W2-Lab 2 Class B -AB-OL 2023-06

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ECPI University, Virginia Beach *

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120

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Electrical Engineering

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Jan 9, 2024

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Experiment 2: Class – B and AB Push-Pull Amplifiers Objectives: After completing this experiment, you should be able to, 1. Observe the Class – B and AB push-pull power amplifier’s output waveforms. 2. Calculate the output power and efficiency. Submission Requirements: Submit the Multisim.ms14 file along with your Experiment 2 lab document. Components needed: Instruments used: 1. Resistors: (2) 10kΩ, 330Ω 1. Digital Multimeter (DMM) 2. Transistor: (1) 2N3904, (1) 2N3906 2. Variable Power Supplies (VPS) 3. Diodes: (2) 1N914B 3. Function Generator 4. Connecting wires (as necessary) 4. Oscilloscope Procedure: 1. The Class - B/AB push-pull power amplifier we build in this lab will include two complementary (NPN and PNP) transistors. 2. Identify the Emitter, Base and Collector terminal of each of the transistor. Below shows an image of a physical devices as well as the NPN form of the schematic symbol. Figure 1: Transistor terminals

3. Build the following circuit in MultiSim. Observe that both emitters and both bases of the transistors are joined together. Figure 2: Class – B push-pull amplifier 3. Set the voltage to 1Vp , ( 2V PP), 1 kHz sinewave . 4. Add an oscilloscope to the circuit simulation. Turn on the Oscilloscope, if necessary . 5. Connect CH0 of the Oscilloscope across the input signal. Q1 2N3904 VCC V 9 V1 Vpk 1 kHz 1 ° 0 RL 330Ω AC_Power Q2 2N3906 VEE V -9.0 CH1 - Output CH0 - Input

6. Connect CH1 of the Oscilloscope across the 330Ω load resistor. Use cursors to display the Vpp of the signals 7. You should be able to observe the following signals. Figure 3: Input and Output signals of the Class-B Push-Pull Amplifier CH0 - Input CH1 - Output

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15. Observe the output voltage is approximately 1.4V lower than the input voltage. This is because both the transistors are off when the signal falls below ± 0.7V. This is called cross-over distortion. 16. Use the following steps to calculate the maximum efficiency of the Class – B/AB push- pull amplifier. a. Maximum peak output voltage V P = V CEQ = V CC = _________4.5v____________. This is maximum possible for the circuit , not the current value in the circuit . b. Maximum peak output current Ip = Ic ( sat )= Vceq / Rl ______13.6mA______. Calculations c. Output power Pout = 0.25 ∗ Ic ( sat )∗ Vcc = ¿ _______3.06V____________. Calculations d. DC input power = Pdc = Ic ( sat )∗ Vcc / π = ____________3.90V___________. Calculations e. Efficiency = Pout / Pdc = ¿ __________________0.78V___________________. Calculations 17. To eliminate the cross-over distortion, diodes are used to bias the transistors. Modify the Class-B push-pull into Class-AB push-pull circuit as shown below.

Figure 4: Class – AB Push-Pull amplifier

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18. Observe the input and output signal voltages using CH0 and CH1 of the oscilloscope. You should be able to see the following waveforms. Figure 5: Input and Output signals of the Class AB Amplifier

19. Observe the input and output voltage levels there are close but not equal. This is because the forward voltage drop of the diode is not exactly matching the V BE of the transistors. The cross-over distortion can be greatly minimized but cannot completely be eliminated.

Experiment Questions: 1. The maximum efficiency of Class-B/AB power amplifier is ___0.78V_______________. 2. What is the advantage in using the Class-B/AB power amplifiers?. To increase the amplification without the issue of overloading the circuit. 3. The conduction angle for Class-B power amplifier is < ___0______ degrees. 4. The conduction angle for Class-AB power amplifier is ____0_____ degrees. 5. What is the Q-point in the Class – B amplifier? __________4.5v,13.6mA__________________. Q-point (Iceq and Vceq) are defined as Ic and Vce when there is no input signal present. Enter actual values. 6. What is the Q-point in the Class – AB amplifier? __________4.5v,13.6mA__________________. Enter actual values. 7. What is the importance of diodes in Class – AB push-pull amplifier? To help reduce the rate of current without overloading the circuit. *********End of the experiment *********

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