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Project 10
Common Collector Amplifier
Objective: This project will show the biasing, gain, frequency response, and impedance properties of a
common collector amplifier.
Components: 2N2222 BJT
Introduction:
The common collector amplifier as shown in Figure 10-1 is one of the most useful small-signal amplifier
configurations. The same biasing scheme and frequency response approximation technique as used for the
common emitter amplifier can also be used for the common collector amplifier. The only change that needs
to be made in biasing is that the voltage across the emitter resistor RE is usually larger for the common
collector to allow a greater output voltage swing. The collector resistor is also usually omitted in the
common collector configuration. The main characteristics of the common collector amplifier are high input
impedance, low output impedance, less than unity voltage gain, and high current gain. This amplifier is most
often used as a buffer or isolation amplifier to connect a high impedance source to a low impedance load
without loss of signal. The load seen by the amplifier's signal source is the input impedance of the amplifier.
With a high input impedance, the CC amplifier loads the source very lightly. Therefore the signal source is
largely isolated
+ 1. This high current gain allows the CC amplifier to increase the power of the signal. These power and
current gains make the CC amplifier a practical choice as an output stage amplifier driving several devices
and/or low impedance loads.
Design:
Design a common collector amplifier with the following specifications:
1. use a 2N2222 BJT and a 12 volt DC supply
2. midband gain VO/VS
3. low cutoff frequency FL between 100 Hz and 200 Hz
5. VO
6. load resistor RL
7. source resistance RS
- p)
Figure 10 - 1: Common Collector Amplifier
Lab Procedure:
1. Construct the CC amplifier of Figure 10-1. Remember RS is installed in addition to the internal 50
resistance of the function generator. Verify the amplifier operation by measuring the Q-point and midband
voltage gain. Monitor the output on the oscilloscope to make sure the waveform is not clipped. Note any
distortion in the output signal.
2. Adjust the input signal level to get a 3.0 volt peak symmetric output voltage swing.
3. Determine the midband current gain IL/IS [measure IS by looking at the current through RS] What is the
overall power gain?
4. Observe the loading affect by replacing RL first by
the output signal and comment on the loading affect.
5. Use computer control to record and plot the frequency response. Find the corner frequencies and
bandwidth to verify that the specifications have been met.
6. Measure the input impedance seen by the source [look at the current through RS and the node voltage on
the transistor side of RS] and the output impedance seen by the load resistor [look at the open circuit voltage
and the current through and voltage
across RL]. Verify that the input impedance specification has been met.
Questions:
1. How can you achieve maximum power transfer from the input signal source to the amplifier circuit? Is
the load resistance a factor in the answer?
2. What value of load resistance results in maximum voltage gain? What load resistance results in maximum
power transfer to the load?
+1. Comment on any differences.
4. Compare the measurements in Lab Procedures 1-6 to the theoretical predictions such as those obtained
using PSPICE®. Note that you must adjust the circuit file to determine the output impedance.
5. What other method could be used to measure RO?