Which of the following is not true for an ideal operational amplifier ?

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VSSC ISRO Technical Assistant Electronics 14 July 2021 Official Paper
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  1. CMRR is infinity
  2. Differential Mode Gain is infinity
  3. Common Mode Gain is Infinity
  4. Input Impedance is infinity

Answer (Detailed Solution Below)

Option 3 : Common Mode Gain is Infinity
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Detailed Solution

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The ideal op-amp has the following characteristics:

  • Input impedance (Differential or Common-mode) = Infinity
  • Output impedance (open loop) = 0
  • Voltage gain = Infinity
  • Infinite Bandwidth
  • Common-mode voltage gain = 0
  • Vout = 0 when both inputs are at the same voltage, i.e. (zero "offset voltage")
  • Output can change instantaneously (Infinite Slew Rate)
  • The purpose of bias current is to achieve the ideal behavior in op-amp which is high CMRR, high differential gain, and high input impedance

 

CMRR (Common mode rejection ratio) is defined as the ratio of differential-mode voltage gain (Ad) and the common-mode voltage gain (Ac).

Mathematically, this is expressed as:

\(CMRR = \frac{A_d}{A_c}\)

Ad = Differential gain

Ac = Common mode gain

NoteIdeally, the common-mode gain of the Op--Amp should be zero., i.e. it must give a zero output for common input at both the inverting and non-inverting terminal. ∴ The CMRR of an ideal Op-Amp is infinity.

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