Junction Field Effect Transistors

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The junction field effect transistor in its simplest form is essentially a voltage controlled resistor. The resistive element is usually a bar of silicon. For an N-channel JFET this bar is an N-type material sandwiched between two layers of P-type material. The two layers of P-type material are electrically connected together and are called the gate. One endof the N-type bar is called the source and the other is called the drain. Current is injected into the channel from the source terminal, and collected at the drain terminal. The interface region of the P- and the N-type materials forms a P-N junction as shown in Figure

The Junction Field Effect Transistor (JFET) exhibits characteristics which often make it more suited to a
particular application than the bipolar transistor. Some of these applications are:

• High Input Impedance Amplifier
• Low-Noise Amplifier
• Differential Amplifier
• Constant Current Source
• Analog Switch or Gate
• Voltage Controlled Resistor

Basic JFET Amplifier Configurations

There are three basic JFET circuits: the common source, the common gate, and the common drain as shown in Figure 1. Each circuit configuration describes a two port network having an input and an output. The transfer function of each is also determined by the input and output voltages or currents of the circuit.