Transcription of Introduction: The Nature of VCRs - Vishay Intertechnology
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Introduction: The Nature of VCRs - Vishay Intertechnology
AN105 Siliconix10-Mar-97 1 T V e e e Introduction: The Nature of VCRsA voltage-controlled resistor (VCR) may be defined as athree-terminal variable resistor where the resistance val-ue between two of the terminals is controlled by a voltagepotential applied to the a junction field-effect transistor (JFET) under certainoperating conditions, the resistance of the drain-sourcechannel is a function of the gate-source voltage alone andthe JFET will behave as an almost pure ohmic drain-source current, IDSS, and minimum re-sistance rDS(on), will exist when the gate-source voltageis equal to zero volts (VGS = 0). If the gate voltage is in-creased (negatively for n-channel JFETs and positivelyfor p-channel), the resistance will also increase. When thedrain current is reduced to a point where the FET is nolonger conductive, the maximum resistance is voltage at this point is referred to as the pinchoff orcutoff voltage and is symbolized by VGS = VGS(off).
Resistors R2 and R3 in Figure 6 couple the drain signal to the gate; the resistor values are equal, so that symmetrical voltage-current characteristics are produced in both quadrants. The resistors must be sufficiently large to provide minimum loading to the circuit: R 2 = R 3 10 [R 1 rDS (max) R L] (3) Typically, 470-k resistors will work well ...
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