Power amplifier equalizer
Circuits and methods for achieving good AM-AM and AM-PM metrics while achieving good power, PAE, linearity, and EVM performance in an amplifier. Embodiments provide an equalization approach which compensates for AM-AM and AM-PM variations in an amplifier by controlling bias voltage versus output power to alter the AM-AM and AM-PM profiles imposed by the amplifier. Differential amplifier embodiments include cross-coupled common-gate transistors that generate an equalization voltage that alters the gate bias voltage of respective main FETs in proportion to a power level present at the respective drains of the main FETs. Single-ended amplifier embodiments include an equalization circuit that alters the bias voltage to the gate of a main FET in proportion to a power level present at the main FET drain. Embodiments may also include a linearization circuit which alters the AM-PM profile of an input signal to compensate for the AM-PM profile imposed by a coupled amplifier.
1. A method of equalizing a differential amplifier including a plurality of field-effect transistors (FETs), the method including:
(a) providing a first main FET including a gate configured to receive a first input signal, a drain configured to provide a first amplified output signal, and a source configured to be coupled to circuit ground;
(b) providing a second main FET including a gate configured to receive a second input signal, a drain configured to provide a second amplified output signal, and a source configured to be coupled to circuit ground;
(c) configuring the first main FET and the second main FET as a differential amplifier;
(d) coupling an equalization circuit to the respective gates and drains of the first and second FETs, wherein the equalization circuit includes:
(1) a first equalization transistor including a drain coupled to the drain of the first main FET, a source coupled to the gate of the second main FET, and a gate configured to be coupled to a first bias voltage source; and
(2) a second equalization transistor including a drain coupled to the drain of the second main FET, a source coupled to the gate of the first main FET, and a gate configured to be coupled to a second bias voltage source; and
(b) altering a bias voltage to the respective gates of the first and second main FETs in proportion to a power level present at the respective drains of the second and first main FETs, as determined by the equalization circuit.
2. The method of claim 1 , wherein the equalization circuit alters the bias voltage to the gate of the first main FET in proportion to the power level present at the drain of the second main FET, and alters the bias voltage to the gate of the second main FET in proportion to the power level present at the drain of the first main FET.
3. The method of claim 1 , wherein the first equalization transistor alters the bias voltage to the gate of the second main FET in proportion to the power level present at the drain of the first main FET, and the second equalization transistor alters the bias voltage to the gate of the first main FET in proportion to the power level present at the drain of the second main FET.
4. The method of claim 1 , wherein the drain of the first equalization transistor is coupled to the drain of the first main FET through at least one intervening FET series-coupled to the drain of the first main FET, and wherein the drain of the second equalization transistor is coupled to the drain of the second main FET through at least one intervening FET series-coupled to the drain of the second main FET.
5. The method of claim 1 , wherein the first and second bias voltage sources provide an adjustable level of bias to the respective first and second equalization transistors.
6. The method of claim 1 , wherein the first and second equalization transistors are sized to be less than or equal to about one-sixth the size of the first and second main FETs.
7. The method of claim 1 , wherein the equalization circuit includes:
(a) a first equalization subcircuit including a stack of n series-coupled transistors coupled to the drain of the first main FET and coupled to the gate of the second main FET; and
(b) a second equalization subcircuit including a stack of n series-coupled transistors coupled to the drain of the second main FET and coupled to the gate of the first main FET;
wherein n is an integer≥1.
8. A method of equalizing a differential amplifier including a plurality of field-effect transistors (FETs), the method including:
(a) providing a first main FET including a gate configured to receive a first input signal, a drain configured to provide a first amplified output signal, and a source configured to be coupled to circuit ground;
(b) providing a second main FET including a gate configured to receive a second input signal, a drain configured to provide a second amplified output signal, and a source configured to be coupled to circuit ground;
(c) configuring the first main FET and the second main FET as a differential amplifier;
(d) coupling an equalization circuit to the respective gates and drains of the first and second FETs, wherein the equalization circuit includes:
(1) a first equalization subcircuit including a stack of n series-coupled transistors coupled to the drain of the first main FET through n−1 intervening FETs series-coupled to the drain of the first main FET, and coupled to the gate of the second main FET; and
(2) a second equalization subcircuit including a stack of n series-coupled transistors coupled to the drain of the second main FET through n−1 intervening FETs series-coupled to the drain of the second main FET, and coupled to the gate of the first main FET;
wherein n is an integer≥1.
9. The method of claim 1 , further including coupling a first stack of one or more FETs series-coupled to the drain of the first main FET, and coupling a second stack of one or more FETs series-coupled to the drain of the second main FET.
10. The method of claim 1 , further including deriving the respective input signals to the differential amplifier from the output signal of a linearizer circuit, the linearizer circuit including:
(a) a reflective hybrid coupler configured to receive an initial input signal to be linearized, the initial input signal having a first amplitude-to-phase modulation (AM-PM) profile; and
(b) a non-linear termination circuit coupled to the reflective hybrid coupler and configured to reflect a modified input signal back through the reflective hybrid coupler as an output signal;
wherein the output signal has a second AM-PM profile shaped to compensate for a third AM-PM profile imposed on the output signal by the differential amplifier.
11. A method of equalizing an amplifier, including:
(a) coupling an equalization circuit coupled to the gate and drain of a main field-effect transistor (FET) configured as an amplifier, the main FET including a gate configured to receive an input signal, a drain providing an amplified output signal, and a source coupled to circuit ground, wherein the equalization circuit includes an equalization transistor including a drain coupled to the drain of the main FET through at least one intervening FET, a source coupled to the gate of the main FET, and a gate configured to be coupled to a bias voltage source, the equalization transistor configured to alter a bias voltage to the gate of the main FET; and
(b) altering the bias voltage to the gate of the main FET in proportion to a power level present at the drain of the main FET, as determined by the equalization circuit.
12. The method of claim 11 , wherein the bias voltage source provides an adjustable level of bias to the equalization transistor.
13. The method of claim 11 , wherein the equalization transistor is sized to be less than or equal to about one-sixth the size of the main FET.
14. The method of claim 11 , wherein the equalization circuit includes a stack of n series-coupled transistors coupled to the drain of the main FET and the gate of the main FET, wherein n is an integer≥1.
15. A method of equalizing an amplifier, including:
(a) coupling an equalization circuit coupled to the gate and drain of a main field-effect transistor (FET) configured as an amplifier, the main FET including a gate configured to receive an input signal, a drain providing a amplified output signal, and a source coupled to circuit ground, wherein the equalization circuit includes a stack of n series-coupled transistors coupled between the drain of the main FET through n−1 intervening FETs series-coupled to the drain of the main FET, and the gate of the main FET, wherein n is an integer≥1; and
(b) altering a bias voltage to the gate of the main FET in proportion to a power level present at the drain of the main FET, as determined by the equalization circuit.
16. The method of claim 11 , further including deriving the input signal to the amplifier from the output signal of a linearizer circuit, the linearizer circuit including:
(a) a reflective hybrid coupler configured to receive an initial input signal to be linearized, the initial input signal having a first amplitude-to-phase modulation (AM-PM) profile; and
(b) a non-linear termination circuit coupled to the reflective hybrid coupler and configured to reflect a modified input signal back through the reflective hybrid coupler as an output signal, wherein the output signal has a second AM-PM profile shaped to compensate for a third AM-PM profile imposed on the output signal by the amplifier.