Fully differential voltage controlled operational transconductance amplifier

DETAILED DESCRIPTION The invention is directed to a simple, reliable, inexpensive, operational transconductance amplifier having both a differential input and a differential output, that substantially reduces control feedthrough.
The invention includes a matched pair of bipolar transistors the bases of which form a differential amplifier input port.
One set of the pair's current conducting terminals are coupled through degeneration resistors to a first current source, the other through active loads to a bias voltage terminal.
A second matched pair of bipolar transistors has its corresponding current conducting terminals connected to the current conducting terminals of the first pair that are coupled to active loads.
The other current conducting terminals of the second pair are coupled through degeneration resistors to a second current source of equal value to the first current source.
The bases of the second matched pair form the terminals of a differential output port for the OTA.
Differential feedback between the matched pairs significantly reduces AC signal distortions.
In the preferred embodiment, the matched pairs are NPN transistors and their emitters are coupled through degeneration resistors to equal current sources and their collectors are coupled through active loads to a bias voltage.
The differential output terminals are coupled to a common mode feedback circuit.
A current steering circuit provides a voltage control input which controls the amount of current flowing into the differential pairs, thereby controlling the transconductance of the amplifier.
A noise reduction circuit, which interposes diodes between a bias voltage terminal and the active loads, may be added to the amplifier.
The new amplifier may be employed to create a voltage controlled low pass filter by placing a capacitor across high impedance nodes created at junctions between the active load and current steering circuits.
These and other features, aspects and advantages of the invention will be apparent to those skilled in the art from the following detailed description, taken together with the accompanying drawings