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UNIVERSITY OF TEHRAN
Electrical and Computer Engineering Department
Electronic Circuits lab I
Experiment 6
BJT Signal Amplifiers II
Common Collector and Common Base Stage
1. Objectives
In this experiment, you will test some general characteristics of the common
base amplifier, including low input resistance, high voltage gain and acceptable
performance at high frequencies. Common collector amplifier, as a general buffer
stage with high input, low output resistance and unity gain is also tested. The
frequency response for both amplifiers is also included.
2. Necessary Equipments
You will need two NPN BJT (2N3904), two 100F capacitors and 100, 1k,
56k, 10k resistors. Other necessary equipments include two DC power
supplies, an oscilloscope, a function generator, a breadboard, an ohmmeter and
enough wires.
3. Pre-lab
A) 3.1 The small signal  model for a common base amplifier is shown below.
Find appropriate expression for input /output resistances and (VO/VS)
voltage gain.
r

v
Vo
gm  v
Rc

RS
Vs
RE
B) Using the expression you derived, calculate the said parameters for the
circuit below. (Output resistance of the current source is very high.) Prior
to that calculate Ra so that the circuit will have a DC bias current of 6mA.
(=100)
C) Find the maximum possible output voltage swing for this circuit. (Hint:
find maximum and minimum values for VO and compare it to DC bias
voltage for VO. Remember to take Q2’s saturation into account, as the
current source must not be saturated.
D) What are the capacitors in the circuit for?
E) Explain how the gain factor will change if each of R1 and R2 are changed
respectively.
3.2
A) The figure below shows the AC small signal  model for a common collector
circuit. Find appropriate expression for output / input resistances as well as
gain factor.
RS
Vs
r

v
gm  v
Rc

Vo
RE
B) Using the expression you just derived calculate the said parameters for the
circuit below. Prior to that find Ra so that the circuit will be biased for:
ICQ = 6mA (=80)
C) Find the maximum swing for VO and the thresholds at which the output
peaks are flattened or distorted. Determine V1 and V2 values when distortion
occurs.
Vo(ac)
V1
t
V2
D) Do R1 and R2 have any effect on the input resistance? If any explain it.
E) How does the load resistance RL affect the gain factor?
F) How does RL affect the distortion thresholds? (Hint: discard the current
source and consider the AC load line.)
4. Procedure
4.1 Common Base Amplifier
4.1.1 Construct Circuit 1. (Similar to previous experiment find Ra so that the
circuit is biased at 6mA.)
Circuit 1
4.1.2 Exert a sinusoid input voltage. (10kHz frequency and 1V peak-to-peak are
recommended.)
4.1.3 Measure the gain of the amplifiers for small signals. (VZ / VS, VZ / VE)
Now increase the input voltage. Pay attention to the distortion in the output
signal. Write the values of input and output voltages when the peaks of the
output signal are distorted. (Hint: you may decrease R1 to gain larger input.)
4.1.4 Measure the input resistance seen from emitter.
4.1.5 Measure the base-emitter AC voltage when the output signal is going to be
distorted.
4.1.6 Measure the frequency response of this circuit similar to the previous one
and calculate fL.
fIN
VO
AV
20
50
100
500
1k
5k
10k
50k
100k
Table 1
4.2 Common Collector Amplifier with Larger Load
Circuit 2
4.2.1 Construct Circuit 2.
4.2.2 Apply sinusoid input voltage. (10kHz frequency and 1V peak-to-peak are
recommended.)
4.2.3 Measure the gain of the amplifier for small signals. (VY / VS, VY / VX)
Now increase the input voltage. Note the distortion in the output signal. Write
the values of input and output voltages when the output peaks are distorted.
Explain why distortion happens.
4.2.4 Measure the input resistance seen from the point X.
4.2.5 Measure the output resistance when:
I.
X is input
II.
S is input
4.3 Common Collector Amplifier with Small Load
4.3.1 Substitute 1kload resistance with a 56 resistance and measure the gain
factor as well as input and output resistances.
4.3.2 Compare these two experiments (especially regarding the output signal
distortion) and explain the effect of the load resistance.
4.4 Comparison
According to your observations compare these three amplifiers in the following
aspects:
1. Voltage gain
2. Input resistance
3. Output resistance
4. Frequency response
What are the applications of these three amplifiers?
You may use PSPICE for frequency response analysis.