Which of the following electrical characteristics is not exhibited by an ideal op-amp?

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Examoogle · Accepted Answer

Option C: Infinite output resistance

Explanation & Concepts:
An ideal operational amplifier (op-amp) is defined by zero output resistance ($R_{out} = 0$), which allows it to act as an ideal voltage source capable of supplying any load current without a drop in output voltage. Infinite voltage gain, infinite bandwidth, and infinite slew rate are all ideal characteristics. Therefore, infinite output resistance is NOT an ideal op-amp characteristic.

Working Principle: In an ideal op-amp circuit, the internal output impedance acts in series with the load resistor. Having $R_{out} = 0$ ensures zero internal voltage drop across the output terminal, enabling total voltage transfer to any connected load regardless of the current drawn.

Key Formulas: $A_v = \infty$ — Ideal Open-Loop Voltage Gain; $R_{in} = \infty$ — Ideal Input Resistance; $R_{out} = 0$ — Ideal Output Resistance; $BW = \infty$ — Ideal Bandwidth; $SR = \infty$ — Ideal Slew Rate

Key Points: Ideal Op-Amp has infinite input resistance ($R_{in} = \infty$) to avoid loading the source signal.; Ideal Op-Amp has zero output resistance ($R_{out} = 0$) so that full voltage is delivered to the load.; Common Mode Rejection Ratio (CMRR) and Power Supply Rejection Ratio (PSRR) are both infinite for an ideal op-amp.

Advantages: Zero output impedance allows driving any load without signal degradation.; Infinite input impedance prevents current drawing from the input source.

Disadvantages: Ideal op-amp characteristics are physically impossible to realize in practical ICs.; Practical op-amps like µA741 have finite output resistance ($R_{out} \approx 75\ \Omega$).

Applications: Ideal op-amp models are used for baseline circuit analysis and feedback design.; Voltage follower / Buffer amplifiers exploit high input resistance and low output resistance.

Additional Info: Option A: Infinite voltage gain ($A_v = \infty$) is an ideal characteristic, allowing precise closed-loop gain control via feedback.. Option B: Infinite bandwidth ($BW = \infty$) ensures amplification of signals from DC to infinitely high frequencies without attenuation or phase shift.. Option D: Infinite slew rate ($SR = \infty$) allows instantaneous response of output voltage to changes in input voltage.

Answer: C is correct — An ideal op-amp exhibits zero output resistance ($R_{out} = 0$), not infinite output resistance.

Exam Tip: Remember the pneumonic for Ideal Op-Amp: 'R-in, Gain, BW, CMRR are INFINITE; R-out, Offset, Noise are ZERO'. This distinction is frequently tested in GATE/ESE exams.

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Option A: Infinite voltage gain

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Option B: Infinite bandwidth

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Option D: Infinite slew rate

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