stupid current mirror - confusion .. #2

Thread Starter

Komisar81

Joined May 21, 2022
2
I have never put a current mirror circuit together before. Today I did, and it doesn't seem to work. I'm using a SSM2220 matched dual PNP, current mirror as seen here -



http://sub.allaboutcircuits.com/images/03282.png

when Vcc is around 12V and Rbias is 10K, I get 1.153 mA bias current. Then if the load resistor is 5.6K, I get 1.297 mA load current ? Now as I understand it, these currents are meant to be the same, but it seems whatever the values of the resistors they never are. What might be the reasons for the difference?

....also does pretty much the same thing when i simulate it, although there are loads of bugs in this simulation program...

have i misunderstood something?
I have also breadboarded the SSM2220P as a current mirror, not exactly the one you have, and found that mirror branch of the circuit does not mirror the Rbias branch current very well, the difference was 4X higher in the Rload branch.

I have breadboarded other matched transistors that did not exhibit this anomaly with differences of 15% at most. My current measurements are in the form of voltage drops across 5% tolerance resistors, so the match was pretty good.

Something peculiar with the SSM2220P. A product I purchased on EBay??

Mod:link to old Thread.

https://forum.allaboutcircuits.com/threads/stupid-current-mirror-confusion.128777/post-1054553
 
Last edited by a moderator:

eetech00

Joined Jun 8, 2013
3,244
I have also breadboarded the SSM2220P as a current mirror, not exactly the one you have, and found that mirror branch of the circuit does not mirror the Rbias branch current very well, the difference was 4X higher in the Rload branch.

I have breadboarded other matched transistors that did not exhibit this anomaly with differences of 15% at most. My current measurements are in the form of voltage drops across 5% tolerance resistors, so the match was pretty good.

Something peculiar with the SSM2220P. A product I purchased on EBay??
Try adding emitter degeneration resistors at each emitter leg. This will help with mitigate the beta difference of the two transistors. Use maybe....350-500 ohm resistor in each leg.

BTW- Crutschow is correct in that your mirror is basic. There are better ones like the "WIlson" mirror mentioned in the earlier post.
 

Thread Starter

Komisar81

Joined May 21, 2022
2
Try adding emitter degeneration resistors at each emitter leg. This will help with mitigate the beta difference of the two transistors. Use maybe....350-500 ohm resistor in each leg.

BTW- Crutschow is correct in that your mirror is basic. There are better ones like the "WIlson" mirror mentioned in the earlier post.
I will try the degeneration resistors. I will also try the Wilson mirror; not with the SSM2220 but with a quad set, as the Wilson mirror requires three transistors.

Thanks for the tips. I will post my circuit once it works properly.
 
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