What is the need of level shifter in op-amp?

Question

What is the need of level shifter in op-amp?

Answer 1

Normally devices must operate within their (approximately) linear range.

If the small signal you want amplified is centred (has a mean DC value) somewhere that the device will not operate, you need to shift it's level, mean DC to make the whole of the small signal of interest in the linear range of the device.  

Answer 2

The amplifiers inside an OpAmp are DC coupled (no AC coupling capacitors).  Each amplifier stage produces AC amplification of the signal but at the same time the DC level is shifted due to the bias voltages.  The output of each stage is designed to have a minimum DC level shift, but some level shift is unavoidable.  

A level shifter placed in between gain stages shifts the DC level back to zero volts (or close to it as required by the design), so that the overall DC path is maintained (input to output).

This means you can amplify a DC signal (at the input) and get the proper DC output.

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