Download 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 Qpoint. 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.
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Drexel University
ECE-E302, Electronic Devices
Lab VIII: Field Effect Transistor
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.
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