Small signal model of a circuit

'Small signal analysis of CE, CB, CC configurations using hybrid π model' is a major topic in 'Micro electronics syllabus'. To do the analysis of a circuit, first we have to draw a model for that particular circuit(CE,CB or CC).  How can we obtain a small signal model for a circuit?- It is explained below. 

Steps to obtain a model from corresponding circuit :

Here we are interested in small signal analysis and so taking only small ac signals into consideration. If you need little more idea, please visit  'Transistor analysis - Small signal analysis and Large signal analysis'. 

I will explain the steps by taking a CE amplifier circuit as example. (Once you get it for a particular circuit, then it will be Easy..Only thing to do is just follow the steps- one by one).Ok..Turn your head to first year class and catch - - - your favorite - - - RC Coupled Amplifier ( it is an example of CE circuit).

fig 1 R C Coupled amplifier

Please Don't mind red line around bias section. .

Step 1: D C voltage sources are replaced by ground(Because of small signal analysis, avoid D C sources)

fig 2

Step 2: D C current sources are open circuited
                NO current sources here.

Step 3: Capacitors are shorted

fig 3

Step 4: Inductors are open circuited
               NO Inductors here.

Step 5: Check for resistors which are parallel to short circuits (here R_E) and remove.

fig 4

Step 6:  Rearrange the components such as to have a common ground 

fig 5

 Step 7: Simplify the circuit and insert transistor model

fig 6

Now fig 6 represents small signal model of fig 1.

Generally analysis of a given circuit is done by finding values(or expressions) of certain parameters such as:
1. Input Impedance
2. Output Impedance
3. Voltage gain          and
4. Current gain.
         
            These parameters give an idea about the behavior of the circuit.

This topic is given in the text book: Micro Electronics by Sedra & Smith. It is presented here with detailed description.