6 Common Applications

Instrumentation amplifiers are frequently used to amplify differential voltages in the presence of common-mode noise. Some common applications include microphone preamplifiers and thermocouple amplifiers.

For a typical two-terminal electret microphone with a dc component to bias the internal JFET, an instrumentation amplifier can be used to amplify the differential voltage, as shown in Figure 6-1. The microphone parameters determine the required biasing resistors. For a microphone with an output impedance of 2.2 kΩ, do not use biasing resistors low enough to load the microphone and cause distortion, and large enough to induce excessive thermal noise. This trade-off must be evaluated and determined by the system requirements. After the biasing resistors are selected, the coupling capacitors can be calculated based on the frequency band required to pass.

Another common application is temperature measurement with a thermocouple. A thermocouple is essentially a low-impedance short that produces a small-signal dc offset based on the Seebeck effect. This offset can be measured using an IA, as shown in Figure 6-2. The thermocouple has a low output impedance; therefore, two biasing resistors are not required. One biasing resistor can provide the necessary path to ground without creating a large offset error due to Ib.