Differential amplifier with high input resistance is used in instrumentation amplifier. Its gain can be precisely set by an internal signal or external resistor. The high common mode rejection ratio makes this amplifier very useful in recovering a small signal buried in large common mode offers and noise.
The instrumentation amplifier is a closed loop device with carefully set gain. The instrumentation amplifier consists of two stages. The input first stage offers very high impedance to both input signals and allows setting gain with a single resistor. The second stage is a differential amplifier with output, negative feedback and ground connections are all brought out.
The input stage consists of two matched op amp. Each stage consists of two matched op amp. Each input V1 and V2 is applied to non inverting input terminal of its op amp. This op amp provides very high input resistance. The output of op amp is connected through a string of resistors.
The two resistors are R and Rg. Output voltage, Vo = (V2-V1) (1+2R/Rg)
Therefore decrease in value of Rg will increase the output voltage Vo. To increase in value of gain the value of Rg has to be decreased.
Even a small value of input voltage can be amplified using instrumentation amplifier.
For transmission purpose for long range, noise also gets superimposed along the original wave. Therefore specific cables are used to reduce noise.