Discussion
An amplifier has open-loop input impedance \( 1 \ \mathrm{k\Omega} \) and a loop gain of \( 24 \). With series feedback the input impedance becomes
Answer: D
Series feedback multiplies by one plus the loop gain:
\[ Z_{if} = 1000 \times 25 = 25 \ \mathrm{k\Omega} \]
so 25 kilohms. Using the loop gain without the added one gives 24 kilohms, a four per cent error that matters little here but matters greatly at low loop gains. Carrying the one plus rather than dropping it is a habit worth keeping, since it is exactly the term that describes the no-feedback case.
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