Abstract: A gain control circuit for generating a differential output signal in response to a differential input signal with a gain controlled by an input control signal employs two asymmetric field effect transistors (FETs) as a linear, voltage-controlled resistor. The FETs are interconnected drain-to-source with the control signal being applied to their gates so that their combined channel current is a substantially linear and symmetric function of the input control signal voltage. The gain control circuit includes two bipolar transistors, with the differential input signal being applied across their bases and the differential output signal being developed across their collectors. A resistor links the emitters of the two transistors, sources are applied to the emitters, and a pair of resistors links each collector to a source of bias voltage. The FET-based voltage-controlled resistor may be connected either between the collectors or between the emitters of the two transistors.

Abstract: An Ft multiplier amplifier employs N similar differential amplifier stages, each including a differential transistor pair. Bases of each pair form stage inputs and collectors of each pair form stage outputs. Inputs of adjacent stages are connected in series by interconnecting the transistor bases of adjacent stages so that each stage receives and amplifies the same input current. Outputs of all stages are connected in parallel so that stage output currents are summed to produce an amplifier output current. Thus the current gain of the amplifier is N times the current gain of each amplifier stage. A set of bias circuits employ voltage dividers across inputs of adjacent stages to provide appropriate bias voltages at the node of connection between the adjacent stages.

Abstract: A control unit automatically adjusts the supply voltage to a differential amplifier so that the amplifier has a common mode output voltage matching an input control voltage. The control unit employs circuit elements analogous to circuit elements forming the differential amplifier to generate a reference voltage in response to the supply voltage wherein the reference voltage is an estimate of the amplifier's common mode output voltage. An operational amplifier receiving the control voltage and the reference voltage as inputs, adjusts the supply voltage so the reference voltage matches the control voltage, thereby ensuring that the amplifier's common mode output voltage matches the control voltage.