ideal Opamp v/s practical Opamp

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Ideal Opamp characteristics

Ideal operational amplifier are characterized by

  • Infinite gain
  • Infinite input resistance
  • Zero output resistance (order of 10’s of ohms)
  • Infinite bandwidth (practically restricted by slew rate)
  • Linear irrespective of entire analog signal range No offsets and, so on

A real Opamp exhibits imperfections from ideal characteristics of an opamp due to practical limitations such as improper matching of transistors in differential stages, due to bulk and parasitic capacitance’s e.t.c. Some of dc error voltages, currents and bandwidth limitations of an opamp are as follows

  • Practically it is of the order of 10^5 and is a function of frequency. Gain decreases linearly with slope of 6db/octave, exhibits the characteristic of low pass filter with gain.

  • Input resistance is of the of mega ohm Order due to differential stage at the front end
  • Output resistance order of tens of ohms.
  • Practically bandwidth of Opamp restricted by slew rate.
  • Practical Opamp exhibits offsets and non linearity

 Input/output characteristics of Practical Opamp

The maximum output of an opamp is limited by dc power supply voltages supplied to opamp. At most output voltage excursions are limited within +Vcc and –Vcc where Vcc is the power supply voltage. Similarly output currents from the opamp are also limited by these saturation voltages. Below is an example of opamp with gain 1,28,571 and power supply voltages equal to 9volts.When input voltages exceeds +or- 70 micro volts opamp starts to exhibit nonlinear characteristics and output voltage saturates at + or- 9volts.

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