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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.