Constant current to capacifor / rising voltage

The op amp circuit is OK, but would be better with a reference voltage (TL431 gives a good 2.5V value) instead of R1, and a PMOS transistor instead of the PNP 2N3906.

For best compliance, use +30V or so (the TL082 can take it).

But, if you want REALLY slow, this is about ramping up the voltage on a capacitor? An integrator (current source into op amp(-), capacitor from output to op amp (-), and opamp (+) grounded) gives op amp output that follows the slow ramp rule.

It only takes a voltage source and resistor to drive that circuit, because the op amp (-) stays at GND due to the feedback, so a resistor and known voltage IS an accurate current source for that load. I've made ramp generators that run for days by feeding low voltage (0.04V) into a few-megohms resistor in such a circuit.

The drawbacks: after it hits the power supply limit, use a reset switch (across the capacitor) to discharge it. And, the 'grounded' op amp (+) input terminal ought to be connected to GND through a resistor that matches the current source resistor... that compensates for any bias current.

infineon BCV61 ?

I'm not yet awake.

Gerhard

BCV-xxx has a lot of 2-transistor thingies.

Motorola - ON Semiconductor - does seem to do monolithic duals with close matching. I pulled this one off the elemt-14/Farnell/Newark web-site.

Analog Devices used to sell tightly matched dual transistors but they weren't cheap.

That's because EEs more often use ICs than actually design them. Few if any op amps are made without a few current mirror sources.

LM13700 is a dual packaged IC mirror, if you drive the differential input with 2 or 3 volts. You get your choice of current sense, positive or negative, from a negative-rail current receiver.

That seems to be a genuine matched-pair monolithic dual, unlike most dual transistors that are two dies in a package. And it's available and cheap.

Thermal image of a dual transistor:

I do some of my best work when I'm asleep. That's sure easy.

It's 3 AM here and I woke up with an idea to research. I'm hungry too.

Look up 'Art of Electronics'. Older editions are available on-line.

RL

Your followup motivated me to search for an online AoE 2nd edition. It was my intention to reminisce about the 2nd's grey backgrounded, bright bulbed, "Circuit ideas" before launching into a full blown gripe about how AoE II's "ultraprecise current source" schematic shown in Figure

The legality of archive's AoE III is unknown to me.

With preliminaries now out of the way, allow me to note my semi-serious quest to create a current source from a 555. Could such a circuit be the cat's meow, or what?

Danke,

Nah, they're two separate dice. Almost no thermal coupling to speak of--there's a spec in there that tells you how much dissipation you can have in the output device before it runs away.

We had a very detailed thread on that some time back, but iirc there was enough info to calculate that the thermal resistance die-to-die was no better than the junction-to-ambient, i.e. 300 K/S or so.

Cheers

Phil Hobbs

And if you don't mind 0.5 MHz bandwidth on a good day. ;)

Cheers

Phil Hobbs

A PWM current source isn't too hard, I shouldn't think. A _linear_ current source, now, that's the ticket. ;)

Cheers

Phil Hobbs

That seems to be true of the Infineon and Nexperia parts but Motorola - ON Semiconductor - does seem to do monolithic duals with close matching. I pulled this one off the elemt-14/Farnell/Newark web-site.

The compound semiconductor transsotrs that you and John Larkin carry on about from tine to time - with the 300V Early voltage asnd tiny collector base capacitance might well work well as the cascode part in a Wilson three transistor current mirror or the four transistor elaboration of it.

Exact is a relative term; thermally linked is also a relative term, you can glue two TO92's together and they ARE linked, thermally and physically: it works better with C1841 NPN transistors than with PN2222's, because the Japanese case has its metal tab oriented differently.

That's a packaging issue.

Discrete transistor designs have the collector as the substrate, so you can't easily electrically separate collectors of two transistors mounted to the same metal plate. To make a good-quality dual, epitaxial silicon grown over oxide can be electrically isolated, OR you need a seven-pin package, so you can bias the substrate (as an IC would do) to create a depletion region. Things like LM13700 have the specified bias on that most-negative pin in order to work, as did the (hard-to-find) transistor multiples of yesteryear (CA3046, anyone?).

A pure-silicon current mirror runs one transistor at 0.6ish volts and low dissipation, and the other at whatever Vce and higher dissipation, so tight thermal coupling is mandatory.

Really good monolithic pairs have many equivalent transistors interleaved in a tricky pattern.

I guess you could bolt two TO-220s or TO-247s metal-to-metal with a thin AlN insulator.

Trouble is, thermal conduction is proportional to the temperature gradient, and there’s no way for a spreader to make the heat flow preferentially into the cooler die.

They really need to be interleaved on a single chip for good matching above, say, 50 uW dissipation.

That’s not at all hard to do, but virtually no one does it anymore.

There are various semicustom array offerings, but the devices available aren’t exactly cutting edge, unfortunately.

Cheers

Phil Hobbs

It takes four transistors to cascode that problem out of existence.

But, that's because they're on complex ICs that have heat sources all around; and those IC transistors are TERRIBLE performers; usually beta of about 40.

Better ways exist; some of the surface mount duals are in very thin packages, less than half a mm thick, and you can glue a heat spreader on top of 'em. Two TO220 tabs will keep the dice two or three mm apart, and you have equal temperature AFTER heating up both tabs... heat doesn't just conduct, it diffuses (long time delay for heating the tabs all the way through).

Riiigghhht. All bipolar IC processes are the same, which is why the uA741 is still the best performing part.

Heat always flows from warmer to cooler.

Two giant chips will hardly heat up, and the thermal conductivity between them will be huge. There wouldn't be much gradient.

Not that I plan to build any discrete current mirrors.

A three transistor Wilson current mirror doesn't.,

The Wilson current mirror minimises the problem by the kind of intelligent design you don't seem to be able to follow.

Very wise. If you can't understand why the Wilson current mirror solves most of the problems you are complaining about you should restrict yourself to approaches you can understand.