How does an instrumentation amplifier differ from a standard differential amplifier?

• A. Instrumentation amplifier uses two op-amps and variable gain with high input impedance.
• B. Instrumentation amplifier uses a single op-amp; differential uses two.
• C. Instrumentation amplifier typically has lower input impedance than a standard differential amplifier.
• D. Instrumentation amplifier uses three op-amps to provide high input impedance and high CMRR with fixed gain.

Answer: (D)

Instrumentation amplifiers are built to measure tiny differential signals in the presence of large common-mode voltages, and they do this with a three-op-amp topology: two input stages that act as high-impedance buffers for each input, and a final differential amplifier that subtracts the buffered signals. The input buffers keep the source from being loaded and present a very high input impedance, while the balanced path to the difference stage enhances suppression of any signal common to both inputs. This arrangement yields two main benefits: the ability to connect sensors without loading them (high input impedance) and strong rejection of common-mode noise (high CMRR). The gain is set by the resistor network linking the stages, typically giving a stable, defined gain in many designs, though some configurations allow gain adjustment. In contrast, a standard differential amplifier using a single op-amp lacks that buffering and balanced, multi-stage path, so it often has lower input impedance and weaker common-mode rejection.