Power detector and method for detecting power
A circuit and method are provided for detecting a power of a signal amplified in a power amplifier. A diode and a voltage bias source are used to shift a voltage of the signal taken at a base of an amplifying transistor of the power amplifier, to generate a positive signal. The positive signal is provided to a base input of an emitter follower exhibiting high input impedance to generate a power detector output which follows the positive signal.
1. A method of detecting a power of a signal transmitted by a power amplifier, the method comprising:
receiving the signal at an input of a power detector, the input of the power detector coupled to one of a base and a gate, of an amplifying device in the power amplifier;
shifting a voltage of the signal positively by a magnitude of a peak voltage of the signal generating a positive signal;
providing the positive signal to a base input of an emitter follower generating a follower signal which follows the positive signal; and
providing the follower signal to an output of the power detector, and to a first capacitor coupled to ground;
wherein the emitter follower presents a high impedance at the input of the power detector.
2. A method according to claim 1 wherein the step of shifting the voltage of the signal comprises:
providing the signal across a capacitor to a cathode of a diode;
wherein the cathode of the diode is coupled to the base input of the emitter follower, and
wherein an anode of the diode is coupled to a voltage bias source.
3. A method according to claim 2 wherein the diode comprises a Schottky diode, wherein the voltage bias source comprises a temperature compensating voltage bias source, and wherein the emitter follower comprises a first transistor.
4. A method according to claim 3 wherein the first transistor comprises a BJT, wherein a collector of the BJT is coupled to a voltage supply, wherein the base input of the source-emitter follower is a base of the BJT, and wherein an emitter of the BJT is coupled to the output of the power detector and the first capacitor.
5. A method according to claim 4 further comprising:
biasing a second transistor coupled between the output of the detector and ground thereby providing operating current for the first transistor.
6. A method according to claim 4 further comprising:
biasing a second transistor coupled between the cathode of the Schottky diode and ground thereby boosting current passing through the Schottky diode.
7. A power detector circuit for detecting a power of a signal transmitted by a power amplifier, the power detector comprising:
an input for receiving the signal coupled to one of a base and a gate of an amplifying device of the power amplifier;
a voltage shifting circuit for shifting a voltage of the signal positively by a magnitude of a peak voltage of the signal and for generating a positive signal;
an emitter follower for receiving at a base input of the emitter follower the positive signal, and for generating a follower signal which follows the positive signal;
a first capacitor coupled to the source-emitter follower for receiving the follower signal, the first capacitor also coupled to ground; and
an output for providing the detector voltage output signal from a combination of a voltage of the capacitor and the follower signal,
wherein the emitter follower presents a high impedance at the input of the power detector.
8. A circuit according to claim 7 wherein the voltage shifting circuit comprises:
a capacitor coupled on a first side to the input of the power detector;
a diode, wherein a cathode of the diode is coupled to a second side of the capacitor and is coupled to the base input of the emitter follower, and
a voltage bias source coupled to an anode of the diode.
9. A circuit according to claim 8 wherein the diode comprises a Schottky diode, wherein the voltage bias source comprises a temperature compensating voltage bias source, and wherein the emitter follower comprises a first transistor.
10. A circuit according to claim 9 wherein the first transistor comprises a BJT, wherein a collector of the BJT is coupled to a voltage supply, and wherein an emitter of the BJT is coupled to the output of the power detector and the first capacitor.
11. A circuit according to claim 10 further comprising:
a second transistor coupled between the output of the detector and ground; and
a second voltage bias source coupled to a base of the second transistor providing operating current for the first transistor at the output of the power detector by appropriate biasing of the second transistor.
12. A circuit according to claim 10 further comprising:
a second transistor coupled between the cathode of the Schottky diode and ground; and
a second voltage bias source coupled to a base of the second transistor for boosting current passing through the Schottky diode by appropriate biasing of the second transistor.