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Oscillator Circuits
49 questions By Tony R. Kuphaldt
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Question 10 of 49
How many degrees of phase shift must the feedback circuit (the box in this schematic) introduce to the signal in order for this common-emitter amplifier circuit to oscillate?
We know that oscillator circuits require “regenerative” feedback in order to continuously sustain oscillation. Explain how the correct amount of phase shift is always provided in the feedback circuit to ensure that the nature of the feedback is always regenerative, not degenerative. In other words, explain why it is not possible to incorrectly choose feedback network component values and thus fail to achieve the proper amount of phase shift.
Reveal answerThe feedback network in this circuit must provide 180 degrees of phase shift, in order to sustain oscillations.
So long as the feedback network contains the correct types of components (resistors, capacitors, and/or inductors) in a working configuration, the components’ values will not alter the amount of phase shift, only the frequency of the oscillation.
Notes:Ask your students to explain why the feedback network must provide 180 degrees of phase shift to the signal. Ask them to explain how this requirement relates to the need for regenerative feedback in an oscillator circuit.
The question and answer concerning feedback component selection is a large conceptual leap for some students. It may baffle some that the phase shift of a reactive circuit will always be the proper amount to ensure regenerative feedback, for any arbitrary combination of component values, because they should know the phase shift of a reactive circuit depends on the values of its constituent components. However, once they realize that the phase shift of a reactive circuit is also dependent on the signal frequency, the resolution to this paradox is much easier to understand.
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Question 11 of 49
How many degrees of phase shift must the feedback circuit (the box in this schematic) introduce to the signal in order for this two-stage common-emitter amplifier circuit to oscillate?
Why is this amount of phase shift different from that of a single-transistor oscillator?
Reveal answerThe feedback network in this circuit must provide 0 degrees of phase shift, in order to sustain oscillations.
Notes:Ask your students to explain why the feedback network must provide 180 degrees of phase shift to the signal. Ask them to explain how this requirement relates to the need for regenerative feedback in an oscillator circuit.
The question and answer concerning feedback component selection is a large conceptual leap for some students. It may baffle some that the phase shift of a reactive circuit will always be the proper amount to ensure regenerative feedback, for any arbitrary combination of component values, because they should know the phase shift of a reactive circuit depends on the values of its constituent components. However, once they realize that the phase shift of a reactive circuit is also dependent on the signal frequency, the resolution to this paradox is much easier to understand.
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Question 12 of 49
Explain what the Barkhausen criterion is for an oscillator circuit. How will the oscillator circuit’s performance be affected if the Barkhausen criterion falls below 1, or goes much above 1?
Reveal answerI’ll let you determine exactly what the “Barkhausen” criterion is. If its value is less than 1, the oscillator’s output will diminish in amplitude over time. If its value is greater than 1, the oscillator’s output will not be sinusoidal!
Notes:The question of “What is the Barkhausen criterion” could be answered with a short sentence, memorized verbatim from a textbook. But what I’m looking for here is real comprehension of the subject. Have your students explain to you the reason why oscillation amplitude depends on this factor.
