higher bias voltage to the center tap of the transformer419to make the input terminals of the power amplifier420have higher DC voltages, therefore, the linearity of the power amplifier420will not be worsened even when the differential input signal Vp and Vn have large swing amplitude. In addition, when the amplitude of the differential input signal Vp and Vn decrease, the control circuit436applies the lower bias voltage to the center tap of the transformer419to make the input terminals of the power amplifier420have lower DC voltages, therefore, the power consumption of the power amplifier420can be improved.

By using the aforementioned AM compensation method, the DC voltages of the input terminals of the power amplifier420can be dynamically adjusted according to the amplitude of the input differential signal Vp and Vn, therefore, the power amplifier420can have better linearity and suitable power consumption.

The voltage sensors412and432can be implemented by the embodiment shown inFIG. 3, without a limitation of the present invention.

Briefly summarized, in the compensation circuit of the present invention, the compensation circuit can dynamically adjust the input capacitance of the power amplifier by referring to the amplitude of the input signal, and the AM-PM distortion and the linearity of the power amplifier can be largely improved. In addition, the compensation circuit further dynamically adjusts the DC voltages of the input terminals of the power amplifier by referring to the amplitude of the input signal to make the power amplifier have better linearity and suitable power consumption.