Electronics Year 1 - Op Amps

Introduction to Operational Amplifiers (Op-Amps)

In summary, the op-amp is a voltage amplifier, typically with two inputs, and a single output. For the sake of simplification, most op-amps can be simplified into the following model.

http://en.wikipedia.org/wiki/Operational_amplifier

There are 5 pins;
V+ This is the noninverting input
V- This is the inverting input
Vout This is the output terminal
Vs+ This pin is attached to a high voltage source
Vs- This is the grounding pin

However, usually they are drawn as follows

http://hyperphysics.phy-astr.gsu.edu/

We do not explicitly show the power-supply connections (Vs+ and Vs-) in circuit diagrams.
It is assumed that the Vout voltage is made in relation to the circuit's ground.

The op-amp multiplies the difference between V+ and V-, and multiplies it by Aol, the open loop gain.
i.e. Vout = |(V+ - V-)| x Aol

This gain parameter, Aol is very large, and is assumed to be infinite, in the case of an ideal op-amp.
For the simplicity of calculation, we simply assume that, because Vout is real,
Vout/Aol = |V+ - V-|

i.e. V+ = V-

This concept is known as the summing point constraint.
This is the default assumption we make whenever calculating feedback loops for ideal op-amps.
By making this assumption, we can apply standard circuit analysis principles.
While there is a difference between V+ and V-, we have to assume that they are the same value for the sake of calculation, such as the following cases demonstrated below.

Using the Op-Amp in circuits

Inverting amplifiers - negative feedback

Inverting amplifiers - positive feedback

Noninverting amplifiers

Non inverting amplifiers always have negative feedback.