Loop gain equalizer for RF power amplifier
The output power of an RF power amplifier is controlled using a feedback loop including a differential integrator for controlling the amplifier's bias voltage. The gain of integration in the differential integrator is varied so as to compensate for variations in the derivative of the power amplifier output power versus the bias voltage.
1. A method of controlling the output power of an RF power amplifier, comprising the steps of:
providing a bias control feedback loop arranged to determine the level of the power output by the amplifier and to control the biasing of the amplifier based on the determined power level, said bias control feedback loop including means for comparing the output power level with a reference value to produce an error signal and for integrating the error signal; and
dynamically varying the gain of integration, applied by the comparing and integrating means, by modulating the gain of integration using a modulating function comprising a function approximately inverse to
(
∂
P
out
∂
V
bias
)
,
wherein P out is the output power of the power amplifier and V bias is the bias voltage applied to the power amplifier by the bias control loop.
2. A method of controlling the output power of an RF power amplifier, according to claim 1 , wherein said modulating function comprises a piecewise linear function approximately inverse to
(
∂
P
out
∂
V
bias
)
.
3. A method of controlling the output power of an RF power amplifier, according to claim 1 , wherein said function approximately inverse to
(
∂
P
out
∂
V
bias
)
constitutes a portion of said modulating function, said portion of the modulating function corresponding to a portion of the range of the amplifier's output power.
4. A method of controlling the output power of an RF power amplifier, according to claim 1 , wherein the providing step comprises the step of providing an operational-amplifier-based differential integrator and the step of varying the gain of integration comprises varying the input resistance to said differential integrator.
5. A method of controlling the output power of an RF power amplifier, according to claim 1 , and comprising the step of providing control means for determining, based on the value of the reference signal the value of said function approximately inverse to
(
∂
P
out
∂
V
bias
)
.
6. A method of controlling the output power of an RF power amplifier, according to claim 1 , and comprising the step of providing control means for determining, based on a sampled value representative of the amplifier output power, the value of said function approximately inverse to
(
∂
P
out
∂
V
bias
)
.
7. An RF power amplifier comprising:
a bias control feedback loop, the bias control loop including means for comparing the output power level with a reference value to produce an error signal and for integrating the error signal;
control means adapted, in use, to vary the gain of integration of the comparing and integrating means, depending on the present level of output power, by modulating the gain of integration using a modulating function comprising a function approximately inverse to
(
∂
P
out
∂
V
bias
)
,
wherein P out is the output power of the power amplifier and V bias is the bias voltage applied to the power amplifier by the bias control loop.
8. An RF power amplifier according to claim 7 , wherein the control means is adapted, in use, to apply a modulating function comprising a piecewise linear function approximately inverse to
(
∂
P
out
∂
V
bias
)
.
9. An RF power amplifier according to claim 7 , wherein the control means is adapted, in use, to apply a modulating function having a portion constituted by said function approximately inverse to
(
∂
P
out
∂
V
bias
)
,
said portion of the modulating function corresponding to a portion of the range of the amplifier's output power.
10. An RF power amplifier according to claim 7 , wherein the comparing and integrating means comprises an operational-amplifier-based differential integrator and the control means is adapted to vary the gain of said differential integrator by varying the input resistance thereto.
11. An RF power amplifier according to claim 7 , wherein the control means is adapted to determine the value of said function approximately inverse to
(
∂
P
out
∂
V
bias
)
based on the value of the reference signal.
12. An RF power amplifier according to claim 7 , wherein the control means is adapted to determine the value of said function approximately inverse to
(
∂
P
out
∂
V
bias
)
based on a sampled value representative of the amplifier output power.
13. An RF transmitter comprising a power amplifier according to claim 7 .
14. A method of controlling the output power of an RF power amplifier, comprising the steps of:
determining the level of the power output by the amplifier;
controlling the biasing of the amplifier based on the determined power level;
comparing the output power level with a reference value to produce an error signal and for integrating the error signal; and
dynamically varying the gain of integration by modulating the gain of integration using a modulating function comprising a function approximately inverse to
(
∂
P
out
∂
V
bias
)
,
wherein P out is the output power of the power amplifier and V bias is the bias voltage applied to the power amplifier.
15. A method of controlling the output power of an RF power amplifier according to claim 14 , wherein said modulating function comprises a piecewise linear function approximately inverse to
(
∂
P
out
∂
V
bias
)
.
16. A method of controlling the output power of an RF power amplifier, according to claim 14 , wherein said function approximately inverse to
(
∂
P
out
∂
V
bias
)
constitutes a portion of said modulating function, said portion of the modulating function corresponding to a portion of the range of the amplifier's output power.
17. A method of controlling the output power of an RF power amplifier, according to claim 14 wherein the step of varying the gain of integration comprises varying the input resistance to an operational-amplifier-based differential integrator.
18. A method of controlling the output power of an RF power amplifier, according to claim 14 and comprising the step of determining, based on the value of the reference signal, the value of said function approximately inverse to
(
∂
P
out
∂
V
bias
)
.
19. A method of controlling the output power of an RF power amplifier, according to claim 14 and comprising the step of determining, based on a sampled value representative of the amplifier output power, the value of said function approximately inverse to
(
∂
P
out
∂
V
bias
)
.
20. A method of controlling the output power of an RF power amplifier, according to claim 15 , wherein said function approximately inverse to
(
∂
P
out
∂
V
bias
)
constitutes a portion of said modulating function, said portion of the modulating function corresponding to a portion of the range of the amplifier's output power.