Although this circuit use more resistor components than in basic standard three op-amps instrumentation amplifier circuit, this circuit has the benefit of easy gain adjustment with only one control resistor. Because the easy gain adjustment feature, this circuit topology has been used inside a single chip instrumentation amplifier ic. The figure below shows the schematic diagram of the circuit.
Though it may not be obvious by looking at the schematic, we can change the differential gain of the instrumentation amplifier simply by changing the value of one resistor: Rgain. We could still change the overall gain by changing the values of some of the other resistors, but this would necessitate balanced resistor value changes for the circuit to remain symmetrical. Please note that the lowest gain possible with the above circuit is obtained with Rgain completely open (infinite resistance), and that gain value is 1. [Circuit’s schematic diagram source: allaboutcircuits.com]