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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?

enter image description here

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    \$\begingroup\$ Not going to try to decipher what's \$\pi\$ and \$x\$ in that picture. \$\endgroup\$
    – pipe
    Jul 5 '16 at 14:10
  • \$\begingroup\$ 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. \$\endgroup\$
    – Mario
    Jul 5 '16 at 14:11
  • \$\begingroup\$ okay. i see my mistake. \$\endgroup\$
    – ucha
    Jul 5 '16 at 14:14
  • \$\begingroup\$ Re: R1||R2 see 2.bp.blogspot.com/-07P0Cs7n2gs/UtqGs6OwL-I/AAAAAAAAf9Q/… :o) \$\endgroup\$
    – JimmyB
    Jul 5 '16 at 14:34
  • \$\begingroup\$ It is a test circuit for finding the output resistance for a common collector circuit - right? \$\endgroup\$
    – LvW
    Jul 5 '16 at 14:47
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Just apply simple KVL for the outermost loop,that has the resistor r-pi, voltage source v-x and the closed switch.

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