Time management is very much important in IIT JAM. The eduncle test series for IIT JAM Mathematical Statistics helped me a lot in this portion. I am very thankful to the test series I bought from eduncle.
Nilanjan Bhowmick AIR 3, CSIR NET (Earth Science)
Jeevesh magnani
since you are asking about OPAMP, I am assuming your basic electronics are very clear. Hence, I will suggest that start reading the concept from a standard book (personal recommendation: Malvino and Bates), see some videos for explanation (good YouTube channel: All about electronics), and then take a set of questions (from standard book or coaching class) that involves combinations of RC circuit, diode, zener, etc. and solve them independently.