the Technology Interface / Spring 97
Abstract: The Common Emitter Amplifier is one of the
three basic transistor amplifier configurations. In this experiment,
the student will build and investigate a basic NPN common emitter
transistor amplifier. It is assumed that the student has had
some background in transistor amplifier theory, including the
use of ac equivalent circuits. The student is expected to develop
his or her own procedure for performing the lab experiment, after
having done a complete prelab analysis, and then analyze, and
thoughtfully summarize, the results of the experiment in a lab
Purpose of the Experiment: to investigate the operation
of a common-emitter NPN transistor amplifier.
Some Circuit Notes:
1) The transistor is a general purpose NPN transistor (2N3904, 2N2222), or equivalent.
2) Assume Beta = 100, VBE = 0.7V, r'e =
25mv/IE in the prelab analysis.
For the circuit shown, predict the following DC parameters: IE,
VE, VB, VC, and VCE,
using the "approximate method". Draw the ac equivalent
circuit, and predict the voltage gain (Av), input impedance
Zi, and output impedance Zo, assuming all
capacitors are "ac shorts". Also draw the "composite"
waveforms expected at points A, B, C, D, and E in the circuit,
assuming Vin (at pt A) is a 100 mv peak, 5 kHz sinewave.
Also, draw a basic "black box" model (equivalent circuit)
for a voltage amplifier, consisting of an input impedance Zi,
a dependent voltage source, and an output impedance Zo.
(Recall that Zi appears across the input of the amplifier,
and Zo appears in series with the output of the amplifier's
dependent voltage source). Refer to this equivalent circuit for
the procedures that follow.
You can measure Zi by inserting a test resistor (e.g:
10K ohms) in series with the signal input to the amplifier, and
measuring how much of the ac generator signal actually appears
at the input of the amplifier (note the voltage divider between
Rtest and Zi in your diagram). For example,
if Rtest = Zi , the amplifier's input signal
will be half of the applied input signal.
You can determine Zo as follows: temporarily remove
the load resistor, and measure the unloaded ac output voltage.
Then replace the load, and remeasure the ac output voltage.
Use these measurements to determine Zo (note the voltage
divider between Zo and RL in your equivalent
Build the circuit shown, and verify ALL of the predictions above.
Use DC coupling and dual trace on the oscilloscope as appropriate.
Then temporarily remove CE , and remeasure the output
Compare your results with your prelab calculations. Explain any
significant discrepancies. Also, explain the purpose or function
of the various components in the circuit; for example, what is
the purpose of the 100 ohm resistor at the emitter? the emitter
T. Floyd, Electronic Devices, Prentice Hall
T. Floyd, Fundamentals of Linear Circuits, Merrill
R. Boylestad and L. Nashelsky, Electronic Devices and Circuit Theory, Prentice Hall
A. Malvino, Electronic Principles, McGraw-Hill
W. Stanley, Operational Amplifiers with Linear Integrated Circuits,