ch2 lecture2

High frequency response
A sketch of the tow stage amp is done, then by sketching a signal model of transistor at high frequency the capacitors at the output consider to be short circuit, while another capacitors will appear due to high frequency, so there is a capacitor between emitter and collector, emitter- base capacitor, and collector-base capacitor.
An exact analysis would first apply KCL at all the nodes and produce a transfer function with a four equations, in which some coefficients would consist of a sum of many products of small-signal model parameters. Many simplifying approximations would be needed to give useful design equations. As the circuit complexity increases, the number of equations increases and the order of the denominator increases, eventually making exact analysis by hand impractical and making lime-constant analysis quite attractive.
The approximate the analysis by using Miller theorm, which reduce the complexity and the number of capacitor, and the number of equation, so making the hand calculation more easy. After that the high 3db frequency is obtained.