Use of JFET as a WR or VDR

Use of JFET as a WR or VDR

AU : May-07, 08, Dec.-05, 06, 07, 08

• Let us consider the drain characteristics of FET as shown in the Fig. 3.7.1 In this characteristics we can see that in the region before pinch off voltage, drain characteristics is linear, i.e. FET operation is linear. In this region the FET is useful as a voltage-controlled resistor, i.e., the drain to source resistance is controlled by the bias voltage VGS. The operation of FET in this region is useful in most linear applications of FET. In such an application the FET is also referred to as a voltage-variable resistor (VVR) or voltage-dependent resistor (VDR).

Application

• The voltage variable resistor property of FET can be used to vary the voltage gain of a multistage amplifier A as the signal level is increased. This action is called automatic gain control (AGC). This is illustrated in Fig. 3.7.2 Here, maximum value of signal is taken, rectified, and filtered to produce a d.c. voltage proportional to the output signal level. This voltage is applied to the gate of JFET, thus causing the a.c. resistance between the drain and source to change. As this resistance is connected across RE, effective RE also changes according to change in the drain to source resistance. When output signal level increases, the drain to source resistance also increases, increasing effective R_E.

Increase in RE causes the gain of transistor Q1 to decrease reducing the output signal. Exactly reverse process takes place when output signal level decreases. Therefore, the output signal level is maintained constant. It is important to note that the d.c. bias conditions of Q1 are not affected by JFET since FET is isolated from Qj by means of capacitor C₂.

Review Questions

1. Draw the chracteristics of JFET and mark its regions of operation.

2. Explain one application of JFET used as variable resistor.

3. Describe how a JFET can be used a voltage variable resistor.