Gain - common cathode stage

[überfuzz]

I'm trying to understand how the calculation of gain in a grounded cathode gain stage is derived. Assuming I have a plate resistor Ra, an internal plate resistance ra, a cathode resistor Rc and an amplification factor mu the amplification of the stage is:
A = -mu*Ra/(Ra+ra+Rc*mu+Rc)

The part where I get confused is Rc*mu+Rc, or Rc(mu+1) as it is presented on many places on the www. I get that the cathode resistor works as an impedance and should be considered, but what's up with the Rc*mu part?

[jazbo8]

It basically says Rc (or Rk in the image below) is multiplied by the gain of the tube, see:



Jaz

[überfuzz]

It basically says Rc (...) is multiplied by the gain of the tube...

Jaz

Dude, you're just quoting the formula.. The rule of thumb i use is, modified to fint a grounded cathode stage is: The voltage drop between the grid and the cathode is amplified, amplify any impedace appearing in series. It might be crude but it helped me back in school, sometime.
If anyone can explain how to derive these impedances I'd go .

[jazbo8]

Did you go through the formulas step-by-step? If so, you can see where the (1+u)*Rc term came from, wasn't that the question?

Jaz