Re: Small Signal Analysis
Well, Lets say you are trying to analyze a first few stages of multi-stage amplifier system. For first stage, signal source could be a transducer producing few micro volts of signal voltage. It is very important for you to preserve the wave shape of the input signal. So you need, out of your first amplifier stage, an input output relation of the form:- output= Gain x Input. i.e equal increments in output corresponding to equal increments in input. But this is possible only if you bias your transistor in linear region. In BJT terms Q point to be set into linear region. Now, Q point is (VCEQ, ICQ). Here, if your input signal doesn't disturb your operating point Q, i.e. input (micro-volts) & supply voltage (A few volts), then the signal could be called as small signal.
As to why is it necessary, the reason is mathematical simplicity in analyzing as pointed out by "andre_teprom" above.