How to draw AC equivalent circuit of CE amplifier?

[abhijath]

in the attached diagram its seen that while drawing AC equivalent of the common emitter circuit, Re is not taken in, the AC equivalent diagram no.2. why?

 

[Jony130]

We simplify assume that CE capacitor ( the capacitor connected across Re resistor) is large enough (very small Xc) so that we can treat him as a "short circuit".

 

[LvW]

It is the only purpose of the capacitor CE across RE to ground the emitter node for the operating frequency range.
Therefore, in the ac equivalent diagram the emitter node is grounded (ac wise).
In addition, please note that in such an equivalent diagram it is common practice also to replace the transistor by its small-signal equivalent model (h-parameters or y-parameters).

 

[CWatters]

The emitter resistor provides feedback that helps stabilise the DC operating point of the transistor. For example if the collector emitter current were to increase for some reason then the emitter resistor causes the emitter voltage to rise. That in turn reduces V_BE which reduces the collector emitter current again.

However a side effect of the emitter resistor is to reduce the gain...

Gain = R_C/R_E

Adding that capacitor doesn't effect the DC biasing but improves the AC gain.

There is quite a good explanation of that here..

http://www.electronics-tutorials.ws/amplifier/emitter-resistance.html

 

[LvW]

I agree, of course, to CWatter´s explanation.
However, I don`t agree to his recommendation (link).
This link is not very serious and the contributions contain many errors and wrong explanations.
Example: The above link contains the sentence:
"Now if the supply voltage Vcc increases, the transistors collector current Ic also increases for a given load resistance"
I think, everybody will agree that this "explanation" shows that the author does not know how a transitor really works.

 

[CWatters]

But it does. As VCC increases the base bias voltage increases. That increases the voltage across the emitter resistor and hence the collector current also increases.

 

[LvW]

But it does. As VCC increases the base bias voltage increases. That increases the voltage across the emitter resistor and hence the collector current also increases.

(Typo?)

I am not sure if this was the idea of the author. The most important reason for RE-feedback is to reduce the influence of tolerances (passive parts and primarily uncertainty of beta).