I am designing a differential amplifier for a Wheatstone bridge which have small voltage variations like 0uV to 20uV. so need to design an instrumentation amplifier. I do not know how to find the maximum gain that can be get from an opamp (I know how to calculate overall gain from resister values used). data acquisition frequency is not a huge matter in my application. 5Hz or 5 samples per second reading would be enough via an microelectronic. 
UA741 how to find maximum gain that can be archive from this opamp(frequency at 5Hz)?
Best Answer
Maximum gain is limited to the open loop gain of the op-amp but, for a linear amplifier you should not try and implement a gain value that gets too close to this value because distortion will be an issue and you will also find that the frequency response may be much less than you require. DC accuracy is also made worse with a larger gain so this should also be considered.
Here is the typical open loop gain of an op-amp: -
At a frequency that is somewhat less than 10 Hz the gain is flat down to DC and very high (10\$^5\$ = 100,000). At about 7 Hz (in the example above) the gain begins to fall at 6 dB per octave and unity gain is seen around 1 MHz.
So if you want a closed-loop gain of (say) 1000, your frequency response will be limited to about 1 kHz and, as your input frequency rises from about 100 Hz, you will notice the output signal gets more distorted because there is less open-loop gain headroom to perform the corrective action that negative feedback brings to the party. Above 1 kHz, the gain will follow the open-loop response.