Figure 
shows an op-amp in the inverting configuration along with
the power supply connections 
and 
. Future circuit
diagrams will show only the signal portion of the circuit with the understanding
that the power supply connections are required for proper operation of the
circuit. To analyze this circuit, we will use Kirchhoff's Current Law (KCL) to determine
the output node voltage 
and the circuit voltage gain given by the formula
It is important to distinguish between the voltage gain of the circuit and the open-loop voltage gain of the op-amp. The op-amp is only part of the amplifier circuit. The open-loop voltage gain A of the op-amp is the voltage gain from the two op-amp inputs to the op-amp output. While the output node of the whole amplifier circuit may be the output node of the op-amp, the input to the amplifier circuit will not be, in general, a voltage applied across the input terminals of the op-amp.
To analyze an op-amp circuit we first look at the op-amp
input nodes (2 and 3). Assuming an ideal op-amp, no current
flows into either of the op-amp inputs (Equation ).
The current through
is zero and therefore 
.
From Equation we know that 
.
From this, the current flowing through resistor 
is
From Equation we know that 
.
To find the gain (of the amplifier circuit), we need to divide the output voltage by the input voltage:
Note that the final gain is negative, thus the name 
. However,
sometimes a negative gain is not desired. In such a case, one could either use
the output of the inverting amplifier as the input to a second inverting amplifier
which would cause the total gain to be positive. However, a simpler method
would be to use the non-inverting amplifier configuration.
