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I am interested on knowing what would be a better solution to create a current source for low current applications (below 20mA), an opamp based current source such as a Howland current pump (link to PDF) using closely matched resistors, or something like a transistor based current source such as Wilson current mirror using matched transistors in a single package.

I am aware that there are a lot of factors that should be considered, to start with, the types of opamps used, the type of load, bjt vs fet, voltage compliance, etc... So in a general way, what I would like to know is if its worth looking into a transistor based solution, does it offer any advantages over opamp current sources in low current applications?, or should one just stick to opamps for simplicity?

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You will never get the ac and dc performance with the Wilson current mirror that the Howland current source can attain. Plus the Howland can sink and source current.

For the Wilson current pump, consider the mirroring BJT's constant current region - there is a voltage on the base set by the "other" BJT (that is acting like a diode) and you know that this sets a current into the base of both transistors - look how compliant the graph is - outside the saturation region. It's not really a good constant current source when Vce changes: -

enter image description here

The Howland current pump (on the other hand will have a compliance graph that is as flat as a pancake meaning you can swish Vce around and the collector current will remain really quite flat (dependent on resistor matching of course).

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  • \$\begingroup\$ That makes a lot of sense, however I had the idea, that the wilson current mirror, the one using a cascode, had that extra transistor to pin down the collector of the current setting transistor, thus eliminating the Early effect on that transistor, but I guess that the transistor on the cascode still has Early effect since it will have to vary its VCE to achieve constant current, am I right? Then in that case whats the advantage of the Wilson Current mirror vs a standard 2 transistor current mirror? \$\endgroup\$ – S.s. Mar 13 '14 at 4:37
  • \$\begingroup\$ By the time you have stacked up several layers of transistors (to get the output compliance) it's probably simpler to use an op-amp and single transistor as a current mirror or, the Howland current source but, you are correct, the W current mirror is a couple of orders of magnitude better than the two transistor type. \$\endgroup\$ – Andy aka Mar 13 '14 at 8:27
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The advantage of the Howland current source is that it can (1) both source and sink current (2) to (or from) a grounded load. The caveat is that it needs good resistor matching. See this Comprehensive app note on Howland current source and its variants.

Wilson current mirror can only sink current. It's made with a few transistors. Inside ICs, transistor matching is easier to achieve than with discrete transistors.

Sticking with OpAmps for simplicity is a good idea (unless circumstances dictate otherwise). Besides, there are other OpAmp constant current sources and sinks that are simpler than Howland.

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