Voltage controlled current source

Assuming you mean the LM324, keep in mind that its input current can exceed 100nA so the lower ranges might not be accurate enough.

P-channel? Any old small FET would do, depending on how much compliance range you need. My usual standby is the BSS84. For the opamp I usually use the LM324 as well but with a resistive divider because it can't get close enough to the positive rail.

"Small" mosfet or jfet is the operant word, because ordinary mosfets, like the one in the EDN article, are "large" in the sense that they can conduct amps of current, etc. They have a high gate capacitance, Ciss, crippling their performance at low currents. The OP can use a medium-sized mosfet, like the bss84, with Ciss = 30pF (not that low), but for 0.1 to 0.5mA currents he'll likely be better off using some common small PNP transistor, like the 2n3906, instead.

Yes, I go for BJT's as well. But for the usual reason: They cost a couple of pennies less ;-)

BJTs do introduce a small error though. Their hfe often ain't that great at low currents and the base current enters into the equation because it also flows across the sense resistor.

Thank you all for your help. When you say you'd choose a BJT instead, is it because of the smaller input capacitance? Or higher transconductance? I thought since the opamp already has a large open- loop gain, the gain of the transistor shouldn't matter so much. Am I right?

My application needs

I'd say yes, inside the feedback loop, the mosfet's low transconductance isn't an issue. Actually, at low currents the mosfet's transconductance/current ratio improves dramatically, but it doesn't get as good as a BJT, with gm/Ic = q/kT = 1/25mV at 25C.

Then consider useing a Darlington transistor, or wire two ordinary ones in the Darlington configuration.

The LM324 input bias current, which is the spec you should be concerned with, can be as high as 250nA at room temp, according to the datasheets by ON Semi, Fairchild, TI, etc. NSC's LM324A datasheet says 100nA but if you're serious about that parameter, I'd select a better opamp.

There are two current-source issues having to do with speed, capacitance, and MOSFETs. The first is gate capacitance, which will determine how fast the opamp feedback loop can be, and how much capacitive gate current shows up on the sense resistor, causing an error at high frequencies. The second is the drain- source output capacitance, which will make a current source look like a voltage source at high frequencies. A BJT has Cobo = 4.5pF max, 3pF typ above 5V. By comparison, a bss84 mosfet, being a reasonably-small part, has Coss = 10pF typ, which is about 3x higher. Other common power MOSFETs will be much higher yet.

Consider, 10pF at 100kHz looks like 160k ohms, which can be an issue if your goal is 100nA predictability.

So, as you can see, there are tradeoffs on each side considering BJT vs MOSFET for low-current circuits.

Or: -------------+--------+--- ! ! !/e ! --------! PNP ! !/ !\\ !/ ------> Repeat if needed ---! NPN -------! NPN !\\e !\\e ! ! ----+----------------+----

The EB voltage is the same as the single transistor and the HFE can be really huge

Thank you (and MooseFET), I'll look into the circuits you suggested.

For my constant current sources, I'm actually using LM234 (3-terminal adjustable current source), not LM324 after all. Sorry about all the confusion! Any comments on these parts?

Thanks a log,

Scott

Keep in mind that they are temperature-dependent and that the inityial accuracy is only 3%.

I just put three of those into the wood stove :-)

Also, mind the 7mV or so offset voltage. This adds to the total error and its impact will be larger for smaller voltages across the sense resistor. With Scott's specs I'd pick a better one.