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Like for instance, the picture above. How to find the output impedance?

My thought was like draw the small signal circuit of them, then do the equivalent. But I am not sure if I need to short the current source of Q2 or not. I know that for Q1, I need to short the current source, cause Vbe for Q1 is zero. What should I do? Thank you.

  • \$\begingroup\$ Why do you want to short the current source? \$\endgroup\$
    – G36
    Oct 13, 2018 at 6:04
  • \$\begingroup\$ Vbe for Q2 is also zero? \$\endgroup\$
    – markable
    Oct 13, 2018 at 6:07

1 Answer 1


You don't have to short anything. In this schematic from a book (Behzad Razavi).

The GND symbol represents the AC ground, not a DC ground. Therefore you must assume that both transistors are properly biased. All, you need is to replace all transistor with the corresponding small-signal model and do the calculations.

Additional to simplify a thing notice that Q2 is work as a diode connected BJT.

And (I assumed that you read the book) you should know by now that the small-signal equivalent circuit for Q2 is a single "resistor" rd:

\$r_d = \frac{1}{gm}||r_\pi||r_o \approx \frac{1}{gm}\$

Hence your circuit reduced to:


simulate this circuit – Schematic created using CircuitLab

Also, you can look here:

BJT common-base output resistance derivation

Calculation of output impedance of CE emitter bias configuration( unbypassed) with r_0


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