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Op-Amp Golden Rules

An ideal operational amplifier with negative feedback follows two simple rules: no current flows into the inputs, and the voltage on the two inputs is equal (a virtual short). These two rules are enough to derive the gain of almost any op-amp circuit without dealing with the internal gain directly.

I+=I−=0I_+=I_-=0no current flows into the inputs
V+=V−V_+=V_-virtual short between the inputs

Symbols

V+, V−V_+,\ V_-voltage on the + and - input respectivelyV
I+, I−I_+,\ I_-current into the inputsA

Example

Non-inverting amplifier with V+=2V_+=2 V:

The virtual short gives V−=2V_-=2 V as well.

The two rules only hold with negative feedback – without it the output saturates.
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Part of Electronics: Operational amplifiers.