Open-loop gain

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The open-loop gain of an operational amplifier is the gain obtained when no feedback is used in the circuit. Open loop gain is usually exceedingly high; in fact, an ideal operational amplifier has infinite open-loop gain. Typically an op-amp may have a maximal open-loop gain of around 10^5. Normally, feedback is applied around the op-amp so that the gain of the overall circuit is defined and kept to a figure which is more usable. The very high open-loop gain of the op-amp allows a wide range of feedback levels to be applied to achieve the desired performance.

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.

The definition of open-loop gain (at a fixed frequency) is

A_{\text{OL}}=\frac{V_{\text{out}}}{\left(V^+-V^-\right)},

where V^+-V^- is the input voltage difference that is being amplified. The dependence on frequency is not displayed here.

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