Operational Amplifier: How Does It Work?

[ali297]

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

 

[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]

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.

 

[cabraham]

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.