Question

where and why we use different models for solving circuit. like hybrid , pi and hybrid+ pi model.?

Answer 1

Here is a good description found at Yahoo! Answers ( http://answers.yahoo.com/question/index?qid=20110210215133AAddXJj)

In BJT's at low frequencies, one can use the T model. It is simplest. At higher frequencies, to bring in effects of feedback capacitance and so on, one might make use of hybrid pi model. The choice of the model depends on the device characteristics some times, and how well you can have a one to one correspondence between the parameters of the device, and how that can be incorporated in the model. A hybrid pi model is almost universal as it can also include feedback parameters and features.
Choose the simplest model first and understand it. Once you find that it has some limitations, it is time to search for better models!