How to design Op Amp amplifier with Electronic Gain Control is discussed in this analog circuit design example. This op amp transistor circuit is designed with two operational amplifiers and a pair of matched JFET transistors. For more examples see    • Electrical Engineering, Analog, Digit...  
As discussed in this circuit analysis video, values of the Reference Voltage Vref and Control voltage Vc have to be properly set so that both junction field effect transistors are enforced to operate in deep Ohmic Region of the transistor drainsource IV curve. This results in transistor behaving as electronically controlled resistors whose value is defined by the voltage of the gate that is then defined by the output voltage of the op amp. A combination of op amp virtual short enforced by negative feedback and proper biasing, Kirchhoff's current law (KCL) and JFET equivalent resistor in deep ohmic region of transistor operation is used to analyze this circuit and find the equation for voltage gain of the amplifier as a function of the reference voltage, control voltage and two resistors in the circuit.