# How to find the output impedance of a two BJT small signal circuit?

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.

• Why do you want to short the current source? – G36 Oct 13 '18 at 6:04
• Vbe for Q2 is also zero? – markable Oct 13 '18 at 6:07

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}\$$