A unity gain buffer (aka voltage follower) is an op amp circuit whose output voltage follows the input voltage. This may not seem to be very useful, but there are all sorts of reasons that you might use one in a circuit. This video describes some of it's uses and then derives the equation for the relationship between input voltage and output voltage (Yes the gain is 1, but that is only when the open loop gain of the op amp is infinite).

This video also shows a simple simulation example using LTSpice

To see other common operational amplifier configurations check out:
https://www.electronx.ca/education/op...

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https://www.electronx.ca