Transconductance control circuit, method of controlling transconductance, and a power amplifier

1. A transconductance control circuit, comprising:

a test transconductance circuit for providing an output current from a reference voltage;

apparatus for deriving a bias current for the test transconductance circuit from the output current, the bias current including a component that varies with temperature and a component that varies with

process, wherein the apparatus for deriving the bias current comprises apparatus for determining a difference current representing the difference between the output current and a reference current, the reference current including a component that varies with temperature, and a current amplifier for amplifying the difference current to produce the bias current; and

apparatus for providing the bias current to other transconductance circuits.

2. A circuit as claimed in claim 1 , wherein the bias current includes a component independent of temperature and process.

3. A circuit as claimed in claim 2 , wherein the relative magnitudes of the component that varies with temperature and the component independent of temperature and process are selectable.

4. A circuit as claimed in claim 1 , wherein the reference current includes a component independent of temperature and process.

5. A circuit as claimed in claim 4 , wherein the relative magnitudes of the component that varies with temperature and the component independent of temperature and process are selectable.

6. A circuit as claimed in claim 1 , wherein the apparatus for providing the bias current to other transconductance circuits comprises at least one current mirror.

7. A circuit as claimed in claim 1 , wherein the reference voltage is a differential voltage and the test transconductance circuit is a differential circuit that produces the output current from the differential reference voltage, wherein the output current is a differential current.

8. A method of controlling transconductance of a transconductance circuit, comprising:

producing an output current from a reference voltage using a test transconductance circuit;

deriving a bias current for the test transconductance circuit from the output current, wherein the bias current includes a component that varies with temperature and a component that varies with wherein deriving the bias current comprises determining a difference current representing a difference between the output current and a reference current, the reference current including a component that varies with temperature, and amplifying the difference current to produce the bias current; and

providing the bias current to other transconductance circuits.

9. A method as claimed in claim 8 , wherein the bias current includes a component independent of temperature and process, and the method comprises selecting the relative magnitudes of the component that varies with temperature and the component independent of temperature and process.

10. A power amplifier, comprising:

a power transconductance circuit;

a load impedance; and

a gain control circuit, comprising: a test transconductance circuit for providing an output current from a reference voltage; apparatus for deriving a bias current for the transconductance circuit, the bias current including a component that varies with temperature and a component that varies with process, wherein the apparatus for deriving the bias current comprises apparatus for determining a difference current representing a difference between the output current and a reference current, the reference current including a component that varies with temperature, and a current amplifier for amplifying the difference current to produce the bias current; and apparatus for providing the bias current to the power transconductance circuit to control the gain of the power amplifier.

11. A power amplifier as claimed in claim 10 , wherein the bias current includes a component independent of temperature and process.

12. A power amplifier as claimed in claim 11 , wherein the relative magnitudes of the component that varies with temperature and the component independent of temperature and process are selectable.

13. A power amplifier as claimed in claim 10 , wherein the reference current includes a component independent of temperature and process.

14. A power amplifier as claimed in claim 13 , wherein the relative magnitudes of the component that varies with temperature and the component independent of temperature and process are selectable.

15. A power amplifier as claimed in claim 10 , wherein the apparatus for providing the bias current to other transconductance circuits comprises at least one current mirror.

16. A power amplifier as claimed in claim 10 , wherein the reference voltage is a differential voltage and the test transconductance circuit is a differential circuit that produces the output current from the differential reference voltage, wherein the output current is a differential current.

17. A power amplifier as claimed in claim 12 or 14 , wherein the relative magnitudes of the component that varies with temperature and the component independent of temperature and process are selected such that the power amplifier has a gain substantially independent of temperature and process.

18. A power amplifier comprising:

a power transconductance circuit;

a load impedance; and

a gain control circuit, comprising: a test transconductance circuit for providing an output current from a reference voltage; apparatus for subtracting the output current from a reference current to produce an error current, wherein the reference current includes a component that varies with temperature and a component independent of temperature and process; a current amplifier for amplifying the error current to produce the bias current; and a current mirror for providing the bias current to the power transconductance circuit to control the gain of the power amplifier;

wherein the relative magnitudes of the component that varies with temperature and the component independent of temperature and process are selected such that the gain of the power amplifier is substantially independent of temperature and process.

19. A transconductance control circuit, comprising:

a test transconductance circuit for producing an output current i ref from a reference voltage V ref ;

apparatus for determining an error current representing a difference between the output current and a reference current;

an amplifier for amplifying the error current to produce a bias voltage for the test transconductance circuit; and

apparatus for providing the bias current to other transconductance circuits;

wherein the transconductance of the transconductance circuit is maintained at substantially i ref /V ref .