Op-amp instrumentation amplifier is given in figure bellow.
Simulate op-amp instrumentation amplifier.
Op-amp instrumentation amplifier is a circuit that combine op-amp differential amplifier with two input buffer amplifiers. This is very successful combination, since op-amp instrumentation amplifier has very high input impedance, very high common-mode rejection ratio, and very high voltage gain. For that reason op-amp instrumentation amplifier is almost always a choice to be input stage of EKG and EEG instruments, and generally whenever we have high input impedance and small signal power. Some measurement instruments also have op-amp instrumentation at input stage. Input impedance is high due to current type feedback input. Procedure for calculate overall voltage gain of an op-amp instrumentation amplifier is as follows.
Current I through resistor R1 is
Potential of point A expressed over current I is
Potential of point B is
Potential of point C expressed over voltage divider R3-R4 is
Voltage on inverting terminal of op-amp OP3 is
Voltage on non-inverting terminal of op-amp OP3 is
Since inverting and non-inverting terminals of an non-saturated op-amp are on the virtually same potential, we have:
Overall gain of op-amp instrumentation amplifier is