1. NATIONAL COLLEGE OF SCIENCE AND TECHNOLOGY
Amafel Bldg. Aguinaldo Highway Dasmariñas City, Cavite
EXPERIMENT 2
Class B Push-Pull Power Amplifier
Pula, Rolando A. October 11, 2011
Signal Spectra and Signal Processing/ BSECE 41A1 Score:
Engr. Grace Ramones
Instructor

2. Objectives:
1. Determine the dc load line and locate the operating point (Q-point) on the dc load line for a
class B push-pull amplifier.
2. Determine the ac load line for a class B push-pull amplifier.
3. Observe crossover distortion of the output waveshape and learn how to estimate it in a class b
push-pull amplifier.
4. Determine the maximum ac peak-to-peak output voltage swing before peak clipping occurs and
compare the measured value with the expected value for a class B push-pull amplifier.
5. Compare the maximum undistorted ac peak-to-peak output voltage swing for a class B amplifier
with the maximum for a class A amplifier.
6. Measure the large-signal voltage gain of a class B push-pull amplifier.
7. Measure the maximum undistorted output power for a class B push-pull amplifier.
8. Determine the amplifier efficiency of a class B push-pull amplifier.

3. Sample Computation
Step 7
Step 9
Step 10
Step 11

4. Materials
One digital multimeter
One function generator
One dual-trace oscilloscope
One dc power supply
One 2N3904 npn bipolar junction transistor
One 2N3906 pnp bipolar junction transistor
Two 1N4001 diodes
Capacitors: two 10 µF, one 100 µF
Resistors: one 100 Ω, two 5 kΩ
Theory
A power amplifier is a large-signal amplifier in the final stage of a communications transmitter that
provides power to the to the antenna or in the final stage of a receiver that drives the speaker. When an
amplifier is biased at cutoff so that it operates in a linear region of the collector characteristic curves for
one-half cycle of the input sine wave (180o), it is classified as a class B amplifier. In order to produce a
complete reproduction of the input waveshape, a matched complementary pair of transistors in a push-
pull configuration, as shown in Figure 15-1,is necessary. In a class B push-pull amplifier, each transistor
conducts during opposite halves of the input cycle. When the input is zero, both transistor conducts
during opposite halves of the input cycle. When the input is zero, both transistors are at cutoff (IC = 0).
This makes a class B amplifier much more efficient than a class Q amplifier, in which the transistor
conducts for the entire input cycle (360o) . the man disadvantage of class B amplifier is that it is not as
linear as class A amplifier. Class b amplifiers are used in high-power applications where a linear amplifier
is required, such as high-power audio amplifiers or linear power amplifiers in high-power transmitters
with low-level Am or SSB modulation.
Figure 15-1 Class B Push-Pull Amplifier with Crossover Distortion
XSC1
Ext T rig
+
R1 V1
_
XFG1 5kΩ A B
20 V + _ + _
Q1
C1
10µF
2N3904
C3
Q2
C2 100µF
R3
100Ω
10µF R2
5kΩ 2N3906
In a class B push-pull emitter follower configuration in Figure 15-1, both transistors are biased at cutoff.
When a transistor is biased at cutoff, the input signal must exceed the base-emitter junction potential

5. (VBE) before it can conduct. Therefore, in the push-pull configuration in figure 15-1 there is a time
interval during the input transition from positive to negative or negative to positive when the transistors
are not conducting, resulting in what is known as crossover distortion. The dc biasing network in figure
15-2 will eliminate crossover distortion but biasing the transistors slightly above cutoff. Also, when the
characteristics of the diodes (D1 and D2) are matched to the transistor characteristics, a stable dc bias is
maintained over a wide temperature range.
Figure 15-2 Class B Push-Pull Amplifier – DC Analysis
XMM1
+ U3
0.000 A DC 1e-009Ohm
-
R1
5kΩ
Q1
V1
D1 2N3904 20 V
1N4001GP
D2
1N4001GP Q2
R2 2N3906
5kΩ
The dc load line for each transistor in figure 15-2 is a vertical line crossing the horizontal axis at VCE =
VCC/2. The load line is vertical because there is no dc resistance n a collector or emitter circuit (slope of
the dc load line is the inverse of the dc collector and emitter resistance). The Q-point on the dc load line
for each transistor is close to cutoff (Ic = 0). The dc collector-emitter voltages for the two transistors in
Figure 15-2 can be determined from the value of VE using the equations
VCE1 = VCC – VE
and
VCE2 = VE – 0 = VE
The complete class B push-pull amplifier is shown in Figure 15-2. Capacitors C1, C2, and C3 are coupling
capacitors to prevent the transistor dc bias voltages being affected by the input circuit or the load
circuit. The ac load line for each transistor should have a slope of 1/RL (the ac equivalent resistance in
the emitter circuit is RL), cross the horizontal axis at VCC/2, and cross the vertical axis at IC (sat) = VCC/2RL.
The Q-point on the ac load line should be close to cutoff (Ic = 0) for each transistor. When one of the
transistors is conducting, its operating point (Q-point) moves up the ac load line. The voltage swing of
the conducting transistor can go all the way from cutoff to saturation. On the alternate half cycle the
other transistor can go all the way from cutoff to saturation. Therefore, the maximum peak-to-peak
output voltage is equal to 2(VCC/2) = vCC. The amplifier voltage gain is measured by dividing the ac peak-
to-peak output voltage (Vo) by the ac peak-to-peak input voltage (Vin). Because the push-pull amplifier in
figure 15-3 is an emitter follower configuration, the voltage gain should be close to unity (1). This is not a

6. problem for large-signal amplifiers because they are used primarily for power amplification rather than
voltage amplification.
Figure 15-3 Class B Push-Pull Amplifier
XFG1
XSC1
V1
20 V Ext T rig
+
R1 _
5kΩ A B
+ _ + _
Q1
C2
10µF D1 2N3904
1N4001GP
C3
D2
1N4001GP Q2 100µF
C1
R3
10µF 100Ω
R2 2N3906
5kΩ
The amplifier output power (PO) is calculated as follows:
The percent efficiency (ŋ) of a large-signal amplifier is equal to the maximum output power (PO) divided
by the power supplied by the source (PS) times 100%. Therefore,
where Ps = (VCC)(ICC). The current at the source (IS) is determined from
where = VCC/2RL and IC(AVG) is the average value of the half-wave collector current.
Note: IRB1 is normally much less than IC(AVG) and can be neglected.
Procedure:
Step 1 Open circuit file FIG 15-1. Bring down the function generator enlargement. Make sure
that the following settings are selected: Time base (Scale = 200 us/Div, Xpos = 0, Y/T), Ch
A (Scale =2 V/Div, Ypos = 0, AC), Ch B (Scale = 2 V/Div, Ypos = 0, AC), Trigger (Pos edge,
Level = 0, Auto). Run the simulation to four full screen displays, then pause the
simulation. You are plotting the amplifier input (red) and the output (blue) on the
oscilloscope. Notice the crossover distortion of the output waveshape (blue curve).
Draw the waveshape in the space provided and note the crossover distortion.

7. Step 2 Open circuit file FIG 15-2. Bring down the multimeter enlargement and make sure that V
and dc (----) are selected. Run the simulation and record the dc base1 voltage (VB1).
Move the multimeter positive lead to node VB2, then node VE, then node A and run the
simulation for each reading and record the dc voltages. Also record the dc collector
current (IC)
VB1 = 10.496 V VB2 = 9.504 V VE = 10.017 V
VA =10V IC = 0 A
Step 3 Based on the voltages recorded in Step 2, calculate the dc collector-emitter voltage
(VCE) for both transistors.
VCE1 = 9.983 V
VCE2 = 10.017 V
Step 4 Draw the dc load line on the graph and locate the operating point (Q-point) on the dc
load line based on the data in Step 2 and the calculations in Step 3.
100
75
50
25
0 5 10 15 20
Step 5 Open circuit file FIG 15-3. Bring down the function generator enlargement. Make sure
that the following settings are selected: Sine wave, freq = 1 kHz, Ampl = 4 V, Offset = 0
V. Bring down the oscilloscope enlargement. Make sure that the following settings are
selected Time base (Scale = 200 us/Div, Xpos = 0, Y/T), Ch A (Scale = 2 V/Div, Ypos = 0,
AC), Ch B (Scale = 2 V/Div, Ypos = 0, AC), trigger (Pos edge, Level = 0, Auto). Based on the
values of VCC and RL, draw the ac load line on the graph in step 4.
Questions: Where was the operating point (Q-point) on the dc load line? Where was the operating point
on the ac load line? Explain.

8. Both are located at the cutoff. This is because of the absence of collectors current.
What was the relationship between the dc load line and the ac load line? Explain.
The dc load line and the ac load line have same quiescent point that is located at the
cutoff.
Step 6 Run the simulation. Notice that there is hardly any crossover distortion of the output
waveshape. Keep increasing the input signal voltage until output peak distortion occurs.
Then reduce the input signal level slightly until there is no longer any distortion. Pause
the analysis and record the maximum undistorted ac peak-to-peak output voltage (VO)
and the ac peak-to-peak input voltage (Vin). Adjust the oscilloscope settings as needed.
VO =20.368 V
Vin = 10.2 V
Questions: What caused the crossover distortion in Step 1? What does the addition of diodes D1 and D2
accomplish?
There is a time interval during the input transition from positive to negative or negative to
positive when the transistors are not conducting, resulting in crossover distortion. When the
characteristics of the diodes (D1 and D2) are matched to the transistor characteristics, a stable dc
bias is maintained over a wide temperature range.
How did the maximum undistorted peak-to-peak output voltage for the class B amplifier, measured in
Step 6, compare with the maximum undistorted peak-to-peak output voltage for the class A amplifier,
measured in Experiment 14, Step 9?
The peak voltage of class B is much greater than class A.
Step 7 Based on the voltages measured in Step 6, calculate the voltage gain of the amplifier.
Questions: How did the measured amplifier voltage gain compare with the expected value for a class B
push-pull emitter circuit?
They are the same
Step 8 Based on the ac load line and Q-point located on the graph in Step 4, estimate what the
maximum ac peak-to-peak output voltage (Vo) should be before output clipping occurs.
Record your answer.
Vo = 10V
Question: How did the maximum undistorted peak-to-peak output voltage measured in step 6 compare
with the expected maximum estimated in Step 8?

9. There is only a difference of 0.184 V or 1.81%.
Step 9 Based on the maximum undistorted ac peak-to-peak output voltage measured in Step 6,
calculate the maximum undistorted output power (PO) to the load (RL).
Step 10 Based on the supply voltage (VCC) and the average collector current (IC(AVG)), calculate the
power supplied by the dc voltage source (PS).
Step 11 Based on the power supplied by the dc voltage source (PS) and the maximum
undistorted output power (PO) calculated in Step 9, calculate the efficiency (ŋ) of the
amplifier.
Questions: How did the efficiency of this class B push-pull amplifier compare with the efficiency of the
class A amplifier in Experiment 14?
The efficiency of class B amplifier is much larger than the previous experiment.

10. Conclusion
After performing the experiment, I can able to say that class B is conducting at half cycle
wave. The quiescent point of class B is located at the cutoff because of the absence of the
collector current. The ac load line crosses the vertical axis saturation current and the cutoff for
each transistor. There is a crossover distortion in the output waveshape if the input voltage is
above the required peak-to-peak input voltage. The voltage gain of the class B amplifier is
almost the same with the unity gain. Finally, power efficiency of class B amplifier is much
greater than the class A amplifier.