if you look at the "equivalent circuit" diagram on an op amp's data sheet, and compare it to a power amp, you will find them very similar. if i were to design a discrete op amp, it would be very similar to a power amp design, except for the output stage, which would be just two small transistors for the op amp, but as many as 3 pairs of successively larger transistors (usually complementary pairs in a darlington configuration) for the power amp, because the power amp needs to source a lot of current to low impedance loads, while the op amp is usually used as a voltage amp