AN105 Datasheet PDF

Vishay AN105

The Nature of VCRs A voltage-controlled resistor (VCR) may be defined as a three-terminal variable resistor where the resistance value between two of the terminals is controlled by a voltage potential applied to the third. For a junction field-effect transistor (JFET) under certain operating conditions, the resistance of the drain-source channel is a function of the gate-source voltage alone and the JFET will behave as an almost pure ohmic resistor.

Part No.	AN105
Datasheet	AN105 Download
Manufacturer	Vishay
Description	FETS AS VOLTAGE-CONTROLLED RESISTORS
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APPLICATIONS

for VCRs A simple application of a FET VCR is shown in Figure 4, the circuit for a voltage divider attenuator. –2.5 V VGS = –3.0 V 200 400 VDS (mV) R VIN –2.5 V –1.5 V 0V –200 – + VCR VGS VOUT Figure 2. NĆChannel JFET Output Characteristics Enlarged Around VDS = 0 V Figure 4. Simple Attenuator Circuit The output voltage is: The graph in Figure 3 is useful in estimating rDS values at any given value of VGS. The resistance is normalized to its specific value at VGS = 0 V. The dynamic range of rDS is shown as greater than 100:1, although for best control of rDS a range of 10:1 is normally used. V IN rDS V OUT + R ) r (1) DS It is assumed that the output voltage is not so large as to push the VCR out of the linear resistance region, and that the rDS is not shunted by the load. The lowest value which vOUT can assume is: 1000 VDS ¬ 0.1 V V IN rDS(on) V OUT(min) + R ) r DS(on) (2) rDS /r DS(on) ( W ) 100 r DS(on) r DS ] 1 – V ńV GS GS(off) 10 Signal Distortion: Causes Figure 2 sh...