Building an instrumentation amplifier: Does the classic instrumentation amplifier topology require three operational amplifiers (not two) along with precision resistors for accurate, high-CMRR gain?

Introduction / Context:
The widely used “classic” instrumentation amplifier (INA) employs three operational amplifiers and precision-matched resistors to achieve high input impedance, precisely programmable gain, and excellent common-mode rejection ratio (CMRR). Stating that two op-amps and seven resistors are required as a general rule under-specifies the standard architecture and can mislead learners about the canonical design.

Given Data / Assumptions:

• Classic INA consists of two input buffer amplifiers and a final differential amplifier stage.
• Resistor matching (often internal laser trimming) is crucial for high CMRR.
• External gain-setting resistor allows easy gain adjustment.
• Some two-op-amp INAs exist but with trade-offs (input impedance asymmetry, lower CMRR at high frequency).

Concept / Approach:
In the three-op-amp INA, the first two op-amps buffer each input, presenting high impedance to the source. The third op-amp, together with a precise resistor network, subtracts the buffered signals to produce a differential output while rejecting common-mode voltage. This topology underlies many integrated INAs that integrate the resistor network for accuracy and stability.

Step-by-Step Solution:

Verification / Alternative check:
Review integrated INA datasheets: many specify an internal three-op-amp core and provide a single external gain resistor to program gain while maintaining trimmed resistor ratios for CMRR.

Why Other Options Are Wrong:

Common Pitfalls:
Equating any differential amplifier with an INA; overlooking resistor matching requirements that dominate CMRR; ignoring input bias current and impedance needs of sensors.

Final Answer:
Incorrect