The open-loop gain of an amplifier is the gain obtained when no overall feedback is used in the circuit. Open loop gain, in some amplifiers, can be exceedingly high. An ideal operational amplifier has infinite open-loop gain. Typically an op-amp may have a maximal open-loop gain of around
Contents
Normally, feedback is applied around an amplifier with high open loop gain so that the effective gain circuit is defined and kept to a desired figure.
Definition
The definition of open-loop gain (at a fixed frequency) is
where
Role in non-ideal gain
The open-loop gain is a physical attribute of an operational amplifier that is often finite in comparison to the usual gain, denoted
Take the case of an inverting operational amplifier configuration. If the resistor between the single output node and the inverting input node is
However, with the use of open-loop gain, the equation becomes:
Notice that the equation becomes effectively the same for the ideal case as
In this manner, the open-loop gain is important for computing the actual gain for a given non-ideal operational amplifier network in situations where the ideal model of an operational amplifier begins to become inaccurate.
Operational amplifiers
The open-loop gain of an operational amplifier falls very rapidly with increasing frequency. Along with slew rate, this is one of the reasons why operational amplifiers have limited bandwidth.