Instrumentation and Summation Configuration-Operational Amplifiers Types Tutorials Series

Instrumentation Amplifier Configuration

Some applications, such as an oscilloscope input, require differential amplification with extremely high input resistance(Circuit is shown above)

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Related Tutorials

Operational Amplifiers Tutorials Series
Op-amp Integrator Network
Op-amp with T-bridge Feedback Network
Voltage Follower and Difference Amplifier Configuration
Linear Op-amp Operation-Inverting And Non-Inverting Configuration

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A3 is a standard difference op-amp with differential gain R2/R1

A1 and A2 are additional op-amps with extremely high input resistances at v1 and v2 (input currents = 0)*Differential gain of input section: 

–Due to the virtual shorts at the input of A1 and A2, we can write iA = (v2 – v1) /RA 

–Also, iA flows through the two RB resistors, allowing us to write v02 – v01 = iA(RA + 2 RB)

– Combining these two equations with the gain of the A3 stage, we can obtain
vOUT = (R2/R1)(1 + [2RB/RA])(v1 – v2) 

– By adjusting the resistor RA, we can adjust the gain of this instrumentation amplifier

Summation Amplifier Configuration

A summation op-amp (shown at left) can be used to obtain a weighted sum of inputs v1…vN 

–The gain for any input k is given by RF/Rk
If any input goes positive, vOUT goes negative just enough to force the input v- to zero, due to the virtual short nature of the op-amp
–Combining all inputs, we have

vOUT = -RF(v1/R1 + v2/R2 + .. + vN/RN) 

–The input resistance for any input k is given by Rk due to the virtual short between v- and v+