Figure 6.1a shows a complete diagram of an operational
amplifier.
A more common version of the diagram is shown in figure 6.1b,
where missing parts are assumed to exist.
The inverting input means that the output signal will be 180 
out
of phase with the input applied to this terminal.
On the diagram 
V (DC) and 
V (DC).
is typically, but not necessarily, 
V.
The positive and negative voltages are necessary to allow the amplification
of both positive and negative signals without special biasing.
Figure 6.1: a) Complete diagram of an operational
amplifier and b) common diagram of an operational amplifier.
For a linear amplifier (cf. a differential amplifier) the open-loop gain is
The open-loop gain can be approximated by the transfer function
where 
is the DC open-loop gain and 
is the
transfer function of a passive low-pass filter.
We can write
where 
and 
Hz.
Two conditions must be satisfied for linear operation:
.
.