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EXPERIMENT 3-3 AMPLIFIER WITH VOLTAGEDIVIDER BIAS
In this experiment you will become familiar with a transistor amplifier using
voltage-divider bias. You'll see what factors determine its gain and distortion.
EQUIPMENT
 VOM
 oscilloscope
 12-VDC power supply
 audio signal generator
 2n2222 a NPN transistor
 Electrolytic capacitors-15 WVDC or greater
o (2) 4.7 µF ; (1) 47 µF
 Resistors 1/2 W ±-. 5%
o 100 KΩ; 15 KΩ; (2) 5 KΩ; (3) 1 KΩ; (1) 10 Ω
E3-3 Circuit for experiment 3-3.
PROCEDURE
1.
Build the circuit of Fig. E3-3. Measure the d-c base-to-ground voltage VB, the
collector-to-ground voltage VC, and the drop across collector resistor RC. Using the
voltage across Rc, determine the value of collector current IC.. Your circuit should
look like the image on the next page – this will aid with the rest of the Experiment. Your
values should be near to the listed ones, if not there is a problem with your circuit.
Record your values.
VB = 1.45VDC, ___________
Vc = 7.9VDC, ___________
VRc = 4.1VDC, ______________
IC = 0.82 mA, _________________
68 Electronic Troubleshooting Modified for CSN’s ET 198B-005 Fall 2010
E3-3 Circuit for experiment 3-3 on the breadboard.
2. Next, with an oscilloscope connected at the outputs, adjust the signal generator until
the a-c output amplitude across RL, reads 2 Vp-p.
Consider the measured voltages the normal values when the amplifier is
operating as it should. Make changes to various circuit components as
indicated in Table E3-3, and record the changed readings. Follow the new
readings with ‘ I’ if the reading increases noticeably (more than 10%), ‘D’ if
the reading decreases, and S if the reading remains essentially the same as
the normal value. You can determine whether Ic changes by observing the
voltage across Rc or RE. If the voltage increases while the resistance
remains the same, the current must have increased. If you think that the
transistor is cut off or saturated, place a check in the appropriate blank in
the table. Use line 1 for normal values and line 2 of the table as an
example.
To decrease the value of a resistor by 50%, simply bridge a resistor of
equal value in parallel with it. To determine the effect of an open component,
temporarily remove one end of it from the circuit. Be sure to return the circuit
to its original setup before proceeding with the next check.
Bipolar Transistors 69
Table E3-3
Change
1.
Reference – Normal Values
2.
open R 1
3.
d ecr ea se R I 5 0%
4.
open R 2
5.
d ecr ea se R 2 5 0%
6.
op en R c
7.
d ec r ea se R c 5 0 %
8.
open R E
9.
d ecr ea se R E 5 0%
10.
open R L
11.
decrease RL 50%
12.
open C1
13.
open C2
Vc
Ic
vo
Satura
ted?
D
I
D
D
open C3
QUIZ
Refer to Fig. E3-3. For each symptom, choose one of the following faults
as the most probable cause, and write it in the space provided.
Faults
a.
b.
c.
d.
e.
f.
g.
h.
i.
j.
k.
l.
R 1 open
R 1 decreased
R2 open
R2 decreased
Rc open
Rc decreased
RE open
RE decreased
C2 open
C2 shorted
open transistor
shorted transistor
Cut
off?
2Vpp
short C2
14.
15.
VB
X
Symptoms
1. The a-c output voltage is lower than normal, but not zero. When you
measure the voltage from collector to ground, it reads high, but the
voltage across RE is normal. VB is also normal.
Probable fault
2. The output voltage is lower than normal. Vc reads high, but less than
Vcc, and VB reads lower than normal, but not zero.
Probable fault
3. The a-c output voltage is zero and VC = VCC. VB measures normal. A
resistance check from emitter to ground reads extremely high.
Probable fault
4. All d-c readings are normal, but the AC signal gain is lower than
normal.
Probable fault
5. No a-c output voltage. Vc is equal to Vcc, and VB reads zero.
Probable fault
6. Vc measures low. The voltage at the emitter is approximately equal to
Vc, and VB is higher than normal. R1 measures normal.
Probable fault
7. Vc is lower than normal. Output looks clipped on the negative half.cycles
when the transistor is driven with a fairly large signal VB reads higher than
normal.
Probable fault
8.
Vc = Vcc, and the voltage from emitter to ground is zero. VB measures
normal.
Probable fault
9. Collector-to-emitter voltage reads zero.
Probable fault
10. Voltage at collector is equal to Vcc. Voltage VB is lower than normal,
and voltage between base and emitter is zero.
Probable fault