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Electronics

Inverting amplifier configuration

  • 3 things to know
  • Paper 3, section B

Overview

The gain formula, the virtual earth, and the phase inversion at the output.

What you need to know

Answer each one before you reveal it.

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Numbers

What does the minus sign in the inverting gain formula mean, how is gain set, and how does the output's polarity compare with the input's?

Numbers

Why is no current assumed to enter the inverting input?

Numbers

What is virtual earth, and at what voltage does it hold the input?

Questions from this subtopic

Each one with the model answer underneath.

  1. 1

    In an inverting amplifier, how is the voltage gain related to the resistor values?

    Model answer

    The gain equals the negative of the feedback resistor divided by the input resistor: G = −Rꜰ/Rᵢₙ

  2. 2

    What does the negative sign in the inverting amplifier gain formula indicate?

    Model answer

    It indicates a 180° phase shift between the input and output signals

  3. 3

    How does the output voltage of an inverting amplifier relate to the input voltage in terms of polarity?

    Model answer

    The output voltage is inverted relative to the input voltage — when the input is positive, the output is negative, and vice versa

  4. 4

    In the inverting amplifier gain formula G = −Rꜰ/Rᵢₙ, what do Rꜰ and Rᵢₙ represent?

    Model answer

    Rꜰ is the feedback resistor (connected between output and inverting input) and Rᵢₙ is the input resistor (connected between the signal source and the inverting input)

  5. 5

    Why is no current assumed to flow into the inverting input terminal when deriving the inverting amplifier gain formula?

    Model answer

    Because the ideal op-amp has infinite input impedance, so no current can flow into its input terminals

  6. 6

    Why is the potential difference between the two op-amp inputs assumed to be zero in the derivation of the inverting amplifier gain?

    Model answer

    Because the ideal op-amp has infinite open-loop gain, which means even an infinitesimally small potential difference between the inputs would produce an infinite output, so feedback maintains the inputs at the same potential

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