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LM4041-N-Q1: current source transistors ?

rawan nagi
Prodigy 10 points
Part Number: LM4041-N-Q1
Other Parts Discussed in Thread: LM4041-N

Tool/software:

I'm trying to build a current source circuit (for small currents in 5uA range) and I found that I can use lm4041 adj with 2 pnp transistors to build it. (2n2905 & 2n3964)

However, for some reason I can not use these 2 transistors (there is no smd version and their size is not suitable for the PCB - I have some restrictions on the size -). so I was wondering if there is any equivalent for them. or at least why were they chosen specifically in the datasheet? (what are the required features that I should look for to replace them?)

  • 0
    TI__Expert 4980 points

    Hello,

    In many cases bipolar devices are adequate and may be preferred due to their low cost and availability. With a bipolar device, the base current will add error to the output signal as discussed in the cascoding section. Using a Darlington-connected bipolar device, as is seen in the LM4041-N datasheet, feeds the error current back into the signal path and reduces the error by the forward current gain (beta) of the input transistor.

    However, in your case if size is the main concern, you could consider a FET. MOSFETS can minimize the output error because of their low gate current. It would be important to consider the current draw you are expecting when making this selection.

    We have this app note which goes into depth on current source and sink design using our devices:

    https://www.ti.com/lit/an/snoaa46/snoaa46.pdf (see Section 3.2 for LM4041-N)

    Thanks,

    Jackson