# Operational Amplifier

## Main Question or Discussion Point

Hi, I have a quick question regarding Operational Amplifier's. I understand in an ideal Op-Amp the current through the + and - input terminals should be zero. What is the point of the Op-Amp if no current goes through the inputs? How does it work?

thanks

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uart
An Op-amp is designed to amplify the input voltage differential. Only the input voltage is relevant, any input current flow is purely parasitic.

Op-amps have separate power supply terminals (which are not always shown on all schematics) to provide the current (and power) that the Op-amp requires. Current through the input terminals is neither required or desirable.

sophiecentaur
Gold Member
The "zero current" is not really zero. It means that the amplifier takes very very little current; a negligible amount compared with the current in the surrounding components. The op amp inputs are designed to be very high impedance and there is a lot of Power Amplification. (Not lots of Watts - just a massive gain from a really tiny signal to a moderate one)

There are many instances where, in order to design a circuit, a sub-circuit (perhaps a chip) can be assumed to have infinite or zero resistance. This is an acceptable approximation and gives the right answer for overall performance. It's a 'trick' that works.

Some input current is needed, but if the source driving the op amp has a reasonably low output impedance, the op amp input current will not be a problem. It can be a problem if a large valued feedback resistor is employed. High values of input bias current can result in output errors.

Also, the input differential is quite small as well, but not zero. A good text on op amps can explain this in detail. I recommend reading peer-reviewed texts, then ask a specific question here if a point made in the text needs clarifying.

By the way, the input current is not "parasitic". The voltage differential could never develop unless charges are transported through the b-e junctions. Current is needed to obtain the diff voltage. But good bjt devices have a low input current. When the circuitry is designed right, the input current produces an error not too severe.