I have some doubts concerning the calculation of the small-signal output resistance of a common-source stage with source-degeneration. This is related to another question I posted but I have refined my question in detail here as to exactly what is troubling me
Calculate the small-signal output resistance of the following circuit including the effect of channel-length modulation and ignoring the body effect.
I have two approaches to find the output small-signal resistance, they both involve,
- Drawing the small-signal model of the circuit
- Zero all independent sources (voltage sources = short, current sources = opens)
- Applying a voltage Vx at the output and measure the resulting current Ix flowing.
- Output resistance will then be Rout = Vx/Ix
Now from here on, I have two approaches - the first approach gives me the correct answer, however the second approach gives me an incorrect answer.
Approach 1 (KVL)
Approach 2 (Resistors)
Why does my second approach give me an incorrect answer? I suspect it is something to do with the fact that the ro now only gets a current Ix flowing through it.? Something that didn't sit well with me in the second approach was that after decomposing the current source into a resistor, I had to get rid of the negative sign of the 1/gm. So current flow direction is strange to me too.