In a traditional differential amplifier with feedback to the -ye (inverting) input of an operational amplifier and a potential divider at the +ve (non-inverting) input. it is tricky to make the amplification variable. This is because the feedback network and the potential divider need to be each other’s image to ensure satisfactory suppression of the common-mode signals (common-mode rejection ratio – CMRR). This means that there must be two variable resistors and these must at all times be equal.
This is not necessary with the present circuit in which one preset can arrange the amplification without affecting the CMRR. It has the following transfer function:
Uo = 2R2/ R1(1 + R2/P1)(U2- U1) [V]
It is clear from this that only P1 affects the difference voltage U2-U1. The common-mode signal does not appear in the function. This is partly because P1 has no effect on it and partly because the resistors With the same circuit reference also have the same value. In theory, that gives total rejection of common-mode signals; in practice, tolerances of the components used will determine the CMRR. To calculate that, however, the formula will have to be, expanded appreciably.
