YEDITEPE UNIVERSITY
DEPARTMENT OF ELECTRICAL AND ELECTRONICS ENGINEERING
EE333 ANALOG ELECTRONIC CIRCUITS LABORATORY
2015-2016 FALL
EXPERIMENT 7: POWER AMPLIFIERS
Objective:
Studying some power amplifier classes and their design considerations.
Equipment:






Oscilloscope
Function Generator
DC Power Supply
BC 547 NPN transistor; BC 557 PNP transistor.
1N4001 Diode
Resistor and Capacitor series.
General Information:
An amplifier receives a signal from some pick up transducer or other input source and provides a
larger version of the signal to some input device or to another amplifier stage. An input transducer signal
is generally small (a few millivolts from a cassette or CD input, or a few microvolts from an antenna)
and needs to be amplified sufficiently to operate an output device (speaker or other power-handling
device). Power amplifiers, primarily, provide sufficient power to an output load to drive a speaker or
other power device, typically a few watts to tens of watts. One method of categorizing amplifiers is by
classes, which depends on their conduction angles. The efficiency of a power amplifier depends on its
class of operation. Briefly;
Class A: The transistor in class A conducts for the entire cycle of the input signal; that is the
conduction angle is 360o. As shown in Fig. 7.1a, the Q-point is chosen at the mid-way between Vcc and
VCEsat.
Class B: The transistor in class B conducts for only half of the cycle of the input sine wave,
resulting in a conduction angle of 180o as shown in Fig. 7.1b. Therefore the class B amplifier is biased at
zero DC current. It has to be used as a push-pull stage for linear amplification.
Class AB: It is an intermediate class between A and B. For class AB operation the resulting
conduction angle is between 180o and 360o. It has to be used as a push-pull stage for linear amplification.
Class C: The transistor in class C conducts for an interval shorter than a half-cycle; that is the
conduction angle is less than 180o. It will operate only with a tuned circuit which provides a full cycle of
operation for the tuned or resonant frequency. Class C amplifiers are used for radio-frequency (RF) power
amplification.
Class D: This operating class is a form of amplifier operation using pulse signals. The signal to be
amplified pulse-width modulates (PWM) a periodic pulse train before being applied to the class-D
amplifier. This high-frequency pulse train is suppressed at the output of the class-D amplifier.
Figure 1 Amplifier operating classes
Figure 2 Comparison of amplifier classes
Figure 3 Power considerations for different amplifier classes
For efficiency of a power amplifier is a measure of the amount of ac power delivered to load.
𝑃𝑂
𝜂 = 𝐴𝐶 . 100 [%]
𝑃𝐷𝐶
Pre-Laboratory Work:
1. Design a class AB amplifier with efficiency not less than 60%. The voltage gain should be at
least 50. In your design use VCC=12V and the DC bias current for voltage amplifier stage is
ICQ=3mA. You can choose transistor parameters arbitrarily. For the circuit you designed, find
the efficiency and voltage gain. Explain shortly how you designed the circuit.
2. Find the maximum input voltage that does not cause distortion for the circuit you have designed.
3. Simulate your design on computer and find the desired values and results in steps 1 and 2.
Procedure:
Figure 4 Class A Amplifier
R1=120 kΩ; R2=10 kΩ; RC=820 Ω; RL= 1.2 kΩ; RE=22 Ω
CS=1 μF; CE=4.7 μF
VCC=12 V
Vin = 100mVp-p sine wave with a frequency of 1kHz
1. Build the circuit in Fig. 4. Draw the input and output signals you see on the oscilloscope screen.
2. Find the voltage gain of the circuit and find the maximum input voltage that does not cause
distortion at the output.
3. Find the maximum efficiency of the circuit.
Figure 5 Complementary symmetry class B push-pull circuit
RL= 1.2 kΩ ; CS= 1μF
VCC= 8V, frequency= 1 kHz
4. Build the circuit in Fig. 5. Note that you should apply an input signal (Vin) having an
amplitude of more than the cut-in voltages of the transistors. Draw the input and output
signals you see on the oscilloscope screen and observe the crossover distortion. Find the
maximum efficiency of the circuit. What is the minimum amplitude of the signal that must
be applied at the input?
Figure 6 Pin connection diagram for BC 547B and BC557B
Figure 7 AB class push-pull amplifier
R1=120 kΩ; R2=10 kΩ; RC=820 Ω; RL=1.2 kΩ; RE=22 Ω
CS=1 μF; C2=1 μF
Vin = 100 mVp-p sine wave with a frequency of 1 kHz
5. Build the circuit in Fig. 7.6. Observe the input and output signals. Find the maximum efficiency
of the circuit. Comment on the efficiency values of each type of power amplifier schemes.