Selectable gain differential amplifier

1. A selectable gain differential amplifier comprising:

a differential amplifier;

a plurality of cascode leg pairs coupled to the differential amplifier, each leg of each said cascode pair including a cascode device, an output node, and a load resistor configured to provide a selectable gain;

a variable voltage generator connected to each said leg configured to set the output node voltage of any active cascode leg pair to a uniform predetermined common mode voltage and the output node voltage of any inactive cascode leg pair to a voltage different from the predetermined common mode voltage; and

a selector circuit configured to select the output of any said active cascode leg pair.

2. The selectable gain differential amplifier of claim 1 in which the selector circuit comprises as a selector buffer circuit including a plurality of emitter follower pairs connected to output nodes of the cascode leg pairs, the selector buffer circuit configured to select the emitter follower pair connected to the active cascode leg pair to provide a buffered output voltage and the other emitter follower pairs are inactive based on the output node voltage of each said leg set by variable voltage generator.

3. The selectable gain differential amplifier of claim 1 in which the selectable gain differential amplifier includes a common emitter or common source differential amplifier.

4. The selectable gain differential amplifier of claim 1 further including a switching voltage generator connected to the control electrode of each said cascode leg pairs configured to turn on and turn off predetermined cascode leg pairs to provide the selectable gain.

5. The selectable gain differential amplifier of claim 1 in which the plurality of cascode leg pairs includes three cascode leg pairs.

6. The selectable gain differential amplifier of claim 3 in which the cascode common emitter differential amplifier includes a pair of emitter degenerative resistors.

7. The selectable gain differential amplifier of claim 6 in which the ratio of the load resistor of each said leg to an associated degenerative resistor is configured to determine the gain.

8. A method of providing selectable gain, the method comprising:

providing a differential amplifier;

providing a plurality of cascode leg pairs connected to the differential amplifier, each leg of each said cascode pair including a cascode switching transistor and a load resistor;

providing a selectable gain;

providing a selector circuit;

setting output node voltage of any active cascode leg pair to a uniform predetermined common mode voltage and the output node voltage of any inactive cascode leg pair to a voltage different from the predetermined current mode voltage; and

selecting the output of any said active cascode leg pair.

9. The method of claim 8 further providing a selector buffer circuit including a plurality of emitter follower pairs connected to predetermined cascode legs of predetermined cascode leg pairs.

10. The method of claim 9 further including selecting the emitter follower pair connected to any active cascode leg pair to provide a buffered output voltage and setting the other emitter follower pairs to be inactive based on output node voltage of each said leg.