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Procedure 4 Complementary class-AB output stage Measurement-4a Use an oscilloscope to monitor the input and output voltage waveforms. Record sketches of the input and output waveforms in your lab notebook. Increase the amplitude of the input signal until the output voltage waveform is clipped on both the positive and negative peaks. Measure and record the output voltage clipping levels. Restore the input signal to a 1.0 kHz 5.0 Vpp amplitude sinewave and increase the frequency until the output voltage waveform falls to 70 percent of its previous amplitude. This is the -3 dB bandwidth of the output stage. Gain of circuit almost unity Clipping level of circuit High frequency -3db falloff of circuit Question-4a (a) Calculate the voltage gain for this output stage. (b) Comment on any distortion that is seen in the output voltage waveform. (c) Calculate the limited value of output current when the short-circuit protection becomes active. Responses 4a a) Voltage gain = Vout/Vin = 4.68/4.96 = .944 b) There is no distortion in the output voltage, only when the circuit is driven into clipping is there distortion. c) Current I = V/R, so at the output during clipping: 15.7volts/100Ω = .157A = 157mA More Set-Up Modify the circuit of Fig. E5.4a to that of Fig. E5.4b by adding an opamp and a feedback loop. Measurement-4b Apply a 1.0 kHz 5.0 Vpp sinewave to the input and use an oscilloscope to monitor the input and output voltage waveforms. Record sketches of the input and output waveforms in your lab notebook. Increase the amplitude of the input signal until the output voltage waveform is clipped on both the positive and negative peaks. Measure and record the output voltage clipping levels. Restore the input signal to a 1.0 kHz 5.0 Vpp amplitude sinewave and increase the frequency until the output voltage waveform falls to 70 percent of its previous amplitude. This is the -3 dB bandwidth of the overall circuit. Gain of circuit with opamp in feedback = unity (side by side comparison) Gain of circuit with opamp in feedback = unity Circuit clipping level with opamp in feedback Circuit -3dB freq falloff with opamp in feedback Question-4b (a) Calculate the voltage gain of the overall circuit. (b) Compare the clipping levels and bandwidth of the circuit to that of the output stage without the opamp. (c) Compare the distortion of the opamp circuit to that of the output stage without the opamp. What are the tradeoffs? Response 4b a) Voltage gain Av = Vout/Vin = 5.05/5.08 = .994 ≈ 1 = Unity b) Clipping level without opamp: 19 in, 15.7 out. Clipping level with opamp: 18.7 in, 15.8 out. With the opamp the clipping level goes up by .1 volt but for a lower input of .3 volts. Bandwidth: Without opamp -3dB @ 1.381MHz. With opamp: -3dB @ 84.89 kHz. c) For the non-opamp circuit: The distortion at -3dB drop off is on the rising edge of the sine wave and is non-symmetrical to the falling edge. There is no phase shift For the opamp circuit: The distortion at the -3dB drop off is at a lower frequency but has symmetrical rising and falling edges. There is also a phase shift to the right (latent output signal) of the opamp feedback circuit. The tradeoffs are higher frequency and in phase signal response but slightly lower gain, clipping level and non-symmetrical distortion for the non-opamp circuit. On the other hand the opamp feedback circuit tradeoffs are: Lower frequency -3dB dropoff and a slight latent phase shift at high frequency for symmetrical distortion, slightly better gain and a slightly higher clipping level. Conclusion: Our class AB amplifier has provided us with an increase of output current and a higher range of input voltage than for a single stage amplifier.