| 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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