System and method for over-voltage protection of a power amplifier
A system and method for over-voltage protection of a power amplifier is provided. A power amplifier is typically employed in a transmitter to amplify signals prior to transmission via a load; the load may include an antenna or a cable. As a result of an impedance mismatch between the power amplifier and its load, excess power from the power amplifier output fails to reach the load and must be dissipated by one or more transistors in the power amplifier. In severe impedance mismatch conditions, this dissipated power may damage or destroy the transistor(s). An automatic gain control (AGC) is provided for detecting a gain difference between the power amplifier and a replica power amplifier. A gain difference may signal an over-voltage situation. The AGC may be configured to adjust the gain of the power amplifier if a gain difference exists to prevent device damage.
1. A system for over-voltage protection, comprising:
a first power amplifier configured to receive a modulated signal and to provide a first amplified modulated signal across a first load;
a second power amplifier configured to receive the modulated signal and to provide a second amplified modulated signal across a second load;
a first power detector configured to provide a first power signal proportional to an average voltage of the first amplified modulated signal;
a second power detector configured to provide a second power signal proportional to an average voltage of the second amplified modulated signal;
a comparator configured to generate a comparison signal based on a comparison between the first power signal and the second power signal; and
a logic network configured to adjust a gain of the first power amplifier and a gain of the second power amplifier based on the comparison signal.
2. The system of claim 1 , wherein a voltage of the second amplified modulated signal is substantially equal to the voltage of the first amplified modulated signal under nominal conditions.
3. The system of claim 2 , wherein nominal conditions exist when the impendance of the first load is substantially equal to an expected impedance.
4. The system of claim 1 , wherein the first power amplifier and the second power amplifier are configured as transconductance devices.
5. The system of claim 4 , wherein the transconductance of the second power amplifier is substantially N times smaller than the transconductance of the first power amplifier.
6. The system of claim 5 , wherein the second load has an impedance that is substantially N times larger than the impedance of the first load.
7. The system of claim 5 , wherein the second load has a resistance that is substantially N times larger than the resistance of the first load.
8. The system of claim 1 , wherein:
the logic network is configured to reduce the gain of the first power amplifier and the second power amplifier if the first power signal becomes greater than the sum of the second power signal and a first threshold value; and
the logic network is configured to increase the gain of the first power amplifier and the second power amplifier if the first power signal becomes less than the difference between the second power signal and a second threshold value.
9. The system of claim 1 , wherein the comparator has hysteresis.
10. The system of claim 1 , wherein the first load comprises at least one of an antenna or a cable.
11. A method for over-voltage protection, comprising:
providing a modulated signal to a first power amplifier to produce a first amplified modulated signal;
providing the modulated signal to a second power amplifier to produce a second amplified modulated signal;
detecting a gain difference between the first power amplifier and the second power amplifier; and
adjusting a gain of the first power amplifier and a gain of the second power amplifier based on the gain difference.
12. The method of claim 11 , further comprising:
converting the first amplified modulated signal into a first power signal proportional to an average voltage of the first amplified modulated signal;
converting the second amplified modulated signal into a second power signal proportional to an average voltage of the second amplified modulated signal.
13. The method of claim 12 , further comprising:
comparing the first power signal and the second power signal to generate a comparison signal.
14. The method of claim 13 , wherein the gain difference between the first power amplifier and the second power amplifier is detected based on the comparison signal.
15. The method of claim 14 , wherein adjusting a gain of the first power amplifier and a gain of the second power amplifier based on the gain difference comprises:
reducing the gain of the first power amplifier and the gain of the second power amplifier if the first power signal becomes greater than the sum of the second power signal and a first threshold value; and
increasing the gain of the first power amplifier and the gain of the second power amplifier if the first power signal becomes less than the difference between the second power signal and a second threshold value.
16. A transmission path that includes a system for over-voltage protection, the transmission path comprising:
a transmitter configured to provide a modulated signal;
a first power amplifier configured to amplify the modulated signal; and
an automatic gain control (AGC) comprising a second power amplifier configured to amplify the modulated signal;
wherein the AGC is configured to detect a gain difference between the first power amplifier and the second power amplifier and to adjust the gain of the first power amplifier and the gain of the second power amplifier based on the gain difference.
17. The transmission path of claim 16 , wherein the AGC further comprises:
a first power detector configured to provide a first power signal proportional to an average voltage of the first amplified modulated signal; and
a second power detector configured to provide a second power signal proportional to an average voltage of the second amplified modulated signal.
18. The transmission path of claim 17 , wherein the AGC further comprises:
a comparator configured to generate a comparison signal based on a comparison between the first power signal and the second power signal.
19. The transmission path of claim 18 , wherein the gain difference between the first power amplifier and the second power amplifier is detected based on the comparison signal.
20. The transmission path of claim 17 , wherein:
the AGC is configured to reduce the gain of the first power amplifier and the second power amplifier if the first power signal becomes greater than the sum of the second power signal and a first threshold value; and
the AGC is configured to increase the gain of the first power amplifier and the second power amplifier if the first power signal becomes less than the difference between the second power signal and a second threshold value.