JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD B.Tech III INDUSTRIAL ELECTRONICS Previous Question Paper (January/February - 2021 - R16)

An instrumentation amplifier is a high-precision differential amplifier designed to amplify small signals while rejecting common-mode noise. The most common configuration is the three-op-amp instrumentation amplifier. This circuit consists of two input buffer stages followed by a differential amplifier stage.

The first stage uses two op-amps with a gain-setting resistor connected between their inverting terminals. This configuration provides high input impedance and high common-mode rejection ratio (CMRR). The second stage is a standard difference amplifier that subtracts the two buffered signals, effectively canceling out common-mode voltages.

Another configuration is the two-op-amp instrumentation amplifier, which is more cost-effective but offers lower CMRR and limited gain flexibility compared to the three-op-amp version. It uses a single gain-setting resistor between the two op-amps, which function as both buffers and a differential gain stage. While it saves one op-amp, it is more sensitive to resistor matching for maintaining high CMRR.

DC amplifiers are prone to drift, where the output changes due to variations in temperature, power supply voltage, or component aging. Stabilization techniques are essential to maintain a stable DC operating point.

1. Differential Amplifier Configuration: Using a balanced differential amplifier (long-tailed pair) helps cancel out common-mode drifts, as both transistors experience similar thermal effects, leading to a reduction in output offset.

2. Negative Feedback: Applying heavy negative feedback significantly reduces the effect of gain variations and improves the stability of the closed-loop gain, although it reduces the overall gain.

3. Chopper Stabilization: This technique involves converting the DC input signal into an AC signal using a mechanical or electronic chopper, amplifying the AC signal with an AC-coupled amplifier, and then demodulating it back to DC. Since AC amplifiers do not suffer from DC drift, this method provides extremely low drift.

4. Temperature Compensation: Using matched transistor pairs or integrated circuits ensures that thermal drifts in one component are compensated by the other. Additionally, thermistors or diodes can be used in the bias network to counteract temperature-induced changes in transistor parameters.