Скачать или смотреть VCA Electronic Gain Control (Part1) Voltage Controlled Attenuator Overview

An example implementation of electronic gain control circuit in the form of a Voltage-Controlled Attenuator (VCA) designed with Op Amp and BJT transistors is presented. In this Part1 video, the overview and DC bias analysis of this electronically-controlled gain/attenuator circuit are discussed. The follow-up Part two and Part three are posted in the Analog Circuits video playlist    • Electrical Engineering, Analog, Digital De...  . This VCA circuit 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. Please see VCA (Part2) Electronic Gain Control Formula and AC analysis    • Voltage Controlled Attenuator (VCA): Elect...   to see the AC signal analysis and deriving the Electronic Gain Control formula for this Voltage-Controlled Attenuator (VCA).