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Thanks for all your suggestions. Sorry I have not given full details earlier.
I have an instrument which is giving a 4-20mA current sink, where I have to connect to another instrument which will accept current source. I can't modify either of the device internal circuitry. So I have to insert a circuit in between these two instruments.
I have tried earlier as per tyassinsuggestion, but it gives 0.3mA added current output. May be I have to try with a matched pair of transistor or will try with MOSFET instead of BJT. I can happily accept If you have any other suggestions.
You neither did now.
* voltage range of the source
* voltage range of the load
* (or available power supply voltage)
* load resistance (range)
* expected precision (you complained about it, so it's urgent to give at least a clue of what you expect)
You don't want to use MOSFETs in a current-mirror as the Vgs mismatch between two devices is much higher than BJTs.
Use a matched pair of BJT transistor for the current mirror.
The normal Vbe mismatch between two discrete transistors can cause the current offset you noted.
Also, you might use a 3-transistor Wilson current mirror (below) which is much less sensitive to the output voltage.
(Change all the transistors to PNP for your purposes).