Some time we need more powerfull amplifier, specially when we deal with transducer that its output is very small such of thermocouple. And the answer Is instrumentation amplifier.
Why difference amplifier did not adequate on such application? The difference amplifer is satisfactory for low impedance source, buts its input impedance is too low for high output impedance sensor. Furthermore , If the input signal very low level and inckude signal noise, the difference amplifier is unable to extract a satisfactory difference signal. The solution is not difference amplifier but instrumentation amplifier.
Instrumentation amplifier has the following characteristic :
a. Instrumentation amplifier has Very high input impedance
b. Large CMMR: CMMR is the ratio of the difference mode gain to the common mode gain. The difference mode gain is the amplification factor for the difference between the input signals, and the common mode gain is the amplification factor for the average of the input signal. For an ideal instrumentation amplifier and difference amplifier , the common mode gain is zero, implying an infinite CMMR. But sure the ideal instrumentation amplifier isnot exist.
c. Instrumentation amplifier has capability to amplify low level signal in noisy environment (due to high CMMR)
d. Instrumentation amplifier has consisten bandwidth over a large range of gain.
Instrumentation amplifier are commercially available as monolithic IC( ex. AD 5234, AD624 or LM 623). In these instrumentation amplifier ICs , a single external resistor is used to set gain.
But theorycally, we can made an instrumentation amplifier using op-amp. There are many instrumentation amplifier circuit design , following figure show an instrumentation amplifier circuit use three op-amp.
