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?


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    \$\begingroup\$ 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.) \$\endgroup\$ – Brian Drummond May 4 at 14:06
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    \$\begingroup\$ 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. \$\endgroup\$ – Bimpelrekkie May 4 at 14:38
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    \$\begingroup\$ @BrianDrummond you should post your comment as an answer. \$\endgroup\$ – The Photon May 4 at 14:48
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    \$\begingroup\$ Yes, just google, constant current source mosfet. Or mosfet current mirror. \$\endgroup\$ – Aaron May 4 at 19:31
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    \$\begingroup\$ @Marcos See example 1 and example 2. \$\endgroup\$ – jonk May 5 at 4:07

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