Electronics > Beginners
BJT/mosfet/jfet minimal usable current?
exe:
Hello!
I'm designing a low current source (from 100nA to 10mA, may be in multiple ranges). I'm trying to choose best pass element for the circuit. So, candidates are:
1) bjt
2) mosfet
2.5) jfet
3) no pass element at all, use opamp output directly
4) use resistor as "pass" element and pull voltage down with an opamp
My question is: are bjts and mosfets suitable for low currents? (assuming minimum required current is above leakage current). Like, any noise, bandwidth? What Hfe/transconductance to expect? I use low-power devices, like 2n3904 and 2n2700. I also have some small signal jfets like j112.
Berni:
Nothing wrong with simply using the output of an opamp at these low currents.
Tho the compliance voltage also plays a big part in it. Its a big difference being able to source current at 1mA 5V and another sourcing 1mA 1000V
With the <1uA currents you will need careful design of your circuit to be able to accurately measure and regulate such a low current. And what kind of current source it is matters too. You can have current sources that only sink to ground, ones that can supply only positive voltages into a load or even 4 quadrant sources where its possible to sink or source current at a positive or negative output voltage. Also high or low side current measurement can have a big impact on design.
exe:
Thank you very much for the answer. Compliance voltage is 12V because I'm afraid of working with high voltages. Originally I wanted a high-side current sensing, but now I'm more into low-side sensing. This will make voltage regulation a bit harder, but I don't need much precision. I'd like to control voltage and current from DAC, so reference voltages are referenced to ground. The whole project is just for fun.
PS I wonder how good tl072 for such applications (I don't have a better opamp at hand with high-impedance inputs).
T3sl4co1l:
Amp will be limited by noise in a single range; with switched ranges (i.e., for a Howland current pump*, a selectable series output resistor), a TL072 wouldn't be bad. I'd think that full range should be reasonable with it.
*The traditional form of this is with symmetrical resistors, but that fully halves the compliance range. Fortunately, the "top" resistors can be reduced proportionally (or something like that, I forget?) to give the same result with compliance range arbitrarily close to the amp's actual output range. Note that this exaggerates errors (increased noise gain), making the output noisier, and making trimming attractive (for amp offset and resistor tolerance).
Otherwise, there are small JFETs, like PN4116 series, which would actually have trouble reaching 10mA so you might pick something not quite as tiny, but that are guaranteed for very low leakage. Small BJTs have good performance, but aren't often rated for it; for example a typical 2N4401 leaks nanoamps at room temperature, but you'll never find a datasheet that agrees. Apparently some RF types are quite good (which makes sense from the small junction size), but be careful about voltage rating (it's often low; that's part of how they get the speed up) and parasitic oscillation (probably use a ferrite bead or resistor in series with the base/gate).
MOSFETs are similar, usually pretty good but not always guaranteed as such, and smaller is better. 2N7000 is pretty much your introductory jellybean MOSFET, and typically leaks ~nA when fully cut off. (Note that subthreshold current flow applies -- there isn't a hard pinch-off for MOSFETs, the drain current just keeps falling exponentially below the quadratic current flow region. Vgs(th) is defined at some modest current in this range; expect to need Vgs(off) much lower than this. For enhancement mode MOSFETs, drain current is usually low enough to be considered leakage current, for Vgs near zero -- no negative bias needed.)
And not that you would, but just for completeness: vacuum tubes leak quite a lot, I think due to their hot operation (weak emission from everywhere, not just the intended cathode surface). The dynamic range (cutoff to full conduction) isn't very good, certainly fewer orders of magnitude than you are asking for here. Though it would be interesting, I guess, to think of ways to trick it into working anyway. Like, the heater voltage could be reduced on the low ranges, to reduce current capacity and gain, and hopefully leakage. Or maybe a high voltage or RF type gives lower leakage, again in normal or reduced operation. If very low bandwidth is acceptable, a high voltage diode could certainly be used, varying the heater voltage to set output current (the thermal time constant of such a heater is fractional seconds).
Tim
MagicSmoker:
I almost dashed off a reply that a Howland current source/sink using a CMOS (or JFET) input op-amp would be much better than a discrete transistor based current source, but then I remembered that the effective output impedance depends on the matching of the resistor ratios as well as their absolute values, and to achieve the effective Zout of 120M (12V / 100nA) would require impractically precise matching and/or high resistance values (defined by me as higher than 10M).
The below circuit at Analog Devices looks more practical, though cleanliness of the pcb and guard rings will almost certainly be needed if any kind of accuracy at 100nA is required:
https://www.analog.com/en/design-center/reference-designs/circuit-collections/precision-nanoamp-bidirectional-current-source.html#cc-overview
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