State ideal and practical characteristics of an op_AMP

Subject: Logic Design

Topic: Biasing of BJT

Difficulty: Medium


Characteristics of Ideal Operational Amplifiers:

  • Open loop Voltage Gain A0 is infinity.
  • Infinity input resistance Ri so that almost any signal source can drive it and there is no loading of the preceding stage.
  • Zero output resistance R0 so that the output can drive an infinite the number of other devices.
  • Perfect Balance, i.e. the differential voltage in inverting and non-inverting terminals are zero.
  • Zero output voltage when the input is zero.
  • Infinity bandwidth so that any frequency signal from 0 to ∞ Hz can be amplified without attenuation.
  • Infinity common-mode rejection ratio so that the output common-mode noise voltage is zero.
  • Infinity slew rate so that output voltage changes occur simultaneously with input voltage changes.
  • Zero drift of characteristics with temperature.

Characteristic of Practical Op-amp

  • The open loop voltage gain A0 is maximum and finite, a typical value for the practical op-amp is considered to be 200,000.
  • The input impedance Zi is maximum and is finite i.e. in the order of 100k or more.
  • The output impedance Z0 is minimum, not zero, in the order of 100 or less.
  • The CMRR is maximum and finite.
  • Bandwidth is maximum and finite i.e. it can amplify dc to 1 MHz signal.
  • A slight drift of characteristics due to the change in temperature , not null.
  • Two terminal may be virtually ground, not Vd = 0 exactly, for all conditions.
  • Maximum slow-rate and has the finite value.
  • The output is negligible due to dc-bias when the input is zero.
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