Prelab 5: Single Stage BJT Amplifiers: Common Base
and Common Collector (a.k.a. Emitter Follower)
Name:
Lab Section:
VCC
RC
iout
VB +
−
iin
Figure 1: Common base amplifier with a current input
1. Draw the small signal model for the common base amplifier shown in Figure 1. Note: The source iin
is a small signal AC current source, not a DC current source; thus, you must include iin in the small
signal model.
2. State (do not derive) the general formulas for input impedance (RIN ), output impedance (ROUT ),
and current gain (Ai = iout /iin ) of a common base amplifier.
1
2
3. For the amplifier shown in Figure 1, calculate the numerical values of RIN , ROUT , and Ai using the
following parameters: IS = 26.03 fA, RC = 1 kΩ, and VBE = 640 mV. Ignore the Early effect and
make approximations when appropriate.
4. How do RIN and ROUT of the common base amplifier compare to those of the common emitter amplifier?
VCC
R1
C
RS
+
ROUT
vin
−
VIN
R2
+
RIN
RE
vOUT
−
Figure 2: Common collector amplifier with biasing network. Note that the common collector is also known
as the emitter follower.
5. What ratio of R1 to R2 is required if the input bias, VIN , is 5.65 V and VCC is set to 12 V? Note:
R1 and R2 approximately forms a “voltage divider” that provides the input DC bias point (just in case
you did not notice). Also, regard the current flowing through RS as negligible for this calculation only.
In addition, recall that a capacitor is a DC open circuit.
3
6. Draw the small signal model for the common collector amplifier shown in Figure 2.
7. State (do not derive) the general formulas for the input impedance (RIN ), output impedance
(ROUT ), and voltage gain (Av ) of a common collector amplifier. Note that the most general formulas
should suffice because the biasing network can be modeled using the Thévenin equivalent circuit
(i.e. a voltage source with one resistor in series).
8. Now use these formulas to calculate the numerical values of RIN , ROUT , and Av for the amplifier
shown in Figure 2. Please also use the following parameters in your calculations: IS = 26.03 fA,
RS = 10 kΩ, RE = 100 Ω, β = 270, and R1 = 1 kΩ. Assuming the ratio of R1 to R2 is correct, the
DC input bias (VIN ) should be 5.65 V. Ignore the Early effect and make approximations when
appropriate. Also, if necessary, apply concepts from the Thévenin equivalent circuit to simplify
the biasing network.
9. In general, how do RIN , ROUT , and Av of the emitter follower compare to those of the common emitter
amplifier?
4
c
University
of California, Berkeley 2008
Reproduced with Permission
Courtesy of the University of California, Berkeley and of Agilent Technologies, Inc.
This experiment has been submitted by the Contributor for posting on Agilents Educators Corner. Agilent has not tested it. All who offer or perform this experiment do so solely at their own risk. The Contributor
and Agilent are providing this experiment solely as an informational facility and without review.
NEITHER AGILENT NOR CONTRIBUTOR MAKES ANY WARRANTY OF ANY KIND WITH
REGARD TO THIS EXPERIMENT, AND NEITHER SHALL BE LIABLE FOR ANY DIRECT, INDIRECT, GENERAL, INCIDENTAL, SPECIAL OR CONSEQUENTIAL DAMAGES IN CONNECTION
WITH THE USE OF THIS EXPERIMENT.
Experiment 5: Single Stage BJT Amplifiers: Common
Base and Common Collector (a.k.a. Emitter Follower)
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
Quantity
1
1
1
2
1
1
2
1
2
1
Table 1: Components used in this lab
3
3.1
Procedure
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
3
2
PROCEDURE
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.
VCC
RC
+
vOUT
VB +
−
−
+
vIN
−
Figure 1: CB amplifier
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.
3. Measuring Input Resistance: For a DC input bias of 0 V, 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. Make sure to remove RS only when finding the input impedance; keep
3
3
PROCEDURE
VCC
R1
C
RS
+
ROUT
vin
−
VIN
R2
+
RIN
vOUT
RE
−
Figure 2: Common collector amplifier with biasing network
it otherwise. With the exception of the output resistance, all other measurements must be conducted
using ICS to minimize error and to save time. (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
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.
Observe 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. Observe 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
3
PROCEDURE
4
c
University
of California, Berkeley 2008
Reproduced with Permission
Courtesy of the University of California, Berkeley and of Agilent Technologies, Inc.
This experiment has been submitted by the Contributor for posting on Agilents Educators Corner. Agilent has not tested it. All who offer or perform this experiment do so solely at their own risk. The Contributor
and Agilent are providing this experiment solely as an informational facility and without review.
NEITHER AGILENT NOR CONTRIBUTOR MAKES ANY WARRANTY OF ANY KIND WITH
REGARD TO THIS EXPERIMENT, AND NEITHER SHALL BE LIABLE FOR ANY DIRECT, INDIRECT, GENERAL, INCIDENTAL, SPECIAL OR CONSEQUENTIAL DAMAGES IN CONNECTION
WITH THE USE OF THIS EXPERIMENT.
Report 5: Single Stage BJT Amplifiers: Common Base
and Common Collector (a.k.a. Emitter Follower)
Name:
Lab Section:
1
Lab Questions
3.1.2–4 CB amplifier parameters:
Av =
Rin =
Rout =
3.1.5 Suppose the source resistance of VIN is 50 Ω. Will the CB amplifier perform well in amplifying the
signal from vin ? Why?
3.2.2 DC voltage values when biased at maximum output voltage swing:
VIN =
VOUT =
3.2.3 CC amplifier parameters:
Av =
1
2
POST-LAB QUESTIONS
2
Rin =
Rout =
Output Voltage Swing =
3.2.4 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?
3.3.3 Is the tone 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.
2
Post-lab Questions
2.1
Common Base Amplifier as a Current Buffer
1. The CB amplifier is rarely used as a voltage or transconductance amplifier while the CE amplifier is
typically used instead. Based on the values of the CB and CE amplifier properties (that you found in
lab), explain why this is the case.
2
POST-LAB QUESTIONS
3
2. The CB amplifier is commonly used as a current buffer. Approximate the current gain of a CB amplifier. (Hint: What is the relationship between the emitter and collector currents of a CB amplifier? )
2.2
Common Collector Amplifier as a Voltage Buffer
1. Even though the common collector amplifier has roughly unity gain, why is it still a useful amplifier
configuration?
2. What are the advantages and/or disadvantages of using a larger RE in a common collector amplifier?
c
University
of California, Berkeley 2008
Reproduced with Permission
Courtesy of the University of California, Berkeley and of Agilent Technologies, Inc.
This experiment has been submitted by the Contributor for posting on Agilents Educators Corner. Agilent has not tested it. All who offer or perform this experiment do so solely at their own risk. The Contributor
and Agilent are providing this experiment solely as an informational facility and without review.
NEITHER AGILENT NOR CONTRIBUTOR MAKES ANY WARRANTY OF ANY KIND WITH
REGARD TO THIS EXPERIMENT, AND NEITHER SHALL BE LIABLE FOR ANY DIRECT, INDIRECT, GENERAL, INCIDENTAL, SPECIAL OR CONSEQUENTIAL DAMAGES IN CONNECTION
WITH THE USE OF THIS EXPERIMENT.