You obviously can build your own op-amp,but it's cheaper to buy an IC, you have got to decide what you're going to use it for,before designing it,just for example;say you want to build a PWM,then look at the data sheet for the LM324N,it will show you 4 transistors,each with a different function,ie; comparitor, 12v reference,ect;this 14 pin chip.together with various resistors and diodes,would serve as the driver for a Mosfet,simple & cheap to populate on a small board.I pay around 50 cents for an IC,there are hundreds to choose from,whether you are building a stereo amp or any thing that requires the function of an intergrated circuit.(if you want to know anything ask Audioguru)