Voltage Controlled Attenuator (VCA): Electronic Gain AC Analysis

AC signal analysis and deriving the electronic gain control formula for this Voltage-Controlled Attenuator (VCA) are presented in this part-2 video of an OpAmp-BJT implementation of VCA circuit. One important application of this VCA circuit is in audio signal processing and Automatic Gain Control (AGC) for audio VCA. In this implementation, the electronic gain/attenuation control circuit is designed with three Operational amplifiers and six NPN Bipolar Junction transistors. In previous Part1 video    • VCA Electronic Gain Control (Part1) Voltag...  , an intuitive circuit overview and DC bias analysis of this electronically-controlled gain/attenuator circuit are discussed. The follow-up Part three presents a variant of this circuit that then enables positive voltage control instead of the negative control voltage (Vc) that is needed in the VCA circuit analyzed in parts one and two. This three-part VCA Electronic Gain Control circuit design is posted in the Analog Circuits video playlist    • Electrical Engineering, Analog, Digital De...  . Note that the presented Voltage-Controlled Attenuator is an example of electronic gain control circuit that is implemented with three operational amplifiers and six NPN bipolar junction transistors. Four matched BJT transistors are used to construct two differential BJT stages. These matched NPN transistors should be in the same IC package and on the same substrate so that they share the same PN junction process and thermal properties and hence have the same PN Junction saturation current and thermal voltage (VT). The bottom NPN BJT transistors provide the DC bias current for the two differential BJT pairs. When applied control voltage Vc is zero, circuit is symmetric (matching transistors) resulting in half of the bias current passing through left and right branches of the differential pair. If a negative control voltage is applied, it steers current away from one branch to the branch of the differential BJT stage. The complete DC bias analysis presented in this video intuitively explains how the circuit is biased and also computes the DC currents and voltages of the nodes in this circuit. It is show that output DC voltage is zero.