JFET as a VVR or VDR | Analog Circuits - Electronics and Communication Engineering (ECE) PDF Download

JFET AS A VVR OR VDR:-

Let us consider the drain characteristics of FET as shown in the fig.

JFET as a VVR or VDR | Analog Circuits - Electronics and Communication Engineering (ECE)

 

In this characteristics we can see that in the region before pinch off voltage, drain characteristics are 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.( In this region only FET behaves like an ordinary resistor. This resistances can be varied by VGS). The operation of FET in the 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).

The drain to source conductance ( rd )

                           gd= JFET as a VVR or VDR | Analog Circuits - Electronics and Communication Engineering (ECE)JFET as a VVR or VDR | Analog Circuits - Electronics and Communication Engineering (ECE)JFET as a VVR or VDR | Analog Circuits - Electronics and Communication Engineering (ECE) JFET as a VVR or VDR | Analog Circuits - Electronics and Communication Engineering (ECE)   for small values of Vds which may also be expressed as


                   gd=gd0JFET as a VVR or VDR | Analog Circuits - Electronics and Communication Engineering (ECE)


where gd0 is the value of drain conductance.

When  the variation of the rwith VGS can be closely approximated by the expression

rd= JFET as a VVR or VDR | Analog Circuits - Electronics and Communication Engineering (ECE) Where ro = drain resistance at zero gate bias. K = a constant, dependent upon FET type.

 

APPLICATION OF VVR:

The VVR 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 AGC automatic gain control. A typical arrangement is shown in the fig.

JFET as a VVR or VDR | Analog Circuits - Electronics and Communication Engineering (ECE)

 

Here maximum value of signal is taken rectified; filter to produce a DC voltage proportional to the output signal level. This voltage is applied to the gate of JFET, thus causing the resistance between drain and source to change. As this resistance is connected across RE, so effective RE also changes according to change in the drain to source resistance. When output signal level increases, the drain to source resistance rd increases, increasing effective RE. Increase in RE causes the gain of transistor Q1 to decrease, reducing the output signal. Exactly reverse process takes place when output signal level is decreased.

:: The output signal level is maintained constant. It is to be noted that the DC bias conditions of Q1 are not affected by JFET since FET is isolated from Q1 by capacitor C2.

The document JFET as a VVR or VDR | Analog Circuits - Electronics and Communication Engineering (ECE) is a part of the Electronics and Communication Engineering (ECE) Course Analog Circuits.
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FAQs on JFET as a VVR or VDR - Analog Circuits - Electronics and Communication Engineering (ECE)

1. What does VVR stand for in the context of JFET?
Ans. VVR stands for Voltage Variable Resistor. It refers to using a JFET (Junction Field-Effect Transistor) as a variable resistor to control the voltage in a circuit.
2. What is the function of a VVR or VDR in a circuit?
Ans. A VVR or VDR (Voltage Dependent Resistor) is used in a circuit to regulate or limit the voltage across a specific component or section of the circuit. It provides voltage-dependent resistance, allowing it to protect sensitive components from excessive voltage levels.
3. How does a JFET function as a VVR or VDR?
Ans. A JFET can function as a VVR or VDR by adjusting the voltage applied to its gate terminal. By varying the gate-source voltage, the channel resistance of the JFET changes, which in turn affects the voltage across the circuit. This voltage-dependent resistance characteristic allows the JFET to act as a voltage regulator or limiter.
4. What are the advantages of using a JFET as a VVR or VDR?
Ans. Some advantages of using a JFET as a VVR or VDR include: - Wide voltage control range: JFETs can provide voltage regulation or limiting over a wide range of voltages. - Fast response time: JFETs can quickly adjust their resistance to changes in the applied voltage, offering fast protection to the circuit. - High accuracy: JFETs provide precise voltage control due to their stable and consistent resistance characteristics. - Low power consumption: JFETs consume low power during operation, making them energy-efficient options for voltage regulation. - Small size: JFETs are available in compact packages, allowing for space-saving designs in electronic circuits.
5. In what applications are JFETs commonly used as VVRs or VDRs?
Ans. JFETs find applications as VVRs or VDRs in various electronic circuits, such as: - Power supplies: JFETs can regulate the output voltage of power supplies to protect sensitive components from voltage fluctuations. - Amplifiers: JFETs can be used to control the gain or amplification of signals in audio or radio frequency amplifiers. - Voltage clamps: JFETs can limit the voltage spikes or transients in a circuit to prevent damage to connected devices. - Voltage regulators: JFETs can ensure a stable and constant voltage output in voltage regulator circuits. - Overvoltage protection: JFETs can act as voltage limiters to safeguard electronic devices from excessive voltage levels.
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