Experiment 5: Single Stage BJT Amplifiers: Common
Collector and Common Base
1
Objective
In lab 4, we explored the properties of a common emitter amplifier. However, even though the amplifier has
an extremely high gain, its high output impedance prevents it from properly driving the speaker. In this lab,
we will investigate the properties of two other single-stage amplifier configurations: the common collector
(CC) and the common base (CB). You will be applying the techniques learned from lab 4 to evaluate the
input impedance, output impedance, and gain for both of these amplifier configurations. If you need a review
of these techniques, please refer to lab 4 now. By the end of this lab, you should be able to model any single
stage amplifier using its two-port model as well as identify the strengths and weaknesses of each single-stage
amplifier configuration.
2
Materials
The items listed in table 1 will be needed. Note: Be sure to answer the questions on the report as you proceed
through this lab. The report questions are labeled according to the sections in the experiment. Also, note
that there is an accompanying post-lab appended to the end of the lab report. CAUTION: FOR THIS
EXPERIMENT, THE TRANSISTORS CAN BECOME EXTREMELY HOT!!!
Component
2N4401 NPN BJT
8 Ω speaker
100 Ω resistor
510 Ω resistor
1 kΩ resistor
2 kΩ resistor
10 kΩ resistor
51 kΩ resistor
10 µF capacitor
10 kΩ potentiometer
screw driver
Quantity
1
1
1
2
1
1
2
1
2
1
1
Table 1: Components used in this lab
3
Procedure
3.1
The Common Base Amplifier
For a common base amplifier using an NPN BJT, the input is applied to the emitter and the output is taken
from the collector.
1. Similar to the CE amplifier, the CB amplifier can also be used as a voltage amplifier. Set up the
configuration as shown in Figure 1. Let VCC = 12 V, RC = 1 kΩ, and VB = 640 mV.
2. Measuring Gain Using ICS: Perform a sweep of VIN from −0.1 V to 0.1 V and plot VOUT vs. VIN .
Find the voltage gain using the slope at VIN = 0 V (when the input has no DC offset). Note: This
technique was used in section 3.1.2 of lab 4. In contrast, section 3.2.2 of lab 4 uses the oscillscope for
measuring gain.
1
3
2
PROCEDURE
VCC
RC
+
vOUT
VB +
−
−
+
vIN
−
Figure 1: CB amplifier
3. Measuring Input Resistance: Find the input resistance by sweeping VIN from −0.1 V to 0.1 V and
plotting IIN . Hint: Section 3.2.1 from lab 4 demonstrates the same technique.
4. Measuring Output Resistance: Find the output resistance using the technique demonstrated in section
3.2.4-5 from lab 4.
5. Suppose the source resistance of VIN is 50 Ω. Will the CB amplifier perform well in amplifying the
signal from vin ? Why?
3.2
The Common Collector Amplifier
For a common collector amplifier using an NPN BJT, the input is applied to the base while the output is
taken from the emitter. Before we begin, there is a chance that your transistor will heat up during
this experiment; if that is the case, please substitute a 51 kΩ resistor for RS . (See Figure 2.)
1. Build the circuit shown in Figure 2, a common collector amplifier with no load attached. Let R1 = 1 kΩ,
R2 = 891 Ω, RS = 10 kΩ, RE = 100 Ω, C = 10 µF, and VCC = 12 V. Hint: To form the 891 Ω
resistance, note that 510 Ω//2 kΩ = 406 Ω, 510 Ω//10 kΩ = 485 Ω, and resistances in series sum up
to form one equivalent resistance. Before you build the circuit, consider this tip that may save
you a lot of time: Build the biasing network at one end of the breadboard and build the CC amplifier
at the other end. Then, use a wire to connect or disconnect the two parts. Such a setup will help
facilitate your measurements, especially for the one on input resistance.
2. A common collector amplifier is typically biased so that it yields the greatest output voltage swing. For
the design in Figure 2, measure the DC bias input voltage, VIN , and the corresponding VOUT value.
3. Now find the voltage gain, input impedance, output impedance, and output voltage swing. Note: For
these measurements, you must disconnect the biasing network from the amplifier and use the techniques
demonstrated in lab 4. With the exception of the output resistance, all other measurements must be
conducted using ICS to minimize error and to save time. Also, remove RS only when finding the input
impedance; keep it otherwise. (See Figure 2.)
4. Another name for the CC amplifier is the emitter follower. Based on the gain that you have found,
why do you think it is given this name?
3
3
PROCEDURE
VCC
R1
C
RS
+
ROUT
vin
−
VIN
R2
+
RIN
vOUT
RE
−
Figure 2: Common collector amplifier with biasing network
3.3
The World’s Second Worst Speaker Amplifier
This part will demonstrate the capabilities of your CC amplifier. Note: Due to impedance mismatch,
the voltage delivered by the function generator does not necessarily match the displayed value.
When in doubt, check with an oscilloscope.
1. Apply a 1 kHz, 2 Vpp sine wave directly to the two terminals of the speaker using the function generator.
Measure the voltage drop across the speaker using the oscilloscope and qualitatively observe the volume
of the 1 kHz tone.
2. Reconnect the circuit as shown in Figure 2.
3. Apply a 1 kHz, 2 Vpp sine wave at the input. Attach the speaker (which is modeled as RL in Figure 3)
as well as the 10 µF capacitor, C. Measure the output waveform, and observe the volume of the tone.
Is it louder, softer, or about the same as when the signal was directly applied to the speaker? Explain
your observations. Hint: Consider the gain and I/O impedances.
VCC
R1
C
RS
+
vin
C
R2
+
−
RE
RL
vout
−
Figure 3: CC amplifier with biasing network and load speaker attached