If a difference amplifier has resistor mismatch from the ideal matching condition, what is the likely effect on the output?

If a difference amplifier has resistor mismatch from the ideal matching condition, what is the likely effect on the output?

• A. The gain deviates from the ideal and a small common-mode gain appears
• B. The gain becomes infinite
• C. The circuit stops functioning
• D. The output is exactly as predicted by the ideal formula

Answer: (A)

When a difference amplifier has imperfect resistor matching, the balance that cancels the common-mode signal is disrupted. In an ideal setup, the common-mode input is rejected completely and only the amplified difference appears at the output. With mismatch, some of the common-mode voltage leaks through, giving a small nonzero common-mode gain at the output. At the same time, the intended differential gain can drift away from its designed value because the resistor ratios are no longer exact. So the output ends up containing the desired amplified difference plus a small contribution from the common-mode input, rather than matching the ideal formula exactly. The circuit still works, but with error in gain and a small unwanted response to common-mode signals.

When a difference amplifier has imperfect resistor matching, the balance that cancels the common-mode signal is disrupted. In an ideal setup, the common-mode input is rejected completely and only the amplified difference appears at the output. With mismatch, some of the common-mode voltage leaks through, giving a small nonzero common-mode gain at the output. At the same time, the intended differential gain can drift away from its designed value because the resistor ratios are no longer exact. So the output ends up containing the desired amplified difference plus a small contribution from the common-mode input, rather than matching the ideal formula exactly. The circuit still works, but with error in gain and a small unwanted response to common-mode signals.