9

FET Amplifiers

9.1 Introduction

We know that the input resistance of the FET (more appropriately of MOSFET) is ideally infinite and the output resistance rd is also very high. The calculation of its gain becomes very simple by assuming them to open circuited w.r.t. the external components.

9.1.1 Small Signal Equivalent Circuit of FET

From Eqn. 6.6.20 it is clear that the drain current ID is function of two variables VGS and VDS described as

 

ID = f (VDS, VGS)                    (9.1.1)

In other words, its value depends partially on both the voltages VGS and VDS. Hence, it can be expressed by partial differential equation, i.e.

For small signal, ...

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