If you want to be a design engineer at Linear, besides knowing circuit fundamentals (R's, C's, L's) you really have to know your transistors. Current mirrors, current sources, diff pairs, cascoding, matching, op-amps, etc. You should also know control systems, noise gain, miller effect, etc. Last but not least, a knowledge of device structure, physics, and non-idealities is necessary. Some companies pride themselves in being able to train monkeys to be design engineers, but Linear only accepts the very best.