W2 -Lab 2 Class B -AB-OG

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ECPI University *

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Electrical Engineering

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Dec 6, 2023

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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. Components needed: Instruments used: 1. Resistors: (2) 10kΩ, 330Ω 2. Transistor: (1) 2N3904, (1) 2N3906 1. Digital Multimeter (DMM) 3. Diodes: (2) 1N914B 2. Variable Power Supplies (VPS) 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. Measure and record the resistor values using Digital Multimeter (DMM) listed in Table 1 below. Listed value Measured value R 1 - 10 kΩ 9.8 kΩ R 2 - 10 kΩ 9.8 kΩ R L – 330Ω 326.9 kΩ Table 1: Listed and measured values 3. Identify the Emitter, Base and Collector terminal of each of the transistor. (Note: Hold the transistor, such that the flat face facing towards you. The leftmost pin is the Emitter, middle pin is the Base and the rightmost pin is the Collector)
Figure 1: Transistor terminals 4. Build the following circuit on the bread board. Observe the both emitters and both bases of the transistors are joined together. Figure 2: Class – B push-pull amplifier 7. Turn ON the power. 8. Set the Supply + to 9V and Supply - to - 9V. 9. Use the Function generator . 10. Set the voltage to 2V PP, 1 kHz sinewave . 11. Turn on the Oscilloscope . 12. Connect CH0 of the Oscilloscope across the function generator. 13. Connect CH1 of the Oscilloscope across the 330Ω resistor. 14. You should be able to observe the following signals. Q1 2N3904 VCC V 9 V1 Vpk 1 kHz 1 ° 0 Supply + (Pin 48) GROUND (Pin 49) RL 330Ω FGEN (Pin 33) Q2 2N3906 VEE V -9.0 Supply - (Pin 50) CH1 - Output CH0 - Input
Figure 3: Input and Output signals of the Class-B Push-Pull Amplifier 15. Observe the output voltage is approximately 1.4V behind 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 efficiency of the Class – B/AB push-pull amplifier. a. Maximum peak output voltage = V CEQ = ____4.5V_____. b. Maximum peak output current = I C = ____13.6mA____. c. Output power = P out = 0.25 * I C (sat) * VCC = ____3.06V______. d. DC input power = P DC = ____3.9V_____. e. Efficiency = = _____0.78V_____. 17. In order 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. CH0 - Input CH1 - Output
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