Electronic – Constant current source drop

constantcurrent

I need some help understanding a little (I guess) thing here.

I was trying to understand a constant current source but something happened that I can't understand.

When I did a simulation in LTspice and I applied 16V or more, like 30V or even more, it works fine I can get around 10mA or very close (at 50V I have 11.2mA for example) but once my Vsource starts to decrease the voltage under 16V I got a bad stability, my (Vbe) starts to drop drastically.

Can someone explain why?

Check images below

*Circuit designed by owel.codes

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Best Answer

Your load requires 15V to maintain 10mA (10mA x 1.5k).

Below about 16V there is no voltage available across Q2 for it to control the current so the current has to drop.

As @mkeith points out the circuit cannot boost the input voltage.

With all constant current circuits, there is a parameter usually called the "Compliance Voltage" which is the range of voltages across the load for which the current is constant.

In this case, the compliance voltage is about 1V less than the input voltage.

With a lower value load resistance the current would be constant down to a lower voltage.

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