BJT: Common Emitter Amplifier Gain

[tomizzo]

So I have a question regarding the gain of common emitter amplifier. Referring to the attached diagram, the voltage gain of the circuit is -RC/RE. However, I don't understand how Vbb can be removed when completing the analysis. I've done the KCL analysis by hand and keep getting a gain function that is dependent on that value of Vbb. But the analysis completed below simply removes that Vbb.

Is the analysis completed in the diagram AC analysis? Therefore the Vbb is removed since it is a constant DC value? Can anyone help clear this up?

Thanks!

Edit: Well it appears that I didn't read the diagram close enough. It states that lower case variables are those of changing values, which I assume means this is AC analysis. Confirmation of this would still be appreciated though.

 

[jim hardy]

Yep that's it. You generally operate over a small part of the curve. But it's not uncommon to include the transistor's internal r_emitter.

That's why bypassing some or all the external R_emitter for AC with a capacitor gives you so much more gain. Bypassing just some of it gives you more predictable gain - when you can explain that to somebody you'll have mastered the common emitter amp.

So tinker with those equations and work the circuit in your head until your intuition comes to agree with them. That sure beats just memorizing them.