THE HONG KONG POLYTECHNIC UNIVERSITY
Department of Electronic and Information Engineering
EN107/1
EN107: OCL Class AB Power Amplifier
Objective
1. To study the circuit performance of an OCL amplifier.
2. To study the effects of biasing on cross-over distortion of a class AB amplifier.
Apparatus
1. OCL class AB amplifier
2. C.R.O.
3. Audio signal generator
4. Twin 30V/2A D.C. power supply
5. One set of headphones
Introduction:
An OCL amplifier is a push-pull amplifier that requires neither transformer nor output
capacitor. Instead, the amplifier requires a twin D.C. power supply and a matched pair of
complementary transistors at the output stage.
As shown in Fig. 1, the cross-over distortion, which is an inherent characteristic of a
push-pull amplifier circuit, is due to the non-linearity of the input characteristics of the
output transistors at the cross-over point at which neither transistor is being turned on. The
result is that a dead zone appears in the output waveform.
This type of distortion can be heard as harmonic noise superimposed on the output
signal, and it can be minimized or even eliminated by providing a suitable amount of bias
voltage to the output transistors.
A class AB amplifier is a modified form of class B push-pull amplifier where each
output transistor is provided with a small amount of bias just sufficient to allow each device
to conduct slightly in order to minimize the cross-over distortion.
THE HONG KONG POLYTECHNIC UNIVERSITY
Department of Electronic and Information Engineering
a)
c) ib1
ib1
Dead zone
Cross-over
point
vb1
vb2
0
θ
π
θ
0 θ
π
π
2π
ωt
θ
ib2
ib2
b)
EN107/2
vi
π
ωt
Fig. 1 a) input characteristic
b) input voltage waveform
c) input current waveform of the push-pull transistor amplifier
without bias voltage.
Circuit
Fig. 2 shows the circuit of the OCL amplifier used. The circuit is classified into two stages
a) the pre-amplifier stage and b) the driver and the output stage.
In the pre-amplifier stage, a differential amplifier is used in which negative feedback
is fed to the base of Q2 while the input signal is fed to the base of Q1. The difference
between these two voltages is amplified and fed to the input of the next stage, the driver,
via the base of Q3.
The driver transistor Q3 has the collector load composed of Q8, R2, R15 and R17 (the
rubber diode). The potential drop across the rubber diode is used to bias the transistors in
the output stage to operate in class AB mode.
THE HONG KONG POLYTECHNIC UNIVERSITY
Department of Electronic and Information Engineering
EN107/3
By turning R17 (a rotary potentiometer) the biasing current to the output transistors can
be reduced to completely cutoff (ie. Class B). For class AB operation, a moderate biasing
current (or quiescent current) is required.
Two Darlington pairs of complementary symmetry transistors are used in the output
stage. R7 and R8 are inserted to provide a convenient way of observing the current
waveforms and measuring the average D.C. current in Q4, Q6 and Q5, Q7.
Procedure
1.
Do not connect power to the OCL amplifier.
2.
Reset the OCL amplifier by
i.
Turn R17 counterclockwise to the end (Class B)
ii.
Switch SW1 to select the 15 Ω load
3.
Plug the headphone into the phono socket.
4.
Connect the signal generator to the I/P of the amplifier. For this experiment, the
maximum I/P voltage is about 1 Vpp. However please set it to minimum at the
moment. Monitor I/P with oscilloscope CH1. Monitor O/P with CH2.
5.
Before connecting the D.C. power supply, select the tracking mode (series mode). The
maximum output current will not exceed 1 A for this experiment. Please limit the
output current accordingly (both +15V & -15V supplies) and then set the outputs to 15
V.
6.
Switch off the power supply and connect it to the amplifier.
7.
Now turn on the supply.
8.
Set the signal generator frequency to 1 kHz. Gradually increase its amplitude until a
20 Vpp voltage is seen on the output (H).
9.
Observe and record voltage waveforms with oscilloscope at (B), (C), R7, R8 and (H).
Note also the DC Levels with respect to GND (0V).
THE HONG KONG POLYTECHNIC UNIVERSITY
Department of Electronic and Information Engineering
EN107/4
Notes: When measuring waveforms across R7 & R8, use ONE probe only. The other
probe must be disconnected from the circuit.
(B) DC voltage = 0.792 V
(R7)
(C) DC voltage = 0.704 V
( R8 )
(H) DC voltage = 0.08 V
THE HONG KONG POLYTECHNIC UNIVERSITY
Department of Electronic and Information Engineering
EN107/5
10. Monitor O/P again. Check if you can identify the cross-over distortion.
11. Gradually reduce Vin until a significant cross-over distortion is seen in Vout and
record the waveform (~ 2Vpp).
12. Reduce Vin to minimum before switching SW1. Now switch to headphone and put
on the headphone. Slowly increase Vin until the volume is at a comfortable level.
13. Turn R17 to and from between Class B and Class AB. Check if you can sense the
cross-over distortion. Caution: keep the supply current below 0.5 A.
14. Repeat (13) with different volume levels (be careful – do not turn on too loud) and
various frequencies. Comment on the sensitivity of the human ear to cross-over
distortion. Caution: remember to reduce Vin before changing the input frequency.
Input frequency
Comment
20 Hz
7.25 Vpp, distortion heard
100 Hz
7.19 Vpp, distortion heard
500 Hz
2.25 Vpp, distortion heard
2 kHz
2.19 Vpp, distortion heard
10 kHz
1.63 Vpp, cannot tell distortion from signal
15 kHz
Inaudible
THE HONG KONG POLYTECHNIC UNIVERSITY
Department of Electronic and Information Engineering
EN107/6
15. Reset the OCL amplifier as in (2). Setup the signal generator as in (8).
16. Monitor both Vin and Vout with oscilloscope.
a) Calculate the voltage gain Av of the circuit where
AV =
Vin = 0.813 Vpp
Vout
Vin
Vout = 19.9 Vpp
Av = 24.48
b) Measure the average D.C. supply currents. Derive it from the D.C. voltage
across R7 and R8.
Voltage across R7 = 0.205 V
current = 0.205 A
Voltage across R8 = 0.199 V
current = 0.199 A
c) Calculate the output power Pout where
Pout =
2
Vout
(r.m.s )
RL
Pout = 3.3 W
d) Calculate the efficiency η of the output stage where
P
P
η = out = out
η = 0.54
Pd .c. Vd .c. I d .c.
17. Reduce Vin to minimum. Turn R17 such that the supply current (quiescent current) is
about 0.1 A.
18. Adjust Vin to give 20 Vpp output. Repeat (16).
Vin = 0.794 Vpp
Vout = 20.2 Vpp
Av = 25.44
Voltage across R7 = 0.233 V
current = 0.233 A
Voltage across R8 = 0.226 V
current = 0.226 A
Pout = 3.4 W
η = 0.49
19. Vary input frequencies from 100 Hz to 40 kHz and plot the gain-frequency response
of the circuit using dB and log frequency. Determine the bandwidth of the amplifier.
THE HONG KONG POLYTECHNIC UNIVERSITY
Department of Electronic and Information Engineering
EN107/7
Frequency
Vin
Vout
Av
Av in decibel
100 Hz
0.8
20.3
25.38
28.09
150 Hz
0.8
20.3
25.38
28.09
250 Hz
0.8
20.3
25.38
28.09
400 Hz
0.8
20.3
25.38
28.09
630 Hz
0.8
20.3
25.38
28.09
1 kHz
0.8
20.2
25.25
28.05
1.5 kHz
0.8
19.9
24.88
27.92
2.5 kHz
0.79
19.4
24.56
27.80
4 kHz
0.79
18.6
23.54
27.44
6.3 kHz
0.79
17.5
22.15
26.91
10 kHz
0.78
15
19.23
25.68
15 kHz
0.78
12.4
15.90
24.03
25 kHz
0.78
8.4
10.77
20.64
40 kHz
0.78
5.9
7.56
20.88
Bandwidth = 12 kHz
20. Record the serial number of the OCL amplifier.
Serial no. 14
THE HONG KONG POLYTECHNIC UNIVERSITY
Department of Electronic and Information Engineering
EN107/8
Problems for further study:
1.
Explain why rubber diode was used to provide base emitter bias for the output
transistors instead of a resistor.
2.
Describe how this amplifier can be modified to work with a single D.C. power
supply and compare the gain bandwidth of these two circuits.
PREAMP
DRIVER
OUTPUT
R13
+15V
390
C1
220u
B
R3
1k
R21
13k
Q3
Q2SA1015
R1
150p
R24
R25
Q2SC1815
Q1
I/P
1.5k
Q2SC1815
Q2
RUBBER
DIODE
R5
510
Tune R17 to vary
the quiescent
current.
SW1
R20
180
R2
1.1k
SPDT
R6
510
R17
1K
Q5
QTIP32C
R18
R19
15
Q7
QMJ2955
F1
Fuse
1
GND
R10
560
H
Q8
QTIP31C
C5
100u
R11
2.2k
Q6
Q2N3055
R15
4.3k
C14
47u
R12
10k
F
Q4
QTIP31C
33k
C3
3.3u
C8
10n
R7
1
560
33k
R23
1k
C17
470u
2
C4
R16
D1
1N5819
V2
15V
C2
100u
C16
220u
6.8k
R9
6.8k
R22
13k
R4
2.2k
R14
C
V3
D2
15V
1N5819
C18
470u
390
Fig. 2 Schematic diagram of OCL amplifier circuit
---END---
G
R8
1
-15V
J1
1
2
3
PhoneJack