Variable differential amplifier

In a traditional differential amplifier with feedback to the -ve (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 tbe potential divider need to be each other's image to ensure satisfactory suppression ofthe 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.