Low current source

Walter Harley · 2005-09-15T22:24:57+00:00

What is a good design for a current source on the order of 50nA, preferably adjustable via a control voltage or variable resistor?In the classic two-transistor circuit shown below, to get 50nA R1 needs to be on the order of 10M, a bit awkward if I want a trimpot rather than a fixed resistor. To get accuracy over varying supply voltage, R2 needs to be at least 10 times R1; 100M resistors are available, but not in my junkbox. ___ I -> ------------ ---o---|___|--o-- V- \ ^ | R1 | --- | | | | | ___ | --- | V+ ---|___|-----o---/ v---------' R2A JFET with resistor from gate to source needs an even higher resistance to go as low as 50nA, because Vgs >> Vbe; and because of the manufacturing spread it is hard to predict the resistance needed. And JFET current sources aren't as "stiff" as the BJT circuit. I -> ------+ +---------o---- V- --- | ^ | | ___ | '---|___|--' R1A Howland current source, using an opamp, also seems awkward; you can use lower value resistors by using a lower control voltage, but if you go too low you run into Vos. Opamps with very low Vos tend to either have relatively high Ib, or relatively high cost.All of those approaches share the same underlying problem: they try to generate a current by putting a relatively large voltage (on the order of 1V) across a resistor.Is there a better way, using standard readily-available components, to make a current source in this range? Maybe a dividing current mirror, or something based explicitly on the exponential nature of the Ic vs. Vbe relationship (e.g., a log converter)?Thanks, -Walter

F

Fred Bloggs 20 years ago

I think that gets you 5V out- it was 5 seconds max slew time and 1V amplitude: View in a fixed-width font such as Courier.

. . . . . 0.33U . +15V +----+----------||-----+ . | | | | . | --10K | | . [1.5M] | / | | | . +--[10K]---+-----|+\\ | | \\ | | | . | | | >+-||-->| / | | | . gnd | +-|-/ | W- | \\ | | | . | | | --- +--------|-\\ | . | | | | | | >-----+ . | | 0.1u| +--------+----|+/ | . Vin>-[9.1K]-|---+-||--+ | | | | . | | | --- | | [1K] . CMOS | | | | / | | | | . IN | [1k] | W+ | \\ | | +-----|>|-----+ . | | +-|>|-+--|| / | | | | . | | | | \\ | | | +-|

Vote

F

Fred Bartoli 20 years ago

"Fred Bloggs" a écrit dans le message de news: snipped-for-privacy@nospam.com...

In that case, if you notice that the A & B points voltages are V(W) +/- one diode drop, then the current sources can even more be simplified and reduced to a simple resistor. I'll also fixe the current divider ratio to say about 50. Then I'll delete the voltage reference and switch the input to the other opamp input. Then we arrive at this simple schematics with Vin being directly a 0/5V logic CMOS output.

+----------------[R]-----------------------------+ | 4.7M | | ___ | | .--|___|-+--+15V | | | A | | | | | | | | .-. ---' | | 100K| | | | | | | | '-' 1M | GND--[R]-+--|+\\ | W ___ C | | >--+--||-+------+------|___|-+--||--+->Vout Vin>-[R]--+-|-/ | A | | | | | | | .-. | | [R] | B | | | | | | +--|>|-+-|-+ GND .-. | +-|+/ 100K| | | | | | GND GND '-' 4.7M | ___ '--|___|-+-- -15V A | | | ---'

Thanks, Fred.

Vote

F

Fred Bloggs 20 years ago

That is a shame- the TL082 has way too much Vio- the pot current was so linear....it can be circumvented but just more parts. Your method is better- maybe some cheap transistor cascodes and the pot at your current src: View in a fixed-width font such as Courier.

. . . . +--|

Vote

F

Fred Bloggs 20 years ago

Maybe this then: View in a fixed-width font such as Courier.

. . +--|-[9.1K]-|---+-||--+ | | | | . | | | [18K] | | [1K] . CMOS | | | | | | | . IN | [1k] +-|>|-+--||-----+ . | | | | | . | | | | +-|

Vote

F

Fred Bloggs 20 years ago

fixing that sink error: View in a fixed-width font such as Courier.

. . +--|-[9.1K]-|---+-||--+ | | | | . | | | [18K] | | [1K] . CMOS | | | | | | | . IN | [1k] +-|>|-+--||-----+ . | | | | | . | | | | +-|

Vote

F

Fred Bartoli 20 years ago

"Walter Harley" a écrit dans le message de news: snipped-for-privacy@speakeasy.net...

really

the

a

Then, to have just one trimpot, you can simply buffer one with another:

1M ( IC(Q1) ~ 10uA ) ___ +Vs-----|___|---+-----+------. | | | | .-. | | | |1M | | | | | Q1 | '-' | >| | | 2N5087 |---+ | /| | | | | .-. | | | | === | | | 22M GND | '-' >| | 2N5087 |----+--->50nA /| Q2 | | === GND

but this doubles the trimpot value, so you can do:

1M ___ +Vs-----|___|-+---------+------. | | | | | .-. | | | | | | | | 10M | | '-' | >| | | 2N5087 |----+--->50nA | /| B | Q2 | \\| | 2N5089 |-------+ 0.4V

Thanks, Fred.

Vote

F

Fred Bartoli 20 years ago

"Fred Bloggs" a écrit dans le message de news: snipped-for-privacy@nospam.com...

one

reduced

delete

Hmmm, it'll work but in case of RadioShack components I guess I'll keep my fixed 18K-1M current divider and variable current sources: the 10K trimpots will produce non negligible current offsets due to the output opamp offset (potentially several mv). The 1M resistor reduces this current offset to a low enough value that you won't noptice its effect.

Thanks, Fred.

Vote

W

Winfield Hill 20 years ago

You move quickly Walter, thanks for that story. Have fun with your radio station. I spent my summer before starting college designing and building a main-console mixer (this was 1960, so I used tubes!) for a small community FM station, so I have some idea how much fun that can be. In my mixer, all ramp-ups and ramp-downs were by hand, by rotating big Gates-style knobs!

Thanks, - Win

Vote

Low current source

Join the Discussion

Didn't find your answer?