An instrumentation amplifier is a specialized type of differential amplifier that is commonly used in precision measurement and signal conditioning applications. Its primary function is to amplify the difference between two input signals while rejecting common-mode signals (signals that appear at both inputs with the same amplitude and phase). This makes instrumentation amplifiers highly effective in amplifying weak signals that are buried in noise or interference.
The operation of an instrumentation amplifier involves several key stages:
Input Stage: An instrumentation amplifier typically consists of three operational amplifiers (op-amps) arranged in a specific configuration. The first two op-amps create a differential input stage. The input signals (usually labeled as Vā and Vā) are applied to the inverting (-) and non-inverting (+) inputs of the first op-amp. The second op-amp also has its inverting input connected to Vā and its non-inverting input connected to Vā. This configuration creates a balanced differential input where the common-mode component (Vcommon = (Vā + Vā) / 2) is cancelled out.
Differential Gain Stage: The output of the second op-amp from the input stage is connected to the non-inverting input of the third op-amp. The feedback network connected to this op-amp determines the gain of the instrumentation amplifier. This stage amplifies the difference between the two input signals, effectively rejecting any common-mode noise.
Feedback Network: The feedback network in the instrumentation amplifier is what sets its gain and establishes its performance characteristics. It usually consists of resistors that create a balanced voltage divider. The resistors can be precisely matched to ensure accurate amplification of the differential signal while maintaining common-mode rejection. The gain of the instrumentation amplifier is typically expressed as a function of the resistor values in the feedback network.
Output Stage: The output of the third op-amp is the amplified differential signal, which can be used as an output or further processed as needed. The output stage ensures that the amplified signal is buffered and can drive external loads without significantly affecting the gain or performance of the amplifier.
Overall, the instrumentation amplifier provides benefits such as high input impedance, high common-mode rejection ratio (CMRR), and tunable gain, making it ideal for applications that require precise measurement and noise rejection, such as medical devices, industrial sensors, and data acquisition systems.