Analog and electronic circuits

In an inverting amplifier using a bipolar-input op-amp, why is a resistor equal to the parallel value of the input and feedback resistors placed between the non-inverting input and ground?

  1. ATo double the gain
  2. BTo cancel the output offset error caused by input bias currents
  3. CTo cause oscillation
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Answer: B. To cancel the output offset error caused by input bias currents

Small input bias currents flow into the op-amp inputs, and the resistance on the inverting side produces a voltage drop that appears as an error at the output. Placing an equal resistance (the parallel value of the input and feedback resistors) on the non-inverting side creates the same drop at both inputs, cancelling it.

  • A. This resistor does not change the gain.
  • B. Correct. It equalizes the bias-current voltage drops at both inputs so they cancel.
  • C. It is not meant to cause oscillation.
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