Experiment 8 Single
Stage Amplifiers with
Passive Loads (BJT)
D. Yee, W.T. Yeung, C. Hsiung,
S.M. Mehta, and R.T. Howe
UC Berkeley EE 105
Fall 2003
1.0 Objective
In this laboratory, we will measure the properties of bipolar single-stage amplifiers,
using the same configurations and measurement techniques as we used for the MOS
amplifiers in the previous lab. Again, two-port parameters properties such as voltage
gain, input impedance and output impedance will be determined. From these measurements you should understand the relative trade-offs between amplifiers. Keep a copy of
your write-up so you can compare data with Experiment 7.
To show your understanding of the lab, your write-up should contain:
•
•
•
•
A table showing the input resistance, output resistance, and gain of the amplifiers
A discussion on trade-offs issues among the three parameters Av, Rin, and Rout
A discussion explaining the advantages and disadvantages of the different amplifiers
A discussion of the differences between similar MOS and BJT stages.
2.0 Prelab
• H & S: Chapters 8.1 - 8.2, 8.7, 8.9
• Biasing will often present a problem when building amplifiers. Below is a npn bipolar transistor which will be configured as a common emitter amplifier. Usually, we
would like to have the DC voltage VOUT to be in the middle of the highest and lowest
allowable voltage. For the circuit in Fig. 1 below, determine the proper voltage for
VOUT (to 2 significant digits) and determine the proper biasing voltage VBIAS needed
to achieve this. Use SPICE rather than hand calculations to confirm this.
• Hand calculations are useful as a starting point in guessing what VBIAS should be.
Use these parameters: βF=100, VAF =100 V, IS = 1 X 1015 A, and VCE(SAT) = 0.2 V.
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Procedure
FIGURE 1.
Bipolar npn Transistor in the Common Emitter Configuration
V CC = 5V
R C = 5kΩ
v OUT
v in
V BIAS
3.0 Procedure
For all the experiments in this lab, use a 5 kHz sine wave with an amplitude of
200 mV as vin.
Most of the circuits here will contain special biasing circuits to set the collector currents
of the npn bipolar transistors. As discussed in Experiment 7, an internal “mirror” circuit
will force the current supply to the transistor to equal that through an external bias resistor. The circuit in Fig. 2 illustrates this point.
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Experiment 8 Single Stage Amplifiers with Passive Loads (BJT)
Procedure
FIGURE 2.
Common Emitter Amplifier with Base Resistor for Procedure 3.1
Lab Chip 3
V CC = 5V
V CC = 5V
PIN 28
Amplifier (to clarify
Fig. 3--the 2-port)
v in
R C = 10kΩ
vB
PIN 26
C = 10µF
PIN 28
RBIAS
IBIAS
v OUT
PIN 25
IC=IBIAS
BIAS
PIN 27
R S = 100kΩ
PIN 14
PIN 14
The user will provide RBIAS across pin 28 and pin 27. The current across the resistor,
IBIAS, becomes the collector current IC for the BJT. IBIAS can be found by use of a voltmeter across RBIAS., with IBIAS equal to the voltage across the resistor divided by its
resistance.
3.1 Common Emitter Amplifier
1. For the circuit in Fig. 2, adjust RBIAS until VOUT = 2.5V. (RBIAS should be about 16
kΩ) Measure the current through RBIAS. Is it the same as the collector current? Also
check VBE and VBC to verify that the transistor is in its forward-active region of oper-
ation. Why is VOUT = 2.5 V a good choice?
2. Using the oscilloscope, measure the small-signal voltage gains.
v out
A v1 = -------vb
v out
A v2 = -------v in
3.1.1
Input Resistance Measurement
1. Why is Av2 lower than Av1? What is the relationship if between Av2 and Av1? Note
that the open-loop voltage gain of the amplifier is Av1, not Av2. (Hint: Draw the
small-signal equivalent circuit.)
Experiment 8 Single Stage Amplifiers with Passive Loads (BJT)
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Procedure
2. Using that relationship, you should be able to find the input resistance. In this case,
the input resistance happens to be rπ.What is the value of rπ?
3.1.2
Output Resistance Measurement
1. Measure the amplitude at vout. Connect the capacitor and the variable resistor to the
output of the circuit as depicted in Fig. 4. and as discussed in Exp. 7. Adjust the
resistance until the amplitude at vout is reduced by one-half.
FIGURE 3.
2-Port Representation of Amplifier to find Rout
C = 10µF
v in
AMPLIFIER
v out
R VAR
3.2 Common Collector Amplifier (Emitter Follower)
1. Figure 4 shows a common collector circuit with a current source bias. IBIAS is the
same current as through RBIAS. Let RBIAS be 100kΩ to determine the bias current.
What is the voltage gain?
v out
A v3 = -------vb
v out
A v4 = -------v in
2. Find the input and output resistance using the methods from earlier sections. The
current source is actually an npn transistor, so its small-signal resistance is just
roc = ro = VA / IC. Note that the voltage gain of the common collector configuration
cannot exceed unity. Do your results agree? In what situation would one use a Common Collector?
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Experiment 8 Single Stage Amplifiers with Passive Loads (BJT)
Optional Experiments
FIGURE 4.
Common Collector Amplifier with Internal Bias Circuit.
V CC = 5V
Lab Chip 4
100 kΩ
BIAS
PIN 25
PIN 23
100 kΩ v B
PIN 24
v OUT
1 ΜΩ
v in
PIN 28
IBIAS
1 MΩ
10 µF
V CC = 5V
PIN 28
IBIAS, roc
PIN 14
PIN 14
4.0 Optional Experiments
4.1 Common Emitter with Emitter Degeneration
1. Connect the circuit shown in Fig. 5. Why is there no biasing problem with this cir-
cuit? Repeat the procedures for the Common Emitter Amplifier. How do the values
of voltage gain, input impedance, and output impedance compare to the corresponding values for the common emitter configuration?
Experiment 8 Single Stage Amplifiers with Passive Loads (BJT)
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Optional Experiments
FIGURE 5.
Common Emitter with Emitter Degeneration
V CC = 5V
Lab Chip 2
R C = 5kΩ
COLLECTOR
PIN 20
v OUT
R S = 100kΩ
BASE v B
PIN 19
v in
EMITTER
PIN 18
R E = 500Ω
1.6V
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Experiment 8 Single Stage Amplifiers with Passive Loads (BJT)