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It's always handy to have a few constant-current injection circuits in one's bag of tricks, so here are just a few that should be at least considered when the occasion calls for one:
The junction FET circuit was a little troublesome at first. It wanted to oscillate. That's why the 0.01 µF is there to quiet it down. The response time was a little slow, but for the application at hand, that didn't matter.
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Those are fine* if the load is benign and you are not concerned about output impedance at high frequency. A ballasted Wilson-style current mirror is usually better in both respects, and you can (still) buy ICs with matched transistors. Alternatively, you could use a cascode with these circuits, but this is more complex and you would lose headroom
*I'm assuming of course that you would substitute the multiple reference Voltage sources with a pair of resistors to a single variable reference
Posted by: George Storm | March 11, 2011 at 05:11 AM
P.S. The usual description of such a circuit would be "constant current source"
Posted by: George Storm | March 11, 2011 at 05:14 AM
Have you looked at precision, temperature drift and bandwidth? And sensitivity to EMC etc?
We manufacture high precision analog Hall sensors for strong fields (6 digits range). As our customers need to make the current source themselves (usually far below 1 mA, some use 5 mA or even 10), it is good that they can find circuits like this on the internet.
I do agree with George Storm: only one reference should be used for optimal temperature behaviour and of course costs.
Thanks for this contribution.
Pieter Hoeben
Posted by: PieterHoeben | March 12, 2011 at 07:07 AM
Hi, George and Pieter.
The single-op-amp current source, so far as know, is shown here as it was presented by Professor Howland at M.I.T. back in 1959.
I put the dual-op-amp based current source into service in amodification to the U.S. Navy V.A.S.T. Microwave Average Power Meter, BB47 to drive thermistor bridges back in 1971 and the op-amp-FET circuit into A/D service in the Bertan High Voltage B-HiVE modules in 1979. In both cases, all went well.
They are by no means the only current sources I've ever used (there are more) nor are they ideal for all cases. They were simply handy to have had around in their respective applications.
Also, the phrase "constant current source" is also correct and that one really goes back a long way. Please see, from September 1955, http://pubs.acs.org/doi/abs/10.1021/ac60105a017
All the best.
Posted by: John Dunn | March 12, 2011 at 07:58 AM