# Finding output resistance

I was trying to find output resistance of a circuit.

The book shows how to obtain Ro step by step. It says is equal to -Vx. I didn't get that point. My question is what is the relationship between Vx and in the below picture? Why?

• Not going to try to decipher what's $\pi$ and $x$ in that picture.
– pipe
Jul 5 '16 at 14:10
• V_pi appears across r_pi. The left side of r_pi (plus V_pi) is at the same potential as the minus side of Vx. The right side of r_pi (minus V_pi) is at the same potential as the plus side of Vx. Therefore Vx=-V_pi. Jul 5 '16 at 14:11
• okay. i see my mistake.
– ucha
Jul 5 '16 at 14:14
• Re: R1||R2 see 2.bp.blogspot.com/-07P0Cs7n2gs/UtqGs6OwL-I/AAAAAAAAf9Q/… :o) Jul 5 '16 at 14:34
• It is a test circuit for finding the output resistance for a common collector circuit - right?
– LvW
Jul 5 '16 at 14:47

## 1 Answer

Just apply simple KVL for the outermost loop,that has the resistor r-pi, voltage source v-x and the closed switch.