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Current Mirror experiment

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sabahan

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CM1.jpg

I did a current mirror experiment with the simple setup,both Q1 and Q2 are MPSA42,the result are satisfying

but when I connect another BJT to the setup,result are all ways off

CM2.jpg

I2 varies according to the Ib and correspond to the beta of the BJT,did I do anything wrong?Or I have to use PNP for the amplifying transistor?
 

The purpose of your circuit isn't obvious.

Q2 is the output side of the current mirror. What do you want to achieve by feeding another current source to the output?
 

I would like to add a constant current sink to my next amplifier project,current mirror seems to be a good choice,but I have no experience on this,so doing some experiment and try to familiar with it first,is just a experiment,trying to learn something from it.
 

So you learned that your modification doesn't do what you expected.
Are you trying to improve the mirror's characteristics?
If so you might look at a Wilson current-mirror which adds a transistor to greatly reduce its variation in output current with output voltage.
 

Hi, the link below is an interesting read, part 7 is about current mirrors, and there is a schematic similar to your second one but the third transistor is in a slightly different place:

**broken link removed**

I recommend the web in general, if you haven't seen it, in my opinion it's really informative.
 

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