Bipolar Junction Transistors and Field Effect Transistor Amplifiers

Drexel University

ECE-E302, Electronic Devices

Lab VIII: Field Effect Transistor

Field Effect Transistor

Objectives

To obtain the volt-ampere characteristics of the Field Effect Transistor (FET) and to demonstrate that the transistor is capable of producing amplification when properly biased.

Theory

The analysis suggests that small sinusoidal signals, vbe, superimposed on the DC voltage VBE, will give a sinusoidal collector current, Ic, superimposed on the DC current Ic at the Q-point. Depending upon the configuration of the resistors in the collector, the emitter, and the load, there will be an ideal Q-point for maximum distortion-free output signal amplitude. Determining these resistor requires constructing an ac load line. These topics will be covered in Electronics II.

Field Effect Transistor (FET)

The FET is a semiconductor device whose operation depends on the control of current by an electric field. There are two types of FETs, the junction field effect transistor (JFET) and metal-oxide-semiconductor field effect transistor (MOSFET). The FET differs from the BJT in the following characteristics:

(a) Its operation depends upon the flow of majority carriers only. It is therefore a unipolar device unlike the BJT which is bipolar.

(b)It is simple to fabricate and occupies less space in integrated circuits.

(c)It exhibits high input resistance (on the order of 1MΩ).

(d)It is less noisy than a bipolar transistor.

The main disadvantage of FET is its small gain-bandwidth product in comparison with that of a conventional transistor. The circuit symbol is shown in Figure 1.

Figure 1. Circuit symbol for an n-channel JFET

The characteristic curves of a FET look similar in shape to those of a BJT, but since the FET has a different basis of operation, the curves are given in terms of other parameters.

Procedures

a)Obtain the I-V characteristics of your FET using the curve tracer. Be sure to record the part number of your FET in your notebook.

b)Connect the circuit for your FET.

c)Measure the current Id, voltages Vds and Vgs.

d)Superimpose the dc load line on the characteristic curves and check whether the FET is operating at the Q-point.

e)Modify your circuit and apply AC input. Determine the voltage gain of the FET amplifier. Obtain a printout of the waveforms.

Report

Determine the voltage gain of the FET from voltage measurement and compare it with the result from theoretic calculation.

1-2