can anybody please explain me how the gain of the op-amp gets affected with the tolerances of the resistors used in the inverting or non-inverting configurations.
Some more information on this particular point. The absolute value of the internal resistors may vary +/-20% from a value, say 20k. But the resistors are trimmed to be matched within +/-.01% or better.Fortunately, instrumentation amplifiers do exist, and they have internal resistors that have been laser-trimmed at the factory to match at very tight tolerances. You would be very hard-pressed to duplicate such performance using discrete components.