what are the parameters that actually define an amplifier to be a small signal amplifier or a large signal amplifier????
if its the input signal then what is the input signal voltage range for small signal as well as large signal..??

 

Small or large signal is not a strict mathematical definition. It depends on the particular circuit you are using and how you are going to analyze it.

Consider this basic transistor amplifier:

To analyze it you can start with an input of zero then calculate the base emitter and collector currents. That's your operating point.

Now if your input signal remains "small" such that the opperating point can be assumed to be constant then you are in a small signal condition. Thus you can approximate the set point currents as constant and get simple equations for gain to predict what the output does with respect to the input.

However, if the signal becomes "large" enough you have to use different techniques to calculate the output as a large signal could change say the emitter voltage drastically enough to make the device respond in a non-linear fashion. Drive the input with a sufficiently negative voltage and you could make the collector go to near zero, and no further (negative) increase in input will make a change in the output. So it is no longer working in a linear fashion.

Here's someone's nice app for seeing a similar amp in action. If you make the input "large" you can see right away what I mean.

http://ngsir.netfirms.com/englishhtm/Amplifier.htm

 

Linearity would not necessarily distinguish small & large signal amplifiers. I don't think ErnieM was necessarily implying that is the case.

A small signal amplifier might be amplifying a signal from a low level transducer to convert a 100 micro-volt level signal to 100 milli-volt signal. I'd probably class that as a small signal amplifier. Tuned front-end radio frequency amplifiers would fall into that category.

An example of a large signal amplifier might be a high fidelity / high power audio amplifier final stage drive for the speakers in a high quality sound studio system.

 

I think we have to be careful here. If the OP is following an academic course on Electronics, he may be expected to use a definition of small-signal = linear operation. This may be linked to small-signal analysis, which is valid only for sufficiently low-level signals than non-linear effects can be neglected.