Volume 4 No.2, Spring 2001 ISSN# 1523-9926

# Cascaded Common Emitter Swamped Amplifier Experiment

Engineering Technology Department
Community College of Allegheny County
West Mifflin, Pennsylvania

ABSTRACT

The Cascaded Common Emitter (CE) Swamped Amplifier has an output voltage that is much greater than that of a single stage swamped amplifier. The advantages of swamped amplifiers include higher input impedance, less loading effect, and higher stability against temperature change. The disadvantage of the CE swamped amplifier is the reduction of the voltage gain compared to regular CE amplifiers that are unstable against changes of temperature. Therefore, to achieve higher voltage gain for swamped amplifiers, the need for cascaded CE amplifier becomes essential. It is assumed that the student has had some previous knowledge of single stage CE amplifier theory to perform this laboratory experiment.

PURPOSE OF THE EXPERIMENT

The purpose of this experiment is to investigate the operation of a cascaded CE swamped transistor amplifier to compute, analyze, measure, compare, and finally summarize the results of this experiment in a laboratory report.

INTRODUCTION

ASSUMPTIONS

Assume VBE = 0.7V, r'e = 25 mV /IE , and b = 120 in your computations.

CIRCUIT NOTES

1. Use general-purpose NPN transistors, such as 2N3904 or 2N2222.

2. Calculate the dB Voltage Gain, by using Av (dB) = 20 log (vout / vin).
(The computer simulation can be performed with different versions of ElectronicWorkbench or other software packages, it is easier to use Electronic Workbench,version 5 or latest version MultiSim 2001 and use the Bode Plotter to measure the dB gain.)

Figure 1 - Cascaded Common Emitter Swamped Amplifier (EWB Version 5)

VOLTAGE GAIN CALCULATIONS

1. For the amplifier circuit shown above, students will draw the dc equivalent circuit and calculate the following DC quantities:  VB, VE, IE, VC, and VCE(Q).

2. Students will draw the ac equivalent circuit and calculate the input impedance (Zin) for each stage, and the unloaded voltage gain of each stage.

3. Students will determine the final output voltage (vout) by using the unloaded voltage gain and Thevenin's method or by using the loaded voltage gain method.

PRACTICAL LABORATORY CIRCUIT MEASUREMENT

In order to measure the ac voltage gain of the amplifier shown, students will first measure the dc voltages with no applied ac input voltage.  If these are close to calculated values, then students will apply the ac input signal to the input of the amplifier, using a dual-trace oscilloscope to check the input and output signals.   If the input signal is amplified, the DMM can be used to measure applied input, signal at base of each transistor, and final output voltage.  Students should determine if the measured output signal (vout) is close to the calculated value?  They should also determine if the output signal is in phase or out of phase with respect to the input signal?

TROUBLESHOOTING

Using a jumper wire and temporarily shorting the swamping resistor RE1, students will observe the effect on the output.   After opening and shorting the 100 uF capacitor across Re1, students will record their observation of the output signal?   Shorting the resistors, Rg, R2, Rc1, and RL one at a time, students will explain their observations of the output signal.

COMPUTER SIMULATION MEASUREMENT USING ELECTRONIC WORKBENCH
(Version 5 or MultiSim version 2001)

Students will:

1. Load the Electronics Workbench and draw the amplifier circuit shown above.

2. Use the digital Multimeter (DMM) in DC mode to measure the DC voltages of VB, VE, VC, and VCE to determine if these values are close to calculated values.

3. Connect the sine wave source to the input of the amplifier and set the sine wave source for 10 mV (rms)/1kHz/0 Deg.

4. Attach the dual-trace oscilloscope to the input and output.  (If they observe an amplified signal, their circuit is working and is ready for measurement.)

5. Measure the input signal at the base of each stage and the final output signal with an AC voltmeters and DMM as shown in the circuit shown above.

6. Use the bode plotter to measure the dB voltage gain

COMPUTER SIMULATION, DISPLAY OF RESULTS

The figure below shows the computer simulation of the laboratory experiment; the oscilloscope shows the input and amplified output signal.  The bode plotter shows the measured dB gain of the amplifier.

Figure 2 - Bode Plot dB Gain and Oscilloscope Display of Input & output Signal (EWB Version 5)

Students will:

1. Compare their practical experiment, computer simulation, and calculated values

2. Explain any significant differences between practical, computer simulation, and calculated values.

3. Describe the function and purpose of each component in the circuit.

4. Troubleshoot the cascaded CE amplifier and report their comments in a laboratory report.

REFERENCES

[1]    Malvino, A. P., Electronic Principles, 5th edition, Prentice Hall: 1993

[2]     Floyd, T. L., Electronic Devices, 3rd edition, Prentice Hall

[3]     Cox, J., Fundamental of Linear Electronics, Delmar

[4]     Boylstad, R. and Nashelsky, Electronic Devices, Prentice Hall

[5]     Mead, R. L., Foundations of Electronics, 2nd edition, Delmar: 1994

[6]     Huffine, B., Common Emitter Amplifier Experiment, the Technology Interface: Spring 1997

http://et.nmsu.edu/~etti/spring97/electronics/ceamp/ceamp.html