[SOLVED] Current mirror currents

[DharmaSlice]

Hi,

I have a current mirror which takes in 40uA on its input and mirrors 33uA in each branch. The branches are made up from unit size transistors of 15.5u/1.6u.

I need to provide 33uA , 20uA and 10uA.

I cant see how to provide the 20uA and 10uA, do I need to make a new input branch and divide the input 40uA to 20uA ? Or can it be provided by the units I have at the moment ?

 

[Lt_Garillios]

Yes, make a new input branch and divide the input of 40uA to 20uA.

 

[DharmaSlice]

Thanks, so (maybe a silly question) This means that my block now requires 2 input currents, or is there another way to have it as the one ?

 

[godfreyl]

Can't you size the transistors differently to get different currents? I assume this is an IC design.

 

[DharmaSlice]

Can't you size the transistors differently to get different currents? I assume this is an IC design.

Yes, except for good matching for the layout its best to build the current mirror up from unit sizes and just use multiples of that then to generate the currents.

Well that's the initial assumption I was making, that I should stick with the same unit size for good matching.

 

[Lt_Garillios]

Yes, except for good matching for the layout its best to build the current mirror up from unit sizes and just use multiples of that then to generate the currents.

Well that's the initial assumption I was making, that I should stick with the same unit size for good matching.

What you can do is use one standard size for each and adjust the number of devices (ie number of transistors) to get the required current.

And yes, you use one 'reference' transistor and 'feed' the gates of the other transistors with that.
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