1/4 LAB
THE CLASS B PUSH PULL AMPLIFIER
PURPOSE AND BACKGROUND The purpose of this experiment is to demonstrate the design and operation of a Class B push-pull emitter-follower power amplifier. The class B push-pull amplifier has a pair of complementary (npn and pnp) transistors, each of which is biased at cutoff i.e. no collector current. Consequently, collector current in each transistor flows only for alternate half-cycles of the input signal. Since both transistors are biased at cutoff, the input signal must be sufficient to forward bias each transistor on the appropriate halfcycle of the input waveform. As a result, crossover distortion occurs. To eliminate crossover distortion, both transistors should, under quiescent conditions, be slightly forward biased so that each transistor is actually biased slightly before cutoff, resulting in a small amount of current called the trickle current. Since we now have neither true class A nor true Class B operation, but rather something in between, this operation is referred to as Class AB operation, although term “Class B” is frequently used to describe this situation. Despite its complexity, a Class b push-pull amplifier can achieve efficiencies up to approximately 78%, which is more than three times better than can be obtained with a similar Class A amplifier without transformer coupling. REQIURED PARTS AND EQUIPMENT Resistors (1/4 W): 100-Ω, three 1-k Ω, two 10-k Ω Capacitors (25 V): 100-μF Diodes: Two 1N4148 silicon Transistors: 2N3904 npn and 2N3906 pnp silicon DC Power Supply, Signal Generator, Oscilloscope, DMM, Breadborad PROCEDURE 1. Wire the circuit as shown in figure. Connect Cannel 1 of oscilloscope at the input and Channel 2 at the output. 2. Apply power to the breadboard and adjust the sine wave output level of the generator at 6-V peak-to-peak at a frequency of 1-kHz. You should observe the amplifier’s output waveform as shown in figure below. Notice that the output waveform is distorted in the vicinity of zero volts. This condition, referred to as crossover distortion, results when the base emitter junctions of both the transistors are not forward biased until input signal exceeds approximately 0.7-V in both directions. Note that the peak-to-peak output voltage is slightly smaller than the input, a difference approximately equal to two base-emitter voltage drops. In addition, since each half of the push-pull circuit is itself an emitter-follower, there is no phase shift between the input and output signals.
2/4
Class-B Push Pull Amplifier
Output Voltage for Class-B Push-Pull Amplifier
3/4 3. Disconnect the power and signal generator leads from the breadboard, and replace the resistors, R2 and R3 with two 1N4148 compensating diodes as shown in figure below. Again connect the power and signal generator to the breadboard.
4. With the signal generator set at 6-V peak-to-peak at 1-kHz, you should find that there is virtually no crossover distortion. The voltage required to forward bias both the transistors is now supplied by the voltage drops of the two silicon diodes, which are also forward biased. 5. With a DMM, individually measure the transistor dc base 1, base 2, and emitter voltages with respect to ground, recording your result in Table 6. Now carefully increase the peak-to-peak input signal so that the output peaks just clip off. With your DMM, measure the rms voltage across 1-kΩ load resistor and compute the rms output power of the amplifier. Record your results in Table 7. In order to measure the dc power supplied to the amplifier while amplifying an input signal, use your DMM to measure the dc collector current IC of either transistor. Compute the dc power supplied and record your results in Table 8. Finally compute the efficiency of your amplifier, and compare it with the theoretical maximum of 78.5% of a class B amplifier. Record your results in Table. If you calculate a value greater than 78.5%, then repeat steps 7 and 8, trying to determine the source of your error.
4/4 WHAT YOU HAVE DONE This experiment demonstrated the design and operation of a Class B push-pull emitter-follower power amplifier and how it compared with that of a Class A amplifier circuit. The circuit required a complementary pair of transistors (one each npn and pnp). The causes of crossover distortion were demonstrated and possible methods to eliminate it were investigated. In addition, the efficiency of the amplifier was determined and compared with that of a Class A amplifier.
Table 1
Table 2
Parameter Measured Value
VB1 VB2
Parameter Measured Value
VE Vo (rms) IC Parameter Calculated Value Po (rms) Pdc %η
