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Question Bank (5 marks) · Q4

Q.Compare the following terms related to a practical Op-Amp in brief with respect to ideal op-amp

a) Band Width
b) Slew Rate
c) Input Offset Voltage
d) Common Mode Rejection Ratio (CMRR)
e) Open Loop Gain
Karnataka PUCTextbookLong· 5mImportance★★★★★est
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[!TLDR]

For every parameter the ideal op-amp value is the extreme (infinite gain, bandwidth, slew rate and CMRR; zero offset), whereas a practical op-amp has a large-but-finite or small-but-non-zero value.

The five terms are compared below (practical values are those quoted for a typical IC-741 op-amp in the Karnataka 2nd PUC Electronics textbook):

TermMeaningIdeal op-ampPractical op-amp
a) BandwidthRange of frequencies amplified without attenuationInfinite (amplifies 0 Hz to ∞\infty)A few MHz
b) Slew rateMaximum rate of change of output voltage, ΔVO/Δt\Delta V_O/\Delta tInfinite (responds instantly)≈0.5 V/μs\approx 0.5\,\mathrm{V}/\mu s
c) Input offset voltageSmall voltage that must be applied between the inputs to make the output zeroZeroA few mV
d) CMRRRatio of differential-mode gain to common-mode gain, Ad/AcA_d/A_cInfinite (rejects all common-mode signals)≈90\approx 90 dB
e) Open-loop gainVoltage gain without feedbackInfiniteOf the order of 10510^5

Brief notes:

  • a) Bandwidth: an ideal op-amp gives amplified output for all frequencies from zero to infinity with no attenuation; a practical op-amp has a limited bandwidth of a few MHz.
  • b) Slew rate: the maximum rate at which the output can change, Slew rate=ΔVOΔt\text{Slew rate}=\dfrac{\Delta V_O}{\Delta t} in V/μ\mus. Ideally infinite (the output follows the input instantly); practically about 0.5 V/μs0.5\,\mathrm{V}/\mu s, which limits how fast a large signal can be handled. …
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