Two Op-Amp Differential Amp
This is an ideal circuit for sensor amplification. Strain Bridge circuits too. It is an instrumentation amplifier, means precision. High input impedance to a differential input signal source. The gain can be set with external resistors to over 1000. CMRR trim to match resistor network.
I measure Instrumentation Amplifier like this – Tera Ohms Impedance, Pico Amps Bias Current. Micro Volts Offset Voltage. Here is a Interactive demo i built for such a configuration. The Input Impedance of this module is very high and is symmetric. This circuit can be used for strain gauges and for four wire measurements. If inputs are in mV use OP07. The merit is that it uses only 2 OpAmps yet has high differential Input Impedance.
The Outputs of Opamps are low impedance but still have limits they cannot drive more than a few mA of Current into the Load. If low ohmic value loads are to be applied use external transistors as amplifiers. If inputs Vn-Vp are floating, outputs may be random or oscillating, it is good to have a bias network of 10M resistors to a potential even zero or common, this enables some Vout when input floats.