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Simple constant current source
MUFF WIGGLER Forum Index -> Music Tech DIY  
Author Simple constant current source
555x555
I’m looking for a simple constant current source where the drain has compliance, and where rail voltage is not heavily relied upon. Ideally, it would need no adjustment and be accurate to within, say, 50uA in a 50-500uA range. I tried a JFET but got different results than with simulation—these current sources are simple, but should be calibrated so really I just wasn’t rewarded for an irrational hope, lol. A two-BJT current mirror with a resistor into a fixed voltage to set the current would work, but then wastes 2x the current, plus two transistors per source...just seems like there is a better way.
guest
2 transistor current mirror is probably the cheapest/smallest. other options:

1. opamp+trasnsitor
2. lm13700
3. voltage regulator/reference set up as current source
4. single transistor, base tied to ground with emitter+resistor to +12V (if you only need negative voltage compliance).
5. at higher currents you can use LED constant current ICs.

do you need a lot of them for your circuit?
Grumble
opamp + current mirror.
555x555
Thanks.

@guest I only need two of them, but the board is very full so I wouldn’t have room for another opamp, for example. I might go with the voltage-regulator as current source...maybe a bit more complex in terms of money or internal complexity, but this should allow for a high PSRR and a small footprint.
555x555
hmmm...seems like the minimum load requirements and/or quiescent current are a problem for the voltage regulator solution...
guest
are the currents on both sources identical? is this SMT or THMT?
555x555
THT and not identical. Might be able to redesign to make them identical and then use a mirror with an extra transistor for #2.

OK: I think I’m just going with the two transistor solution. Mirrored at +5V reference, resistor to ground setting the current. I’d rather pull the current from V+, but oh well.

It just seems to me there ought to be a simpler way...idk why I’m still thinking that.
guest
the LM334 is the classic solution, but it has temperature dependencies (which can be compensated out with a diode). if the power consumption is your only concern with a 3 terminal voltage regulator solution, they do make micropower varieties:

https://www.digikey.com/product-detail/en/microchip-technology/MCP1700 -3001E-TO/MCP1700-3001E-TO-ND/2651274

these are 4uA quiescent.

the frequency response of the voltage regulator probably isnt as good as a current mirror, but its regulation is probably better.
Synthiq
Here is another relatively simple alternative if both currents have the same polarity.The currents in output transistors Q2 and Q3 are set by the emitter resistances and can be set independently. The currents doesn't change too much over supply voltage and temperature.

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