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A current mirror may be thought of as an adjustable current regulator, the current limit being easily set by a single resistance. It is a rather crude current regulator circuit, but one that finds wide use due because it issimple.
The same bias (Vbe or Vgs) will flow the same current if the fabrication process and the current-flowing area are the same. Especially, if two transistors are adjacent on the chip, their parameter fluctations will be ignorable.
The reason why the current mirror http://en.wikipedia.org/wiki/Current_mirror **broken link removed**
is used is not the fact that is a reglable current generator but the fact that is able to add or substract an important offset voltage,
while keeping the signal amplitude unchanged and without distorsions.
Imagine for example an analogic signal with voltage swing between 0...+1V. Using a current mirror the voltage swing could be adjusted to say +11...+12V or as well to -100...-101V. It's the simplest and cost effective way except the operational amplifier (or together with the OA) for large voltage offseting. **broken link removed**
The second reason is the matched current generation with n outputs using n matched transistors (or more) and one reference. **broken link removed** and the whole article **broken link removed**
Current mirror consists of 2 perfectly matched transistors(either BJT or MOS) with their gate connected.. In one of them,the gate and drain wil be shorted so that both wil be at the same level and since Vds > Vgs - Vt,the transistor wil be in saturation region..
We can see that even the 2nd transistor wil have the same Vgs and Vds values as the first transistor.. Hence the currents in the both transistors wil be same.. This is because,we assume both the transistors to be matched..
Thus we can use the transistor with the gate shorted to the drain as reference current and can use it to control the current to the other transistor to which load is usually connected!
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