Field-effect Transistor Amplifier
In this chapter, we parallel the approach we used for BJT transistors, this time concentrating on the field-effect transistor. After studying this material, you will
- Understand the difference between FETs and BJTs.
- Learn the differences between various forms of FETs.
- Know how to bias FETs for linear operation.
- Understand the small-signal models and how to use them.
- Be able to analyze FET amplifier circuits.
- Be able to design FET amplifier circuits to meet specifications.
- Understand how computer simulation programs model FETs.
- Know how FETs are fabricated as part of integrated circuits.
The modern field effect transistor (FET) proposed by W. Shockley in 1952, differs from that of the BJT. The FET is a majority carrier device. Its operation depends on using an applied voltage to control the majority carriers (electrons in n-type material and holes in p-type) in a channel. This voltage controls the current in the device by means of an electric field.
Field-effect transistors are three-terminal devices, but in contrast with the bipolar transistor, it is the voltage across two terminals that controls the current flowing in the third terminal. The three terminals in an FET are the drain, source and gate.
In comparing FETs to BJTs, we will see that the drain (D) is analogous to the collector and the source (S) is analogous to the emitter. A third contact, the gate (G), is analogous to the base. The source and drain of an FET can usually be interchanged without affecting transistor operation.
We discuss two classes of FET in detail, these being the junction FET (JFET) and the metal-oxide semiconductor FET (MOSFET).
The chapter begins with a discussion of the characteristics of MOSFETs and JFETs and a comparison of these characteristics. We then examine the ways of using these devices in circuits, and the techniques for biasing the various amplifier configurations.
As we examine analysis techniques in detail, we present computer simulation models. This is followed by detailed sections dealing with analysis techniques and with design methodology.
The chapter concludes with a brief discussion of other specialty devices.
The TINA and TINACloud circuit simulators, supporting this resource, include a lot of sophisticated MOSFET and JFET computer simulation models and circuits to be used for circuit simulation.