Bootstrap class-D wideband RF power amplifier
A high-power, high-frequency radio frequency power amplifier includes an output stage and a single-phase driver. The output stage is arranged in a Class-D amplifier configuration and includes a first depletion mode field effect transistor (FET), a second depletion mode FET, and a bootstrap path that couples the output of the output stage to the gate of the second FET. The first and second depletion mode FETs are switched out-of-phase and between fully-ON and fully-OFF states, under the direction of the single-phase driver. The single-phase driver directly controls the ON/OFF state of the first depletion mode FET and provides a discharge path through which the input gate capacitor of the second depletion mode FET in the output stage can discharge to turn OFF the second depletion mode FET. The bootstrap path provides a current path through which the input gate capacitor of the second depletion mode FET can charge to turn the second depletion mode FET ON.
1. An amplifier, comprising:
a first field-effect transistor (FET) having a gate, a drain configured to be coupled to a power supply, and a source;
a second FET having a gate, a drain coupled to the source of the first FET, and a source;
a bootstrap path including a resistive element coupled between the gate and source of the first FET;
a third FET having a gate, a drain coupled to the gate of the first FET, and a source; and
an input terminal that is capacitively coupled to the gates of both the second and third FETs and which is configured to receive a common switch drive signal that drives both the second and third FETs,
wherein the source of the second FET is configured to be biased by a first bias voltage and the third FET is configured to be biased by a second bias voltage lower than the first bias voltage.
2. The amplifier of claim 1 , wherein the gates of the first and second FETs have larger areas than an area of the gate of the third FET.
3. The amplifier of claim 1 , wherein the second bias voltage is lower than the first bias voltage by at least one FET threshold voltage.
4. The amplifier of claim 1 , wherein the first, second, and third FETs are gallium nitride FETs.
5. In a Class-D output stage that includes a first field-effect transistor (FET) and a second FET, a method of switching the first FET ON and OFF, comprising:
providing a bootstrap path including a resistive element between the source and gate of the first FET;
switching the second FET, which has its drain coupled to the source of the first FET, and a third FET, which has its drain coupled to the gate of the first FET, OFF, in response to a common switch drive signal, to allow a voltage across the bootstrap path to rise to a level greater than a threshold voltage of the first FET and thereby switch the first FET ON;
biasing the source of the second FET to a first bias voltage;
biasing the source of the third FET to a second bias voltage that is at least one threshold voltage lower than the first bias voltage; and
switching the second and third FETs ON, in response to the common switch drive signal, to force the voltage across the bootstrap path to a value less than the threshold voltage of the first FET and thereby switch the first FET OFF.
6. The method of claim 5 , wherein the gates of the first and second FETs have larger areas than an area of the gate of the third FET.
7. The method claim 5 , wherein the source of the second FET is configured to be biased by a first bias voltage and the third FET is configured to be biased by a second bias voltage lower than the first bias voltage.
8. The method of claim 5 , wherein the first, second, and third FETs are gallium nitride FETs.