Power amplifier saturation detection
In a portable radio transceiver, a power amplifier system includes a saturation detector that detects power amplifier saturation in response to duty cycle of the amplifier transistor collector voltage waveform. The saturation detection output signal can be used by a power control circuit to back off or reduce the amplification level of the power amplifier to avoid power amplifier control loop saturation.
1. A power amplifier system comprising:
a power amplifier including at least one transistor with an output transistor terminal in electrical communication with a power supply;
a limiter in electrical communication with the output transistor terminal, the limiter configured to clip a positive cycle portion of a radio frequency (RF) signal output by the power amplifier and to provide a negative cycle portion of the RF signal to an averaging filter; and
a comparator circuit configured to produce a saturation detection signal indicating whether the power amplifier is operating in a saturation state, the comparator circuit including a comparator in electrical communication with the averaging filter and a current source in electrical communication with the comparator.
2. The power amplifier system of claim 1 further comprising a power amplifier controller configured to adjust a power control signal based on the saturation detection signal and to provide the power control signal to the power amplifier.
3. The power amplifier system of claim 2 wherein the power amplifier is configured to adjust a power level of the RF signal based on the power control signal.
4. The power amplifier system of claim 1 further comprising:
a second power amplifier including at least one transistor with an output transistor terminal in electrical communication with a second power supply; and
a second limiter in electrical communication with the output transistor terminal of the at least on transistor of the second power amplifier, the second limiter configured to clip a positive cycle portion of a second RF signal output by the second power amplifier and to provide a negative cycle portion of the second RF signal to a second averaging filter.
5. The power amplifier system of claim 4 wherein the comparator circuit is further configured to produce a second saturation detection signal indicating whether the second power amplifier is operating in the saturation state.
6. The power amplifier system of claim 5 wherein the comparator circuit further includes a switching circuit configured to cause the comparator circuit to output one of the saturation detection signal and the second saturation detection signal based on a band-selection signal.
7. The power amplifier system of claim 6 wherein the switching circuit is further configured to:
place the averaging filter in electrical communication with a first input of the comparator and the second averaging filter in electrical communication with a second input of the comparator when the band-selection signal causes the comparator circuit to output the saturation detection signal; and
place the averaging filter in electrical communication with the second input of the comparator and the second averaging filter in electrical communication with the first input of the comparator when the band-selection signal causes the comparator circuit to output the second saturation detection signal.
8. The power amplifier system of claim 6 wherein the comparator circuit further includes a second current source in electrical communication with the comparator, the current source is active and the second current source is inactive when the band-selection signal causes the comparator circuit to output the saturation detection signal, and the current source is inactive and the second current source is active when the band-selection signal causes the comparator circuit to output the second saturation detection signal.
9. The power amplifier system of claim 4 wherein the second power amplifier is configured to operate in a different frequency band than the power amplifier.
10. The power amplifier system of claim 1 wherein the power amplifier is of a power amplifier type other than a collector voltage amplifier control (COVAC) type power amplifier.
11. A transmitter comprising:
a modulator; and
a power amplifier system including a power amplifier, a limiter, and a comparator circuit, the power amplifier including at least one transistor with an output transistor terminal in electrical communication with a power supply, the limiter in electrical communication with the output transistor terminal, the limiter configured to clip a positive cycle portion of a radio frequency (RF) signal output by the power amplifier and to provide a negative cycle portion of the RF signal to an averaging filter, and the comparator circuit configured to produce a saturation detection signal indicating whether the power amplifier is operating in a saturation state, the comparator circuit including a comparator in electrical communication with the averaging filter and a current source in electrical communication with the comparator.
12. The transmitter of claim 11 wherein the power amplifier system further includes a power amplifier controller configured to adjust a power control signal based on the saturation detection signal and to provide the power control signal to the power amplifier causing the power amplifier to adjust a power level of the RF signal based on the power control signal.
13. The transmitter of claim 11 wherein the power amplifier system further includes:
a second power amplifier including at least one transistor with an output transistor terminal in electrical communication with a second power supply, the second power amplifier configured to operate in a different frequency band that the power amplifier; and
a second limiter in electrical communication with the output transistor terminal of the at least on transistor of the second power amplifier, the second limiter configured to clip a positive cycle portion of a second RF signal output by the second power amplifier and to provide a negative cycle portion of the second RF signal to a second averaging filter.
14. The transmitter of claim 13 wherein the comparator circuit is further configured to produce a second saturation detection signal indicating whether the second power amplifier is operating in the saturation state, and the comparator circuit further includes a switching circuit configured to cause the comparator circuit to output one of the saturation detection signal and the second saturation detection signal based on a band-selection signal.
15. The transmitter of claim 14 wherein the switching circuit is further configured to:
place the averaging filter in electrical communication with a first input of the comparator and the second averaging filter in electrical communication with a second input of the comparator when the band-selection signal causes the comparator circuit to output the saturation detection signal; and
place the averaging filter in electrical communication with the second input of the comparator and the second averaging filter in electrical communication with the first input of the comparator when the band-selection signal causes the comparator circuit to output the second saturation detection signal.
16. The transmitter of claim 14 wherein the comparator circuit further includes a second current source in electrical communication with the comparator, the current source is active and the second current source is inactive when the band-selection signal causes the comparator circuit to output the saturation detection signal, and the current source is inactive and the second current source is active when the band-selection signal causes the comparator circuit to output the second saturation detection signal.
17. A wireless device comprising:
a front-end module; and
a transmitter in electrical communication with the front-end module, the transmitter including a power amplifier system, the power amplifier system including a power amplifier, a limiter, and a comparator circuit, the power amplifier including at least one transistor with an output transistor terminal in electrical communication with a power supply, the limiter in electrical communication with the output transistor terminal, the limiter configured to clip a positive cycle portion of a radio frequency (RF) signal output by the power amplifier and to provide a negative cycle portion of the RF signal to an averaging filter, and the comparator circuit configured to produce a saturation detection signal indicating whether the power amplifier is operating in a saturation state, the comparator circuit including a comparator in electrical communication with the averaging filter and a current source in electrical communication with the comparator.
18. The wireless device of claim 17 wherein the power amplifier system further includes a power amplifier controller configured to adjust a power control signal based on the saturation detection signal and to provide the power control signal to the power amplifier causing the power amplifier to adjust a power level of the RF signal based on the power control signal.
19. The wireless device of claim 17 wherein the power amplifier system further includes:
a second power amplifier including at least one transistor with an output transistor terminal in electrical communication with a second power supply, the second power amplifier configured to operate in a different frequency band that the power amplifier; and
a second limiter in electrical communication with the output transistor terminal of the at least on transistor of the second power amplifier, the second limiter configured to clip a positive cycle portion of a second RF signal output by the second power amplifier and to provide a negative cycle portion of the second RF signal to a second averaging filter.
20. The wireless device of claim 19 wherein the comparator circuit is further configured to produce a second saturation detection signal indicating whether the second power amplifier is operating in the saturation state, and the comparator circuit further includes a switching circuit configured to cause the comparator circuit to output one of the saturation detection signal and the second saturation detection signal based on a band-selection signal.