The George Washington University
School of Engineering and Applied Science
Department of Electrical and Computer Engineering
 ECE 20 - Spring 2001

Experiment # 6 (Part I)

Bipolar Junction Transistors
Common Emitter (CE)






Equipment:
You must make up a complete equipment list and have your instructor review it before you start.
 

Objectives:

Figure # 1




1.- (HW) Analysis

In Exp. #5, the DC Bias Analysis and general formula for Unloaded Voltage Gain (Avo) for circuit of Figure #1 were calculated.  The more detailed analysis analysis on Common Emitter Amplifier for this circuit will be conducted in this section.

When this circuit is operated in CEC:

    1. Find ROUT
    2. RIN
    3. Unloaded Voltage Gain, Avo
    4. Av (RL=ROUT)
    5. Ai (RL=ROUT).
    6. Maximum input voltage vin max that the amplifier can accept before the output distorts (unloaded).
To get a credit for this analysis, you must show all the steps below:
1.  Do DC Bias Analysis
2.  Draw Small Signal Equivalent circuit for CEC
3.  Find Rin, then Rout @Vin=0
4.  Find Vout and Vin to get Av
5.  Find Av @ RL=Rout
6.  Find Ai
7.  Find Vinmax

The text book and lab handout should give you a good example on how to perform the analysis.
 

2.- Verification

Build and fully test the circuit shown in Figure #1. By applying a sinusoidal signal such that the small signal approximation holds, measure:

    1. RIN (input impedance) and ROUT (output impedance) of the assembled circuit.  Read the lab handout to properly understand how to measure Rin and Rout.

      2.  
    2. Unloaded Voltage gain, Avo.  Label the plot - "Plot 2a - Avo for CEC"

      3.  
    3. Voltage Gain for RL equal to 2*ROUT, ROUT, ROUT /2, and ROUT /4.  Print the output plot for each case and label each plot as "Plot 2b - Av @ RL= [RL value] for CEC"

      4.  
    4. Find the maximum input voltage that the amplifier can accept before the output distorts (Unloaded case). Print the plot and label it as "Plot 2c - Vin max before output distorts for CEC"

      5.  
    5. Determine the phase relationship between the input and output voltages.
Hint: Connect a large capacitor between VCC and ground in order to remove all the noise from the source. The noise is amplified and mixes with the output due to the input AC signal vs.
 
 
 

3.- (HW) Design

Design a Beta-Stabilized capacitively coupled Common Emitter voltage amplifier (with shorting capacitor on Emitter) similar to the one shown in Fig # 1. Use ORCAD to verify that all the specifications have been achieved..

Design Specifications of the Amplifier

VCC = 24 VDC ± 0.5 VDC

Vsinusoidal   = 100 mVpeak

Av   =  -15

RL   = 10 kW

RIN  =  5 kW
 
 
 

4.- Assembly, Test and Verification of Specifications

Build and test your design. Measure and verify that your design meets all the given specifications.

  1. Measure VBE, VE, VB, , VCE, IB, IE, and IC with no small signal input (Vs).

    2.  
  2. Connect the small signal input and measure Av, Rin (input impedance) and Rout (output impedance) of the assembled circuit.

    3.  
  3. Find the maximum input voltage (Vin max) that the amplifier can accept before the output distorts.

    4.  
  4. Measure the phase relationship between the input and output voltages.

    5.  
  5. Print out the output plot of Vout and Vin.  Label this plot "Plot 4a - Design of CEC - Vout and Vin)". Annotate in the plot the Voltage gain Av you get from the circuit.

 
 

5.- Conclusion

  1. Why is there always a big difference between the value the function generator was set for and the actual input signal?

    2.  
  2. Compare the measured results to your design calculations and specifications. Explain any differences.

    3.  
  3. What can you say about the general characteristics of CEC amplifier( Rin, Rout, Av, Ai etc)