Differential Amplifier is an input stage of every op-amp

[Patrick Chung]

Generally, if input stage of an operational amplifier is realized as BJT transistor differential amplifier, op-amp has smaller slew rate, smaller voltage offset and better CMRR. If it's realized as JFET transistor based differential amplifier, voltage offset is higher, CMRR is smaller but slew rate is better.

Read more:

http://www.cirvirlab.com/index.php/tutorials/167-bjt-transistor-based-differential-amplifier-and-common-mode-rejection-ratio-cmrr.html

and also simulate:

http://www.cirvirlab.com/simulation/Differential_Amplifier_Simulation_Online.php