Slavko Vorkapitch said:

Hi - I am reading up on small signal models where for example a shunt

capacitor is placed across the resistor to ground at the emitter end

of a basic class A amplifier - so I understand the desireability of

eliminating unwanted AC signals but wouldn't the connection to ground

eliminate ALL signals - both DC and AC without a shunt capacitor ?

Maybe this is too mathematically simple ..

Thanks in advance,

El Squid

I'm having trouble understanding what you are asking.

Are you asking us to explain why a common emitter amplifier without an

emitter resistor still amplifies?

If you want the reason, based on some rules of thumb (which are based on the

physics), the answer is easy:

let Zc = 1/(2.PI.F.C.j), where F is the signal frequency.

this is the impedance of the cap.

then if Re is parallel to Zc, then Rt = Re||Zc = (Re.Zc)/(Re + Zc)

You can see that if Zc gets small, (which it will when the frequency goes

up, since its inversely proportional to F) then Rt goes down.

Since the gain of the amp is Rc/(re + Rt), that higher frequency makes the

gain go up. re is just the intrinsic emitter resistance, which is 25/Ic mA.

If there isn't any Rc, then just set that to zero in the above equations,

and note that the gain goes up, since then Rt = 0, so G = Rc/re.

This is all based on a transistor model called the ebers-moll equation,

which you can search for if you need more information.

Regards,

Bob Monsen