Used to amplify small voltages of mV with adjustable gain.
Here the difference voltage is 100mV.
And the output is 4.74V peak.
Lets consider the lower terminal as V1
And upper terminal as V2
Hence the output is A*(V1-V2)
The gain A is adjustable over range of 50 to 500. Here the gain is kept at 50. Change the gain using the potentiometer. A series resistance of 390ohm is added so as to prevent short circuit when potentiometer is made minimum and the minimum gain of 50 us maintained.
