the Technology Interface / Spring 97

# Common Emitter Amplifier Experiment

by

Bill Huffine
huffine@uscolo.edu
Department of Engineering Technology

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 report.

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.

Prelab:

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 circuit).

Lab:

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 voltage.

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 bypass capacitor?

Figure 1 - NPN Common Emitter Amplifier

Suggested References:

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, Prentice Hall