doubt regarding current mirror

[srinpraveen]

I am learning current mirrors. I have a curious doubt. Why is there a need to increase the Rout of a setup to improve the performance of current mirror? Ie. How is Rout and performance of a current mirror related? After all, the reference Ids current on the 1st transistor is copied to the second transistor's output making both those Ids currents of those transistors equal right? Why is improvement of Rout involved in this?

 

[yonutz88]

here is explained pretty well: Current mirror - Wikipedia, the free encyclopedia

 

[srinpraveen]

I am still not sure after seeing wiki. So I am asking. I want to confirm this. Is it because current mirrors are being used as current sources. And current sources are expected to provide constant current irrespective of the voltage across them and hence the output resistance of the current mirror should be infinitely high to make the current flow irrespective of the voltage at the output...Am i right?

 

[emresel]

Hi;
You are right but simple current mirror is not as you expected from theory. As the load resistor increases they don't work.
i advise you to check some modified current mirrors with 3 transistors and so on...
See here https://users.ece.gatech.edu/mleach/ece3050/notes/bjt/bjtmirr.pdf
I may also say mosfets are more robust and close to theory.

 

[srinpraveen]

Well that article from gatech was a bit tough for me but definitely worth reading it..Thanks emresel.

 

[allanvv]

If you just want a quick answer: the current is only properly mirrored if the second transistor has the same Vds or Vce as the first one. If you plot the output current vs Vds/Vce, then you get a graph like: https://upload.wikimedia.org/wikipe...0.jpg/320px-Bjt_current_mirror_out_iv_450.jpg

The output resistance is the slope of the right part of the curve. The higher the output resistance, the flatter it is and the more ideal current source it is.

 

[srinpraveen]

@Allanvv thanks a lot. That was very helpful.