Note on Basic Op-amp Characteristics

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An operational amplifier(often op-amp) is a DC-coupled high-gain electronic voltage amplifier with a differential input and, usually, a single-ended output. In this configuration, an op-amp produces an output potential (relative to circuit ground) that is typically hundreds of thousands of times larger than the potential difference between its input terminals. It has high input impedance and low output impedance and has the capability to amplify the dc or ac signal. An op-amp can be originally designed to perform mathematical operations like summation, subtraction, multiplication, differentiation, integration etc. Nowadays , it can be used for sign changing, rectification, an oscillator circuit, scale changing , voltage regulation , active filters,rectification, wave generators etc.


The amplifier's differential inputs consist of a non-inverting input (+) with voltageV+and an inverting input (–) with voltageV; ideally the op-amp amplifies only the difference in voltage between the two, which is called the differential input voltage. The output voltage of the op-ampVoutis given by the equation:

whereAOLis the open-loop gain of the amplifier (the term "open-loop" refers to the absence of a feedback loop from the output to the input).

Fig:circuit symbol of op-amp

Fig:circuit symbol of op-amp

Op-amp characteristics:

An ideal op-amp is usually considered to have the following properties:

  • Infinite open-loop gain G =vout/vin
  • Infinite input impedance Rin, and so zero input current
  • Zero input offset voltage
  • Infinite output voltage range
  • Infinite bandwidth with zero phase shift and infinite slew rate
  • Zero output impedance Rout
  • Zero noise
  • Infinite common-mode rejection ratio(CMRR)
  • Infinite power supply rejection ratio.


1.R.F. Coughlin, F.F. Driscoll(1982)Operational Amplifiers and Linear Integrated Circuits (2nd Ed)



  • The Operational Amplifier, or Op-amp as it is most commonly called, can be an ideal amplifier with infinite Gain and Bandwidth when used in the Open-loop mode with typical DC gains of well over 100,000 or 100dB.
  • An operational amplifier also has zero output impedance, ( Z = 0 ).
  • Op-amps sense the difference between the voltage signals applied to their two input terminals and then multiply it by some pre-determined Gain, ( A ).This Gain, ( A ) is often referred to as the amplifiers “Open-loop Gain”.

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