i need to create an instrumentation amplifer with 1000 gain. If i have 1000 gain in the first stage and a unity gain in the second stage, i get a upper frequency of 1k Hz in the frequency response. When I divide the gain into 100 in the first stage and 10 in the second stage, i get around 10k Hz in the frequency response.
Why is there a difference.

 

It is the gain bandwidth product (GBW or GB). If a opamp has 1MHz BW at unity gain, it will only have a BW 1Khz at a gain equal to 1000.
Read more here http://en.wikipedia.org/wiki/Gain-bandwidth_product

 

You will do even better if you build two amplifiers with a gain of about 32.