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I find this formula for an op-amp written in a book:

Unity Gain Bandwidth=A*Fo

Where, A is the open loop gain of the opamp

Fo is the break frequency

What I inter from this expression is that the open loop gain of an apamp is generally defined at break frequency. But I could not find it written explicitly. I did not want to make any conclusion without being sure. Can someone tell if what I infer is correct?

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What you infer is correct. At DC the open loop gain is in the realm of 100,000 to 1,000,000 for most op-amps and this would naturally mean that the break (3 dB point) is a few hertz in frequency. Here's an example: -

enter image description here

It brings in the term gain-bandwidth-product i.e. the product of open-loop gain and bandwidth. For a regular op-amp having a 20 dB per decade slope (single order), the product is virtually a constant.

Picture taken from this slide show.

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