Analog and electronic circuits

Which combination correctly describes an ideal op-amp?

  1. AInput impedance: infinite; output impedance: 0
  2. BInput impedance: 0; output impedance: infinite
  3. CInput impedance: 0; output impedance: 0
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Answer: A. Input impedance: infinite; output impedance: 0

An ideal op-amp is treated as having infinite input impedance (no current flows into its inputs), zero output impedance (the output voltage does not depend on the load) and infinite open-loop gain. From these assumptions, a negative-feedback circuit has a 'virtual short' in which the two input voltages are equal.

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  1. In an inverting amplifier using an ideal op-amp, with input resistor Ri = 10 kΩ and feedback resistor Rf = 100 kΩ, what is the voltage gain?
  2. Which is closest to the cutoff frequency of an RC low-pass filter with R = 1 kΩ and C = 1 µF?
  3. In a non-inverting amplifier using an ideal op-amp, with input-side resistor R1 = 10 kΩ and feedback resistor R2 = 90 kΩ, what is the voltage gain?
  4. Which correctly describes a voltage follower using an op-amp?
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