Low current source

A noisy leaky-input opamp that illustrates Walter's last comment.

All this dancing around and purchasing specialized parts, just to help Walter avoid purchasing some high-value resistors, sheesh!

Whew, 120pA, that's massive! 15uV is nice, but what a price to pay.

But I think opamps are a good way to go; Here's my entry, which has two opamps. One is wired as a difference amplifier, and if one uses an INA105 he'll get the required four precision resistors all ready to go. My circuit uses one of NSC's LMC6061 low-offset CMOS series of opamps, which are limited to 16V supply total, hence the unusual supply voltages, but no problem, just whip out some of those three- terminal regulators.

. Win's Programmable Current Source . . +25V . __|_ __ INA105 . V -->----|+| \\/ | . Prg | | >--+-------------, . ,--|-|__/\\ | . | | \\ +13V Rprg . GND -3V | _| | . | / +|--10k--+----O -->

. Monitor ---

You shouldn't hesitate to get some high-value resistors in your junkbox, in my opinion, but it's certainly possible to create very small currents without their use, if you wish. The trick is to take advantage of the Ebers-Moll formula, which tells you that (at room temperature) for every 58mV you reduce the base-emitter voltage the collector current will drop by 10x. If you take two matched PNP transistors (Analog Devices MAT03), wire one as a diode and bias it at 5uA, to get an Ebers-Moll reference voltage, and operate the second transistor's base from the first, minus 116mV, it'll put out 50nA for you. Piece of cake! If you use a say 20k trimpot in place of the 13k resistor, you can adjust the output current from 5nA to 5uA. The circuit draws 14uA from a fresh 9V battery.

. 50nA current source without high-value resistors . . 9V --+--- 13k ---+--- 1.0M ---, . | | | . e e gnd . b --+-- b . c | c MAT03 . | | | . +-----' '----------O -->

. | 50nA . 1.8M . | . gnd

Now, tell us why you need 50nA. What're ya goin' to do with it?

--
 Thanks,
    - Win

The LMC6000 series of CMOS opamps do have amazing low-leakage input-protection diodes, but they're small and vulnerable. The 10k resistor limits input current during over-voltage, to help protect the protection diodes.

--
 Thanks,
    - Win

Walter Harley schrieb:

What about using a CMOS chopper opamp (like good ol' TC7650, or TC913) as inverting (floating) current source?

Using an input voltage of 50 mV with an input resistor of 1M, you get

50 nA in the feedback path. The TC913, for example, is specified with max 15µV Vos and an input bias current of max 120 pA at 25°C - both should be sufficient.

--
Dipl.-Ing. Tilmann Reh
http://www.autometer.de - Elektronik nach Maß.

No claims, period ;-)

...Jim Thompson

--
|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
|  Analog/Mixed-Signal ASIC\'s and Discrete Systems  |    manus    |
|  Phoenix, Arizona            Voice:(480)460-2350  |             |
|  E-mail Address at Website     Fax:(480)460-2142  |  Brass Rat  |
|       http://www.analog-innovations.com           |    1962     |
             
I love to cook with wine.      Sometimes I even put it in the food.

Thanks, Win. I like both your solutions, but this is the one that seems simplest. (Tilmann, your post hasn't shown up on my news server yet, so I can't respond to it yet.)

This circuit is sensitive to the power supply voltage, but that is probably okay for my purposes. If I was worried about it, I could replace the 1.8M and 1.0M resistors with current sources. I'd then have three different ways to program the current: linearly with the "1.8M" current source, and exponentially with the 13k resistor or the "1.0M" current source.

I did flirt with the circuit you show in fig 2.52 of AoE, the Wilson current mirror with emitter resistor on the load side of the matched pair. But in that circuit, the programming transistor's base current flows into the load, so it's not good for high divisors. And then I went to bed :-)

It's probably not a good technique for my present problem, it's just something that I came across while I was solving a problem, and I didn't know a good answer so I figured I'd ask.

The problem that triggered the question is that I need a linear voltage ramp from 0 to about 1V and back down, over an adjustable period from 0.1s to 5s, with the ascent and descent rates separately adjustable. So, my first thought was to switch a capacitor between two current sources (with voltage clamps). I don't like the idea of using electrolytics for timing. 50nA into a 0.47uF cap charges 1V in about 10 seconds.

Since I can have +/-15V supplies, even using a current source at all is probably overkill; a high-valued resistor to the supply would give a reasonably linear charge, between 0V and 1V. But that would still leave me the problem of finding a 300M trimpot :-)

A "modern" approach would be to use a microcontroller and a DAC; 8-bit resolution is fine for what I need. I bought the equipment to start fooling around with microcontrollers a couple years ago, and loaned it all to a friend... Probably time to get it back.

FWIW, the reason for the ramp is to control a stereo audio VCA chip. In the end what this is all about is making a device that fades an audio signal down, and then back up, in response to a logic signal; it's for a little community radio station that runs on autopilot overnight. So yet another approach would be to use a digital potentiometer. But those have the problem of "zipper noise" when used as faders.

Thanks, -walter

The JVC radio I'm listening to has a motor-driven pot.

...Jim Thompson

--
|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
|  Analog/Mixed-Signal ASIC\'s and Discrete Systems  |    manus    |
|  Phoenix, Arizona            Voice:(480)460-2350  |             |
|  E-mail Address at Website     Fax:(480)460-2142  |  Brass Rat  |
|       http://www.analog-innovations.com           |    1962     |
             
I love to cook with wine.      Sometimes I even put it in the food.

Thanks. Yes, that would be another good way to make a voltage ramp. I would have to think a little about how to clamp the voltages at the desired end points, without introducing leakage across the capacitor.

I asked how to make an adjustable low-current current source without using high-value resistors, because that was what I wanted to learn. It is true that it does not happen to be especially relevant to the task at hand; but it was an interesting (to me) question nonetheless.

For increasing and decreasing audio volume with voltage-controlled gain, sure, you don't need precision. Neither do you need matched IC transistors, just select two matched discrete parts (testing at low currents to avoid thermal effects). If you need a little more accuracy you can stabilize the voltages with a 2.5V reference, and if you want you can add a third non-critical PNP to eliminate Early effect errors by operating the matched transistors at similar Vce.

The output of such a circuit is PTAT. That is, assuming the top two BJTs are at the same ambient temperature, the resulting current is proportional-to-absolute-temperature, or about +0.3%/C, as you can see by playing with the Ebers-Moll equations. If that was an issue, you could use a PTC resistor for R1. I doubt you need any of this.

. R1 R2 . V+ --+---+--- 13k ---+--- 270k ---, . | | | | . | e e -2.5V . | b --+-- b . \\_|_ c | c matched . /_\\ | | | pnp pair . | +-----' | . 2.5V | | | . | 100k | . | | e cascode . | +-------- b pnp . | | c . | 300k '-----------O -->

. | | 50nA . +---+----- -2.5V . | . '-- R_bias -- gnd

--
 Thanks,
    - Win

Yeah, I actually did think about that alternative; it would be appealingly simple. I think they'd be worried about a motor-driven device, though, in terms of long-term reliability. The device's function is to enable legal IDs at night when no board operator is around; a worst-case failure could equate to a lost license. Even in the best case, over time the pot will get scratchy.

One advantage of the VCA approach is that the broadcast mixing consoles themselves generally use VCAs. So it's familiar technology for the chief engineer and thus easy to get approved. I can show him the section of the console schematic that I copied the circuit from :-)

But also, the motor-driven pots I've seen have a fixed rotational rate, and aren't usually fast enough to go from zero to full on in less than a couple of seconds.

What are you using for a VCA, maybe from

??

Why not just use a DAC, and vary the clock rate, to generate your ramp?

I'm leery of using low currents in the "outside world". They're fine on-chip, but in the "outside world" a dry fart will conduct 50nA ;-)

...Jim Thompson

--
|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
|  Analog/Mixed-Signal ASIC\'s and Discrete Systems  |    manus    |
|  Phoenix, Arizona            Voice:(480)460-2350  |             |
|  E-mail Address at Website     Fax:(480)460-2142  |  Brass Rat  |
|       http://www.analog-innovations.com           |    1962     |
             
I love to cook with wine.      Sometimes I even put it in the food.

In article , Winfield Hill wrote: [....] What's the 10K for ?

--
--
kensmith@rahul.net   forging knowledge

Not really, you grossly exaggerate.

--
 Thanks,
    - Win

Winfield Hill schrieb:

Please excuse me, but he did not mention *anything* except for the current itself. These types were just examples, there surely are better suited devices around meanwhile.

I think both chips are far more common and less specialized than the MAT03 you suggested for the BJT current source... :-) At least I can get them here very easy and cheap, even in really small quantities.

Note that this was the maximum specification. And there are better ones around, meanwhile. Besides, if it it was 120 pA, that's only 0.24% of the sourced current - and we still don't know anything about Walter's requirements.

At least here a 100M resistor is much harder to get than the opamps I mentioned... Especially if you don't accept 20% tolerance and horrible TC.

I think due to the many missing details in the OP, we simply took different approaches - I took the simple, very cheap one, since Walter complained about expensive parts, while you took the precise one.

As you already requested, Walter should provide some more details now...

--
Dipl.-Ing. Tilmann Reh
http://www.autometer.de - Elektronik nach Maß.

I like that, and it's easily modified for a logic-controlled slow rise and fall operation, which I think is what he wants:

. calculate R for . IC about 0.5/1uA . . +Vs -- R --+--|>|----+-----. . | silicon | | . --|

Either those, or SSM2164. THAT have large MOQ and no USA distributors, a problem for one-off projects.

Well, you guys eat more Mexican food down there than we do up here in the Northwest, I think. But I share your concern.

A uC and a DAC is a possible solution; in fact I just spent a while poking around datasheets trying to figure appropriate components, and I just finished coding the algorithm. There are two reasons I hesitate.

First, I want separate rates for ramp up and ramp down, so I can't just vary the clock. I want the rates settable by trimpots. So, I need to incorporate measuring the trimpots; usually AIUI the way that's done with uC's is to charge a cap with a current source, compare the trimpot voltage with the cap voltage, and count clock ticks. Sheesh, by that time, why not just use a current source for my ramp?

Second, the disadvantage of a uC is that if it fails for whatever reason in the field, it can't just be replaced; it has to be replaced and reprogrammed, effectively meaning that the manufacturer - me - is on the hook forever for replacement parts.

I could use discrete logic: two clocks with rates set by trimpot, switch between them with a gate, feeding an 8-bit up/down counter. Have to be careful to get the clocking right between the counter incrementing and the DAC latching, and make sure that the power-on resets the counter to zero.

-walter

I'll let it, if you'll measure it ;-)

...Jim Thompson

-- | James E.Thompson, P.E. | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona Voice:(480)460-2350 | | | E-mail Address at Website Fax:(480)460-2142 | Brass Rat | |

| 1962 | I love to cook with wine. Sometimes I even put it in the food.

You don't want current sources and a capacitor, instead think about using a capacitor in an opamp integrator configuration. Go ahead and use a low-cost chopper amp, and use low voltages to a modest-sized integrator resistor, trimpot-controlling the voltage to this resistor with conventional low-value stuff. You can create a slew-lmited circuit that goes from one voltage to another, as requested externally, but at the slow slew rate.

--
 Thanks,
    - Win

That's odd. They're located in Massachusetts. In fact, I vaguely recall buying a few of their chips about a year ago... Circuit Specialists ???

They're not very electrically conductive, but they do BURN ;-)

Using a DAC doesn't require a uC. Use 555's, or something similar, plus up-down counters.

...Jim Thompson

--
|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
|  Analog/Mixed-Signal ASIC\'s and Discrete Systems  |    manus    |
|  Phoenix, Arizona            Voice:(480)460-2350  |             |
|  E-mail Address at Website     Fax:(480)460-2142  |  Brass Rat  |
|       http://www.analog-innovations.com           |    1962     |
             
I love to cook with wine.      Sometimes I even put it in the food.