Which statement about a gain-of-one amplifier is true?

Which statement about a gain-of-one amplifier is true?

• A. The op-amp must have equal input bias currents.
• B. The output impedance is zero.
• C. The circuit must use a transformer for isolation.
• D. Circuit input and feedback resistors must be of equal value.

Answer: (D)

Unity gain means the output voltage has the same amplitude as the input (though it may be inverted in sign in some configurations). For an inverting op-amp, the closed-loop gain is Av = -Rf/Rin. To have a magnitude of one, you set the feedback resistor equal to the input resistor, giving Av = -1. That makes the circuit a unity-gain stage in magnitude, with inversion. In a non-inverting unity-gain buffer, you’d simply wire the output to the inverting input and don’t rely on a resistor network at all. The other statements aren’t required for unity gain: equal input bias currents aren’t necessary, real amplifiers don’t have zero output impedance in practice, and isolation by a transformer isn’t a requirement for achieving unity gain.

Unity gain means the output voltage has the same amplitude as the input (though it may be inverted in sign in some configurations). For an inverting op-amp, the closed-loop gain is Av = -Rf/Rin. To have a magnitude of one, you set the feedback resistor equal to the input resistor, giving Av = -1. That makes the circuit a unity-gain stage in magnitude, with inversion. In a non-inverting unity-gain buffer, you’d simply wire the output to the inverting input and don’t rely on a resistor network at all. The other statements aren’t required for unity gain: equal input bias currents aren’t necessary, real amplifiers don’t have zero output impedance in practice, and isolation by a transformer isn’t a requirement for achieving unity gain.