JNTU B.Tech III SIGNAL CONDITIONING CIRCUITS Previous Question Paper (September - 2021 - R13)

A 3-op-amp instrumentation amplifier is designed using two input buffer stages and a final differential amplifier stage. The first stage consists of two op-amps (A1 and A2) configured as non-inverting amplifiers with a common gain-setting resistor R_g. The second stage is a standard difference amplifier (A3) with four resistors (R_f and R₁) to provide a fixed gain and common-mode rejection.

The circuit operation is as follows:

• The input buffers provide high input impedance, preventing loading of the signal source.
• The gain of the first stage is given by A_v1 = (1 + (2R₁/R_g)), where R₁ is the resistor connected between the inverting input and output of the buffer op-amps.
• The second stage acts as a differential amplifier with gain A_v2 = (R_f/R₁).
• The total gain of the instrumentation amplifier is the product of the two stages: A_total = (1 + (2R₁/R_g)) × (R_f/R₁).
• By adjusting R_g, the total gain can be varied without affecting the common-mode rejection ratio (CMRR).

The Wheatstone bridge is used for precise resistance measurement through two primary modes: balancing and deflection.

Balancing Method: In this method, the bridge is adjusted until the galvanometer (or detector) indicates zero current, signifying that the bridge is balanced. The condition for balance is R₁R₄ = R₂R₃. By knowing three resistances, the unknown resistance is calculated. This method is highly accurate as it is independent of the detector's sensitivity, but it is slow because it requires manual or automatic adjustment of a variable resistor.

Deflection Method: In this method, the bridge is not balanced. Instead, the change in the unknown resistance causes an unbalance in the bridge, resulting in a current flow through the galvanometer. The magnitude of the deflection is proportional to the change in resistance. This method is faster and suitable for dynamic measurements or monitoring changes in resistance (e.g., in strain gauges or thermistors), but it is generally less accurate than the null-balance method because it depends on the calibration and sensitivity of the indicating meter.