EXPERIMENT 5 - CASCADED AMPLIFIER
In this experiment, you will study a two-stage capacitively coupled amplifier.
You will measure its gain with and without feedback.
EQUIPMENT
 12-VDC power supply, signal generator, oscilloscope, DMM
 (2) NPN silicon transistors (2N2222)
 (2) 75-KΩ, 1/2-W ± 5% resistors, (2) 12-KΩ ± 5% resistors, (1) 10-KΩ ± 5%
resistor, (2) 4.7-KΩ, ± 5% resistors, (3) 1-KΩ ± 5% resistors, (1) 10-Ω, ± 5%
resistor,
 (1) 25-KΩ, pot
 (2) 20-µF, 25-WVDC
capacitors
 (2) 5-µF, 25-WVDC
capacitor
PROCEDURE
1. Build the circuit of Fig. E5-1A. Apply d-c power, but do not apply an a-c signal
Multistage Amplifiers 125
yet. Measure the d-c collector voltage of each transistor to make sure that both
stages are operating normally.
XDC = V
C1
= 7.40V, _________
YDC = V C2 = 7.40V, ___________
If both collector voltages measure between 6 and 8 V from ground, the
circuit is probably working normally, so go on to the next step.
Otherwise recheck your wiring.
For Reference also check the DC voltages at the Base and emitters of
both transistors.
VB1= 1.6V, ____ ; VB2= 1.6V, ____ ; VE1= 0.98V, _____ ; VE2= 0.98V, _____
2.
Now connect the signal generator to the input. Set the amplitude of Vs to 2 V pp at a frequency of about 1 KHz. This will make vin = 2 mV as a result of the
attenuator RA—RB.
Measure the a-c signal at each collector (test points X -
3.
vO l =
vO l and Y - vO2).
vO2 =
vO l should be much larger than vin, and vO2
should be much larger than vO l. The output signal vO2 should not be clipped
If your circuit is working correctly,
but may be distorted.
Determine the gain at the first stage (vin is the AC signal at the base of Q1).
Av1= vO1 /vin =
Determine the gain at the second stage.
A v2=
vO2/ vO l
Now find the overall gain.
Av(total) =
vO2/ vin =
4.
Now test the amplifier with feedback. Shut off power and remove the 20µF
capacitor CE1 from the emitter of Q1. Then connect the capacitor in series with the
feedback resistor (25-KΩ pot) from the collector of Q2 to the emitter of Q1. See
drawing below. Adjust the pot to its maximum resistance, and apply power.
5.
Adjust the input signal amplitude to 1.00 VP-P. Ensure that the pot is at
maximum resistance by checking that the AC signal at test point Y is at its
maximum value. Measure vin at the base of Q1, measure the output signal at
point measure Y, and calculate the gain.
vin =____ vO2 = _____
6.
Av(total) = vO2/ vin =
Amplifier with feedback output.
a. Adjust the feedback pot to its lowest value and measure AC output voltage
and calculate the gain.
vO2 =
Av(total) =
vO2/ vin =
NOTE: By adjusting RF higher and lower, the output signal amplitude
increases and decreases. RF makes an excellent gain control.
b. Adjust the feedback pot for the maximum non-distorted AC output signal
vO2 =
7.
Av(total) =
vO2/ vin =
Remove the feedback pot and reinstall CE1. Set the input signal back to 2VPP.
8.
Record the amplitudes of the a-c waveforms at points X and Y, vb2, and vb1,
in row 1 of the Table below. Also, record the DC voltages on the collectors and
bases of the transistors (VC1, VB1, VC2, VB2). The DC voltages should be the same
as in step 1; if not trouble shoot and repair.
Condition
vb1
XP-P
Normal
YP-P
VC1
VB1
vb2
YDC VB2
R1 open
R4 open
CE1 shorted
Q2 open
9.
Disconnect R1 from the circuit, and repeat all measurements. Record your
measured values in row 2 of the above table.
10. Reconnect R1 back into the circuit, and remove R4 from the circuit. Repeat all
measurements, recording your values in row 3 of the table.
11. Replace R4 in the circuit, and connect a clip lead from the emitter of Q 1 to
ground. This simulates a shorted capacitor C E1. Repeat all measurements,
recording your values in row 4 of the table.
12. Remove the short from the circuit. Disconnect the collector of Q 2 from the
circuit. This simulates an open transistor. Repeat all measurements, recording the
values in row 5 of the table.
Lab Quiz 1
a. A change in resistance value in the first stage affects the d-c readings in
(a)the first stage only. (b)the second stage only. (c)both stages.
b. A change in resistance in the first stage affects the a-c readings in (a)the
first stage only. (b)the second stage only. (c)both stages.
c. A change in resistance in the second stage affects the d-c readings in
(a)the first stage only. (b)the second stage only. (c)both stages.
d. Shorting CE1 causes Q1 to (a)turn off, (b)turn on hard—as can be seen by
the fact that VC1 went - (c)up, (d)down - due to (e)more, (f)less current flow
through RC1.
e. Opening R4 causes Q2 to (a)turn off. (b)turn on hard.
f. Opening R1 causes Q1 to (a)turn off. (b)turn on hard.
g. Refer to the flowchart of Fig. 5-3 and consider the readings in row 5 of
Table E5-2, the open transistor Q2 would have been found by taking the
(a)YEs, (b)No route from the box labeled "WAVEFORM X OK?"
h. Referring to question g, when reaching the block labeled "VC2 OK?" the(a)YEs, (b)No—route would be followed.
i. Referring to question h, a good indication of an open transistor is when
(a)Vc is normal. (b)Vc is much lower than normal. (c)Vc is equal to Vcc.
j. If Q2 were shorted, VC2 would have been (a)normal. (b)much lower than
normal. (c)equal to Vcc.
13. Reconnect the collector of Q2. Measure the Frequency response of the
amplifier circuit by measuring the AC output voltage at the frequencies listed in
the table. First adjust the input until the AC output is 1.00 VPP at 1 kHz and
record vin (as across RB, but measured across RA and RB ). Ensure that at each
frequency that vin remains the same at each frequency before measuring the AC
output voltage. NOTE: at both low and high frequencies if the magnitude of the
output is lower than1/10 of what it was at 1kHz then stop going higher or lower on
that row of data.
vin =
Run
100
200
400
1K
2K
4K 10K
20K
40K
100K 200K 400K 1M
1
2
3
14. Replace the CC with a 0.1 µF cap and repeat the measurements in the second
row. First adjust the input until the AC output is 1.00 VPP and record vin (as
measured across RB). Ensure that at each frequency that vin remains the same at
each frequency before measuring the AC output voltage.
vin =
15. Reinstall the 5 µF cap as CC and connect a 0.1 µF cap from test point X to
ground. Repeat the measurements in the third row. First adjust the input until
the AC output is 1.00 VPP and record vin (as measured across RB). Ensure that at
each frequency that vin remains the same at each frequency before measuring the
AC output voltage.
vin =
16. Plot the frequency response data on Semi-log paper and identify the
frequencies on the horizontal axis, gain in dB on the right side and as a ratio on
the left side of the graph (See last page of the Lab). Also identify the upper and
lower 3dB points (voltage gain is at 70.7% of midrange) on the plot.
Lab Quiz 2
a. What is the bandwidth of the circuit using the measurements of row 1?
b. What is the effect on the low frequency response when the coupling
capacitor is made smaller?
c. What is the effect on the low frequency response when the coupling
capacitor is made smaller?
d. What is the effect on the low frequency response when a small
capacitance is shunted from the collector of Q1 to ground.
e. What is the effect on the high frequency response when a small
capacitance is shunted from the collector of Q1 to ground.