Cascaded Common Emitter Swamped Amplifier Experiment

M.R. Chitsaz-zadeh
mchitsazzadeh@ccac.edu
Engineering Technology Department
Community College of Allegheny County
West Mifflin, Pennsylvania

ABSTRACT

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

Students should first draw the dc and ac equivalent circuit for this amplifier in order to calculate the ac emitter resistance (r'e), input impedance of the base, input impedance of the stage, and voltage gain for each stage.  For cascaded CE stages, the input impedance of the second stage comprises the load for the first stage.  Therefore, for cascade amplifiers there is a loading effect on the preceding stage.  In other words, the unloaded voltage gain is higher than the loaded voltage gain.   In this experiment students will construct a cascaded CE swamped amplifier to stabilize and boost the overall voltage gain.  Secondly, students will compute the unloaded and loaded voltage gain by using Thevenin's method.   Third, students will construct the circuit shown by using the breadboard and measuring voltage gain.   Fourth, students will simulate this laboratory by using the student edition of Electronic Workbench, version 4, 5,  MultiSim version 6.1,  or latest MultiSim version 2001.   Then students will be expected to measure voltage gain of each stage and the overall voltage gain in order to compare the measured values in actual laboratory with measured values by computer simulation and calculated values.  Finally, students will tabulate and summarize the results in a laboratory  report where they may also choose to discuss the differences between measured and calculated  values in the laboratory report.

ASSUMPTIONS

Assume V_BE = 0.7V, r'_e = 25 mV /I_E , 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 (v_out / v_in).
   (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:  V_B, V_E, I_E, V_C, and V_CE(Q).
     

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

3. Students will determine the final output voltage (v_out) 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 (v_out) 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 R_E1, students will observe the effect on the output.   After opening and shorting the 100 uF capacitor across R_e1, students will record their observation of the output signal?   Shorting the resistors, R_g, R₂, R_c1, and R_L 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 V_B, V_E, V_C, and V_CE 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)

COMMENTS

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