An operational amplifier (op-amp) is a device with two inputs and
a single output. The output of the amplifier 
is given by the
formula:
Where A is the open-loop voltage gain of the amplifier,
is the non-inverting input voltage
and 
is the inverting input voltage. Both 
and 
are node
voltages with respect to ground.
Typically, the open-loop voltage gain
A is on the order of 
.
A resistor is placed between the output node and
the inverting input to provide feedback and adjust amplification.
When an op-amp circuit behaves linearly, the op-amp adjusts its output
current such that
the voltage
difference between the two inputs is nearly zero.
Another important feature of the op-amp is that its input resistance is
very large and may be taken as infinite in many applications.
The most common type of op-amp is the 741 which has an input resistance
of 2 M
. This is large enough
to be considered infinite in most applications. Because of the high input
resistance, only a very small current flows into
either input of an op-amp.
In practical op-amp circuits, the current flowing into either of the inputs
is usually on the order of 
.
In the case of an ideal op-amp, where the single
assumption is made that the open-loop voltage gain A goes to infinity,
Equations 
and can be used to analyze most of the
properties of op-amp circuits.
