Current Mirror experiment

[sabahan]

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



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?

 

[FvM]

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?

 

[crutschow]

The second circuit is acting the way it is designed.
What do you want it to do?

 

[sabahan]

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.

 

[crutschow]

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.

 

[d123]

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.