Constant current source drop

Question

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

Answer 1

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.

Answer 2

You probably want to take a look at a DC Sweep simulation, which is very easy to do in CircuitLab:

^{simulate this circuit – Schematic created using CircuitLab}

If we run a sweep over V1.V we can see how the output current I(RL.nA) varies with input voltage:

From the top subplot, we can see that we get a mostly-flat current when Vcc > 18 or so. But when Vcc < 18 (approximately), we're on a very different part of the curve, where there's no constant-current behavior.

On the bottom subplot I've plotted Q2's Vbc voltage V(Q2.nB)-V(Q2.nC). Q2 is PNP so we want to keep Vbc > 0 in order to keep the collector-base junction reverse biased, as it should be in the active region. However, the DC sweep shows that if we reduce Vcc below 18 or so, Q2's base-collector junction becomes forward biased and starts conducting, putting the transistor in saturation.

You can open the simulation above and add other voltages or currents to the DC Sweep plot, and they may help you find the intuition you're looking for.

Answer 3

Just a minor tip. You can use a "DC Sweep" simulation to check which voltage is required for the circuit to work as a constant current source: