Current Mirror on the wall ...

[PerranOak]

I had just about got to grips with the simple current mirror.
Then I saw an example with one input but multiple outputs - excellent!
However, this suddenly included another BJT (Q1 on the attached) in the input side WITH NO EXPLANATION AT ALL!
Why do they do this just as I am getting it!

[EPTech]

Hi There,

This looks more like an equivalent circuit or a circuit for simulation to me. I suggest you find a better circuit.

http://sub.allaboutcircuits.com/images/03129.png

[Andy Watson]

One input transistor, three output transistors, the output current will be three times the input current. 1:3
Two input transistors, three output, the current will be mirrored 2:3

Although, in practice, variation in Vbe/temperature will probably lead to very disappointing results - unless you can get transistors on the same die.

[PerranOak]

Thanks both.
It was really just to understand the concept.
So the "extra" transistor is just to alter the ratio?

[Ian.M]

In real life, as the transistors need to be on the same die to match their Vbe, (both process variations and temperature), the current ratio is controlled by the ratio of their emitter junction areas.

[w2aew]

Thanks both.
It was really just to understand the concept.
So the "extra" transistor is just to alter the ratio?

Yes. This is often done on an IC because it's generally easy to match and ratio transistor sizes/areas etc. one of the 'arts' in IC design is to recognize the component tolerance are pretty poor, but devices match and ration very well - so you design circuits to take advantage of ratios and scaling rather that absolute values.