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32 questions By Tony R. Kuphaldt
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Question 13 of 32
Compare the voltage gains of these two opamp circuits:
Which one has the greater AV, and why?
Reveal answer
This opamp circuit has the greater voltage gain, because its [(Zfeedback)/(Zinput)] ratio is greater.
Notes:It is common to see impedances represented as boxes, if their constituent components are not germane to the operation of the circuit.
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Question 14 of 32
Describe what will happen to the impedance of both the capacitor and the resistor as the input signal frequency increases:
Also, describe what result the change in impedances will have on the op-amp circuit’s voltage gain. If the input signal amplitude remains constant as frequency increases, what will happen to the amplitude of the output voltage? What type of filtering function does this behavior represent?
Reveal answerAs the frequency of Vin increases, ZC decreases and ZR remains unchanged. This will result in an increased AV for the amplifier circuit.
Follow-up question: normally we calculate the cutoff frequency of a simple RC filter circuit by determining the frequency at which R = XC. Here, things are a little different. Determine the voltage gain (AV) when R = XC, and also determine the phase shift from input to output.
Challenge question #1: explain why the phase shift from input to output for this circuit is always constant, regardless of signal frequency.
Challenge question #2: explain why this type of circuit is usually equipped with a low-value resistor (R1) in series with the input capacitor:
Notes:The answer I’ve given is technically correct, but there is a practical limit here. As we know, the intrinsic gain of an op-amp does not remain constant as signal frequency rises. Ask your students to describe the impact of this phenomenon on the circuit’s performance at very high frequencies.
On another note, this same op-amp circuit is known by a particular name when used with DC input signals. Ask your students what this design of circuit is called.
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Question 15 of 32
Describe what will happen to the impedance of both the capacitor and the resistor as the input signal frequency increases:
Also, describe what result the change in impedances will have on the op-amp circuit’s voltage gain. If the input signal amplitude remains constant as frequency increases, what will happen to the amplitude of the output voltage? What type of filtering function does this behavior represent?
Reveal answerAs the frequency of Vin increases, ZC decreases and ZR remains unchanged. This will result in a decreased AV for the amplifier circuit.
Challenge question: explain why this type of circuit is usually equipped with a high-value resistor (R2) in parallel with the feedback capacitor:
Notes:This same op-amp circuit is known by a particular name when used with DC input signals. Ask your students what this design of circuit is called. When receiving a DC input signal, what function does it serve? The answer to this is key to answering the “challenge” question.
