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Chapter 17. JFETs

Introduction

The acronym FET stands for Field Effect Transistor; the J stands for Junction.

JFETs are transistors with a very high input resistance. They have three terminals: Drain, Gate and Source. JFETs come in two flavours: N-channel and P-channel:

A JFET looks like a voltage-controlled current source; the current source between the Drain and the Source is controlled by the voltage across the Gate and the Source.

The ratio dID/dVGS is called the forward transfer admittance, symbol yfs.

To make an N-channel JFET work, the Gate voltage must always be less than the Drain and Source voltages. That means that VGS must be negative. If VGS becomes more negative, ID will decrease. The voltage at which the drain current becomes zero, is called the pinch-off voltage.

A JFET only behaves like a voltage-controlled current source if VGD is less (more negative) than the pinch-off voltage. Otherwise, the JFET will behave like a voltage-controlled resistor. (For an explanation, take a look inside a JFET.)

Let's take a look at the BF245A, an N-channel JFET. According to the datasheet, yfs = 3mA/V (or 3mS, millisiemens). ID = 1mA if VGS = -1V. If VGS increases by 0.5V, ID will be 1 + 0.5∙3 = 2.5mA.

We can use this behaviour to create an amplifier.

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