JNTU B.Tech III INDUSTRIAL ELECTRONICS Previous Question Paper (February/March - 2022 - R16)

An instrumentation amplifier is a high-precision differential amplifier designed to amplify small signals in the presence of high common-mode noise. It typically consists of three operational amplifiers: two non-inverting buffers at the input stage and a differential amplifier at the output stage.

The circuit configuration uses two input op-amps (A1 and A2) with a gain-setting resistor R_g connected between their inverting terminals. The non-inverting terminals receive the input signals V_1 and V_2. The output stage (A3) is a standard difference amplifier with equal resistors R to provide a gain of 1 for the differential signal.

The operation is as follows: The input stage provides high input impedance and high common-mode rejection. The voltage gain of the instrumentation amplifier is given by:

By adjusting the value of R_g, the gain can be varied without affecting the common-mode rejection ratio (CMRR). This makes it ideal for sensor signal conditioning where high gain and stability are required.

The ideal characteristics of an operational amplifier are:

• Infinite Input Impedance: Prevents loading of the signal source.
• Zero Output Impedance: Ensures that the output voltage remains constant regardless of the load connected.
• Infinite Open-Loop Gain: Allows for precise control of closed-loop gain using external feedback components.
• Infinite Bandwidth: Enables the amplification of signals across all frequencies without attenuation.
• Infinite CMRR: Ensures that common-mode signals are completely rejected.
• Zero Offset Voltage: The output is exactly zero when the input difference is zero.