Compare Ideal and practical OPAMP

Subject: Liner Integrated Circuits

Topic: Introduction to operational amplifiers

Difficulty: Low

1 Answer

Comparing Ideal and Practical OPAMP

Ideal OPAMP Practical OPAMP
An Ideal OPAMP has In a Practical OPAMP
Infinite voltage gain, so that it can amplify input signals of any amplitude. Voltage gain is not infinite, but typically 10^5 to 10^8, so it is not able to amplify input signals smaller than 100 uV.
Infinite input resistance, so that almost any signal source can drive it and there is no loading of preceding stage. Input resistance is typically 10^6 to 10^12 ohm (for FET input Op-Amps such as uAF771), so still it draws some current and not all source can drive it.
Zero output resistance, so that output can drive an infinite number of other devices. Output resistance is typically 75 ohm for standard Op-Amps, so it has limit to deliver current to output devices.
Zero output voltage when input voltage is zero. It is not able to give zero at output when input is zero, due to mismatching of input transistors.
Infinite bandwidth, so that any frequency signal can be amplified without attenuation. Op-Amp has its own Gain-Bandwidth product, so input frequency should not exceed from that particular frequency range at desired gain.
Infinite common-mode rejection ratio, so that the output common-mode noise voltage is zero. CMRR is typically 90 dB, so still it gives output voltage even if both input terminals are shorted.
Infinite slew rate, so that output voltage changes occur simultaneously with input voltage changes. Slew rate is typically 0.5 to 90 V/uS(for improved Op-Amp such as LM318), so output cannot be change simultaneously with input, there is some delay.
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