is given
by the formula: 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,
