Hi Robertt,
The corner frequency has to do with the noise in the transistor. Noise is modeled by referring it to the input, then assuming that the transistors themselves are noiseless, then fc it is simply the frequency at which this "input referred" voltage noise source is no longer dominated by a huge 1/f factor, the lower this fc the better . It depends on the process and also on the biasing conditions. It can be found analytically by citcuit analysis, and HSpice can find it also. Read Analysis and Design of Analog Integrated Ckts for more info.