For the following instrumentation amplifier, I am having some confusion understanding the gain process for the 1st stage. I understand that the 2 input amps are essentially 2 non-inverting amplifiers tied together.
However what causes the gain for V1 and V2 to be the difference between V1 and the common-mode voltage?
**For example if V1 was 2.49V and V2 was 2.51 V, with the common-mode voltage being 2.5 V, if the gain from the 1st stage was 100, output labelled A would be 100*(2.5-2.49)+2.5 and hence it would be 3.5V. At least I believe that is what it is.
Essentially what I do not understand is why the gain of the 1st stage isn't just V1 * 100, but what causes output A. Since output A only multiplies the gain by the difference of the common-mode voltage from V1 or V2. Then the outputs add this value to the common-mode voltage.
Basically what causes the 1st stage (the 2 buffers) to only multiply the difference of the common-mode voltage from V1 by the gain? Then the output (A and B) adds the common-mode signal and this gain calculation. Why isn't it just V1 * Gain of a non-inverting amplifier for output A (like a normal single non inverting amplifier?)
Please include any textbooks or links that might possibly explain this as well as I lack some fundamental understanding for amplifiers.