FET Amplifier Study Materials

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FET Amplifier Study Materials

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INTRODUCTION

Field Effect Transistor (FET) amplifiers provide an excellent voltage gain and high input impedence. Because of high input impedence and other characteristics of JFETs they are preferred over BJTs for certain types of applications.

There are 3 basic FET circuit configurations:

i)Common Source

ii)Common Drain

iii)Common Gain

Similar to BJT CE,CC and CB circuits, only difference is in BJT large output collector current is controlled by small input base current whereas FET controls output current by means of small input voltage. In both the cases output current is controlled variable.

FET amplifier circuits use voltage controlled nature of the JFET. In Pinch off region, ID depends only on VGS.

Common Source (CS) Amplifier

A simple Common Source amplifier is shown in Fig (a) and associated small signal equivalent circuit using voltage-source model of FET .

Voltage Gain

Source resistance (RS) is used to set the Q-Point but is bypassed by CS for mid-frequency operation. From the small signal equivalent circuit, the output voltage

                        VO = -RDµVgs(RD + rd)

Where Vgs = Vi , the input voltage,

Hence, the voltage gain,

AV = VO / Vi = -RDµ(RD + rd)

Input Impedence

From Fig (b) Input Impedence is

                        Zi =  RG

For voltage divider bias as in CE Amplifiers of BJT

                        RG =  R1 ║ R2

Output Impedance

Output impedance is the impedance measured at the output  terminals with the input voltage V­I = 0

From the Fig. (b) when the input voltage Vi = 0, Vgs = 0 and hence

                        µ Vgs = 0

The equivalent circuit for calculating output impedence is given in Fig..

Output impedence  Zo = rd ║ RD

Normally rd will be far greater than RD . Hence Zo ≈ RD

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