# Mirror Currents

#### eestudent123

Joined Feb 10, 2012
3
Hello,

I have a question regarding the circuit attached below. Assuming that bettas are infinite and that Is2=N*Is1 since the emitter area of Q2 is N times larger than the emitter area of Q1.

My intuition tells me that Iout=N*Iref, but I dont know how to put it in equations....
There is no xurrent flowing along Vdd, is thia correct?
Where should I start from?

Thank you,

#### Mark_T

Joined Feb 7, 2012
47
This is a current mirror circuit, but in a mirror circuit beta is matched usually very closely by choosing adjacent transistors at the wafer fab house. So N=1. In yours the current will vary with N as described. Current does flow in the bases as they are forward biased and it is these that balence the the current as if the transistors are matched the base currents will be the same.
By the way if beta's are infinite then you can't have N times it. Also you'd have a short circuit everywhere to ground!

#### crutschow

Joined Mar 14, 2008
25,459
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By the way if beta's are infinite then you can't have N times it. Also you'd have a short circuit everywhere to ground!
No. A BJT with infinite beta will still operate as a normal transistor, with an output current proportional to the base-emitter voltage (it still would have a finite transconductance gain).