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I have a question regarding this constant current source circuit:

LED constant current source

I have seen this circuit with two BJTs (like in the figure), but sometimes T2 is a MOSFET (I assume for lower Rdson resistance and better switching performance).

However, my questions is: could both transistors T1 and T2 be MOSFETs? Is it recommendable or, would the circuit stop working properly?

Thanks!

Marcos
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    Vbe on a bipolar transistor is fairly predictable : thus, so is the current in R. Vgs on a MOSFET ... much less so. The circuit would work, but the current would vary widely between different MOSFETs. (As the circuit is linear, Rds(on) and switching are immaterial.) –  May 04 '20 at 14:06
  • Thanks for your comment. It is very helpful. You are right that in steady state, Rds(on) and switching do not make much sense. I was thinking about something like this slight modification of the circuit, where you can turn on and off the LED with an input signal: http://lednique.com/wp/wp-content/uploads/2017/11/Simple-constant-current-driver.png – Marcos May 04 '20 at 14:11
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    *would the circuit stop working properly?* It depends, if you just replace the NPNs with MOSFETs, there's a chance the circuit will stop working. If you choose the MOSFETs and resistor values carefully then something that works can be made. Maybe the current would vary more than when using NPNs, that does not need to be an issue depending on your needs. – Bimpelrekkie May 04 '20 at 14:38
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    @BrianDrummond you should post your comment as an answer. – The Photon May 04 '20 at 14:48
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    Yes, just google, constant current source mosfet. Or mosfet current mirror. – Aaron May 04 '20 at 19:31
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    @Marcos See [example 1](https://electronics.stackexchange.com/a/360220/38098) and [example 2](https://electronics.stackexchange.com/a/481317/38098). – jonk May 05 '20 at 04:07
  • Thank you all for your comments. @jonk, these two links explain everything and are extremely useful. Thanks for that. – Marcos May 06 '20 at 08:34
  • @Marcos No problem. I'm glad they helped. – jonk May 06 '20 at 08:35

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