Compensation of trapping in field effect transistors

1. A circuit, comprising:

a field effect transistor (FET);

a reference transistor having an output coupled to an output of the FET;

an active bias circuit coupled to the reference transistor and configured to generate an input signal for the reference transistor in response to a drain current of the reference transistor and to apply the input signal to an input of the reference transistor; and

a summing node coupled to an input of the FET and to the input of the reference transistor, wherein the summing node adds the input signal to an input signal of the FET.

2. The circuit of claim 1 , wherein the FET and the reference transistor each comprise respective source, drain and gate terminals, wherein the drain terminals of the FET and the reference transistor comprise the respective outputs of the FET and the reference transistor, and wherein the drain terminal of the FET is RF-coupled to the drain terminal of the reference transistor through a coupling capacitor.

3. The circuit of claim 2 , further comprising:

an RF shunt capacitor coupled between the gate terminal of the reference transistor and ground.

4. The circuit of claim 2 , wherein the gate terminal of the reference transistor is DC coupled to the gate terminal of the FET.

5. The circuit of claim 4 , further comprising:

a buffer coupled between the gate terminal of the reference transistor and the gate terminal of the FET.

6. The circuit of claim 5 , wherein the buffer comprises an operational amplifier circuit having a voltage follower configuration.

7. The circuit of claim 4 , further comprising:

a low pass filter coupled between the gate of the reference transistor and the gate of the FET.

8. The circuit of claim 1 , further comprising:

a drain current monitor circuit configured to detect a level of the drain current flowing in the reference transistor, wherein the drain current monitor circuit is coupled to the active bias circuit.

9. The circuit of claim 8 , further comprising:

a low pass filter coupled between the reference transistor and the drain current monitor circuit.

10. The circuit of claim 1 , wherein the reference transistor and the FET comprise Group III-nitride based high electron mobility transistors.

11. The circuit of claim 10 , wherein the reference circuit and the FET are formed on a single substrate and share a common epitaxial structure.

12. The circuit of claim 1 , wherein the reference transistor and the FET are biased with a common drain bias voltage.

13. The circuit of claim 1 , wherein the active bias circuit is configured to control a gate voltage of the reference transistor to maintain the drain current at a constant level through the reference transistor.

14. The circuit of claim 1 , wherein the active bias circuit is configured to generate the input signal in response to a change in the drain current of the reference transistor, wherein the change in the drain current of the reference transistor is caused by carrier trapping in the reference transistor.

15. A transistor amplifier, comprising:

a power field effect transistor, FET, having a source terminal, a drain terminal and a gate terminal;

a reference transistor having a source terminal, a drain terminal and a gate terminal;

an RF coupling capacitor connected between the drain terminal of the reference transistor and the drain terminal of the power FET;

a drain current detection circuit coupled to the drain terminal of the reference transistor and configured to detect a change in a drain current of the reference transistor; and

an active bias circuit coupled to the drain current detection circuit and configured to generate an input signal for the reference transistor in response to the change in the drain current of the reference transistor;

wherein the input signal is applied to the gate of the reference transistor; and

wherein the input signal is added to an RF input signal of the power FET.

16. The transistor amplifier of claim 15 , wherein the reference transistor and the power FET are biased for Class A operation or a reduced conduction angle bias, such as Class AB operation.

17. The transistor amplifier of claim 15 , further comprising:

an RF shunt capacitor coupled between the gate terminal of the reference transistor and ground.

18. The transistor amplifier of claim 15 , wherein the gate terminal of the reference transistor is DC coupled to the gate terminal of the power FET.

19. The transistor amplifier of claim 18 , further comprising:

a buffer coupled between the gate terminal of the reference transistor and the gate terminal of the power FET.

20. The transistor amplifier of claim 19 , wherein the buffer comprises an operational amplifier circuit having a voltage follower configuration.

21. The transistor amplifier of claim 18 , further comprising:

a low pass filter coupled between the gate of the reference transistor and the gate of the power FET.

22. The transistor amplifier of claim 15 , wherein the power FET and the reference transistor comprise Group III-nitride based high electron mobility transistors.

23. A method of compensating carrier trapping effects in a field effect transistor (FET), the method comprising:

detecting a change in drain current of a reference transistor that is coupled to the FET;

generating an input signal for the reference transistor in response to the change in the drain current of the reference transistor;

applying the input signal to an input of the reference transistor; and

adding the input signal as a compensation signal to an RF input signal of the FET.

24. The method of claim 23 , wherein the power FET and the reference transistor comprise Group III-nitride based high electron mobility transistors.