Common Emitter Amplifier Experiment including computer simulation by Bill Huffine huffine@uscolo.edu Department of Engineering Technology University Of Southern Colorado 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. Additionally, the use of Electronics Workbench as computer simulation tool is described, to further enhance the learning process. 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, V BE = 0.7V, r' e = 25mv/I E in the prelab analysis. 3) The computer simulations below refer to the use of Electronics Workbench. Prelab: For the circuit shown, predict the following DC parameters: I E, V E, V B, V C, and V CE, using the approximate method. Draw the ac equivalent circuit, and predict the voltage gain (A v ), input impedance Z i, and output impedance Z o, assuming all capacitors are ac shorts. Also draw the composite waveforms expected at points A, B, C, D, and E in the circuit, assuming V in (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 Z i, a dependent voltage source, and an output impedance Z o. (Recall that Z i appears across the input of the amplifier, and Z o 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 Z i 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 R test and Z i in your diagram). For example, if R test = Z i, the amplifier s input signal will be half of the applied input signal. You can determine Z o 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 Z o (note the voltage divider between Z o and R L in your equivalent circuit). Lab Measurements: In-lab circuit measurements: Build the circuit shown and verify ALL of the dc and ac predictions above. Use DC coupling and dual-trace on the oscilloscope as appropriate. Then temporarily remove C E, and remeasure the ac output voltage. Compare your results to your predictions. Computer simulation using Electronics Workbench: Draw the circuit shown (Figure 1) using Electronics Workbench. Connect the amplifier input to a sinusoidal signal generator as shown in the figure. Use the DVM (digital voltmeter) in Electronics Workbench to measure the DC voltages on the transistor terminals, and compare these to the predicted and measured values. Then connect the oscilloscope to the amplifier s input and output terminals (see Figure 2), and compare these measured results to your predicted and experimentally measured values. Finally, try changing some of the circuit s component values, and see how this affects the output signal; for example, by temporarily removing C E, you can see that the output signal dramatically decreases; by shorting out the emitter swamping resistor Rs, the output signal (and non-linear distortion) increase significantly. By changing the base bias resistors, it may be possible to further optimize the amplifier for a larger peak output signal. In summary, by using Electronics Workbench, note how easy it is to change any component at will, and immediately see the effects, which further encourages and enhances the student s learning process.
Comments: Compare your experimental and computer simulation 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? etc
Figure 1 - NPN Common Emitter Amplifier
Figure 2 - Computer Simulation Results
Footnote: Most of this article was based on my original article appearing in the Spring 1997 issue of ET, the Technology Interface. The use of Electronics Workbench, by Interactive Image Technologies, was added here, to additionally show how computer simulations could be used to further enhance student understanding, and to encourage student experimentation. Suggested References: Electronic Devices, by Floyd, Prentice Hall Fundamentals of Linear Circuits, by Floyd, Merrill Electronic Devices and Circuit Theory, by Boylestad and Nashelsky, Prentice Hall Electronic Principles, by Malvino, McGraw-Hill Operational Amplifiers with Linear Integrated Circuits, by Stanley, Prentice Hall