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: