ELECTRICAL AND COMPUTER ENGINEERING
THE CITADEL
ELEC 313 Lab 9
Common Emitter Transistor Amplifier
OBJECTIVE:
to construct and measure the v/i characteristics of a common-emitter amplifier
EQUIPMENT:
transistor -- 2N2222A
resistors -- 470 Ω, 1 kΩ, 22 kΩ, 100 kΩ; capacitors -- 0.1 F (x2), 10 F
resistive decade box -- Heathkit IN-3117
triple-output DC power supply -- Hewlett Packard E3631A
function generator -- Keysight 33210A
multi-meter -- Hewlett Packard 34401A or Keysight 34460A
oscilloscope -- Keysight DSO-X 2022A
PRE-LAB:
Figure 1. Common-emitter transistor amplifier (without emitter bypass capacitor).
1.
For the circuit of Figure 1, compute the value of R2 that causes 3 mA to flow in the emitter
of the 2N2222A transistor. Assume β = 150 and the transistor turn-on voltage is Vf = 0.70 V .
Round this value for R2 to the nearest value in the Equipment list (above).
Copy this circuit into your lab notebook and write the values for the base voltage (VB), collector
voltage (VC) and emitter voltage (VE) next to their respective terminals on the schematic.
2.
Calculate the theoretical input and output resistances (Rin and Rout).
3.
Simulate the circuit in PSpice. Use part “Q2N2222” in the “EVAL” library for the transistor.
Apply a 500-mV AC source (“SOURCE” library  “VAC” part) to Vin
and a 1 M resistor from Vout to ground. (Remember to type “1Meg” instead of “1M”.)
Perform an AC sweep and plot |Vout|/|Vin| for frequency = 10 Hz to 100 kHz (logarithmic).
Export this trace to a CSV file (“File” menu  “Export”  “Comma Separated File”)
so that you may overlay your simulated data and your measured data on the same axes (later).
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Print out this plot, print out your schematic, and paste/staple both into your lab notebook.
Note the gain Av of the amplifier at f = 30 kHz . (Calculate it and write it on your plot.)
4.
Add a 10-F capacitor from the emitter to ground and repeat the PSpice analysis.
Export this trace to a different CSV file.
Print out a new plot of |Vout|/|Vin| and paste/staple it into your lab notebook.
Note the new gain Av of the amplifier at f = 30 kHz . (Calculate it and write it on your plot.)
5.
Construct the circuit of Figure 1 (without the emitter bypass capacitor).
Leave the connections for Vin , Vout , 12 V, and ground open.
PROCEDURE:
1.
Ensure that the resistors, capacitors, and transistor are the proper values.
2.
Complete the circuit of Figure 1.
Measure and record the DC voltage at each terminal of the transistor (VB, VC, VE).
3.
Connect the output of the function generator to Vin .
Set the function generator to output a sine wave at a frequency of 100 kHz (in high-Z mode).
Connect Channel 1 of the oscilloscope to Vin and Channel 2 to Vout .
4.
Set the amplitude of Vin to 500 mVpp as measured on the oscilloscope.
Record the peak-to-peak amplitude of the output waveform, Vout . Calculate Av = Vout/Vin .
Note if the waveform Vout is inverted with respect to the waveform Vin .
At the following additional frequencies, record Vin , Vout , and calculate Av = Vout/Vin :
50 kHz, 20 kHz, 10 kHz, 5 kHz, 2 kHz, 1 kHz, 500 Hz, 200 Hz, 100 Hz, 50 Hz, 20 Hz, 10 Hz
5.
Connect the decade resistance box to the output and set the frequency of Vin back to 30 kHz.
Adjust the resistance until Vout reads as one-half the open-circuit value of Vout (measured in step 4).
Record this value as the output resistance of the amplifier, Rout .
6.
Disconnect the function generator from Vin .
Remove the decade box from the output and reconnect it between the function generator
and the circuit input (i.e. reconnect it in series with the function generator).
7.
Adjust the resistive decade box so that the new measured Vin (after the decade box) is one-half
the open circuit value of Vin (from step 4). Record this resistance as the input resistance Rin .
8.
Remove the decade resistance box and reconnect the function generator directly to Vin .
9.
Slowly increase the amplitude of the generator while observing the output waveform.
Record the values of Vin and Vout at which the waveform begins to clip.
10. Insert the 10-F capacitor (emitter bypass capacitor), from the emitter to ground.
Ensure that the arrow (or “–” symbol) on the electrolytic capacitor is pointed towards ground.
11. Repeat step 4 with Vin set to its minimum value (100 mVpp).
12. Complete the Data Analysis section before leaving the lab.
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DATA ANALYSIS:
1.
Compute percent error for VB, VC, VE (vs. pre-lab calculations).
2.
Using Excel or Matlab, plot Av vs. frequency (logarithmic) for the circuit without the emitter
bypass capacitor. Overlay your simulated and measured data on the same plot.
3.
Using Excel or Matlab, plot Av vs. frequency (logarithmic) for the circuit with the emitter bypass
capacitor inserted. Overlay your simulated and measured data on the same plot.
QUESTIONS:
1.
Do your simulated and measured Av-vs.-frequency plots match? Why or why not?
2.
Do your values for Rin and Rout match theory/simulation? Why or why not?
3.
What causes Vout to clip for high values of Vin ?
4.
What is the effect of the bypass capacitor on the DC bias voltages?
5.
What is the effect of the bypass capacitor on the gain of the transistor circuit?
LAB REPORT:
Complete your lab report in the format requested by your instructor.
See Lab #1 for a complete list of requirements.
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