1mA Constant Current Source?

OBW0549

Joined Mar 2, 2015
3,509
1. R2 is way too large, as it will only allow about 12 μA through ZD1; while ZD1 will "kinda-sorta" function as a Zener diode at that current, it won't be very stable or predictable. I'd make R2 about 1.2 kΩ which will give you about a mA of bias current for ZD1.

2. Since Vbe of a transistor has a negative temperature coefficient of about -2 mV/°C, there will be a small positive temperature coefficient to the 1 mA output current (800 nA/°C, roughly).

Other than those two things, I can't see anything that stands out unless you're out to achieve very high precision.
 

dl324

Joined Mar 30, 2015
11,282
What are the pitfalls of the 1mA constant current source shown below?
As previously noted, the zener current is too low for the zener to regulate voltage.

What are the requirements? Will supply voltage vary? Do you require temperature stability? Do you need exactly 1mA? How much headroom do you need? ...
 

crutschow

Joined Mar 14, 2008
25,267
You can use a TL431 voltage reference to make a fairly low-cost and stable current sink as shown here (Figure 14).
Unfortunately there's no easy way to make it into a high-side current source to a grounded load.
 

OBW0549

Joined Mar 2, 2015
3,509
Another approach you might consider is a simple current mirror, as in the attached. R1 sets the output current, while emitter ballast resistors R2 and R3 mitigate the effects of any Vbe mismatch between Q1 and Q2. Assuming the +4.5V supply doesn't vary, output current will be constant within a couple percent over a load voltage range of 0V up to within a couple of hundred millivolts of the supply voltage.

Mirror.png

Mirror.png
 
Last edited:

atferrari

Joined Jan 6, 2004
4,069
Maybe not what you have in mind.

I played with several variations. This is one, bipolar, from the Walt Jung's book, with buffered Vsens.

Sure it must be simplified and could be to a high degree. The unconnected resistor is there to play with two ranges of input signal. Worked OK. Sensible to spikes in Vdd. Solved with good filtering.

Para tracecom 01.png

/EDIT to add:

Load is R31 which for tests I made 4 Ohms.

EDIT/
 

dl324

Joined Mar 30, 2015
11,282
If supply voltage will vary, you could combine the TL431 reference from @crutschow with the current mirror from @OBW0549; replacing the ballast resistors and 2 transistors with a dual transistor that would give better matching.
 
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