accidental differential pairs

May 18, 2013 27 Replies

on

PG

il

just such a system together, and didn't have much trouble getting 0.1% accu racy.

uency converters - presumably for the slow A/D converter market - which sta y highly linear even at very low oscillation rates, which is nice when you have to cope with a large dynamic range.

second transistor inverted for minimum off-set voltage - slow as a wet wee k, but adequate for industrial control. Somebody else's idea, and I got stu ck with showing that it worked.

Not at all. I thought that it was a cute idea at the time, and was happy to have the job of reducing it to practice - which wasn't trivial. My boss's first approach didn't work at all well - too much difference in the propaga tion delay between the - different - switches he'd had in mind. Using the s ame switches in both halves of teh circuit - V-to-F and PWD - made it work quite a bit better.

In general I'm always happier to use an idea that has been shown to work th at one that somebody has dreamed up (even when that somebody is me). Less c hance of inconvenient surprises.

Bill Sloman, Sydney

150

poorly

Sure, if the swing is small enough. Though I suppose care mostly near zero amps anyway.

Cheers

Phil Hobbs

Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC Optics, Electro-optics, Photonics, Analog Electronics 160 North State Road #203 Briarcliff Manor NY 10510 USA +1 845 480 2058 hobbs at electrooptical dot net http://electrooptical.net

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The thermal compensation is almost independent of swing. Since the diff-pair transistors cool by the same amount (from nominal at "maximum heat"), it's good to close to saturation or cutoff.

about 150

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poorly

It looks like it can stand an input swing up to maybe 50 mV before it gets seriously nonlinear. And V- can be low, like -1V, and the max pair current can be low, 1 mA or less.

Interesting problem, for such a simple circuit.

John Larkin Highland Technology Inc www.highlandtechnology.com jlarkin at highlandtechnology dot com Precision electronic instrumentation Picosecond-resolution Digital Delay and Pulse generators Custom timing and laser controllers Photonics and fiberoptic TTL data links VME analog, thermocouple, LVDT, synchro, tachometer Multichannel arbitrary waveform generators

about 150

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poorly

And so easy to calculate. ...Jim Thompson

| James E.Thompson | mens | | Analog Innovations | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona 85048 Skype: Contacts Only | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at http://www.analog-innovations.com | 1962 | I love to cook with wine. Sometimes I even put it in the food.

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heat"),

It only works for small signals, though, because once you get away from V_CE = VCC/2, the differential dissipation is no longer quadratic in deltaV_BE. It does help over a considerable range.

Cheers

Phil Hobbs

Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC Optics, Electro-optics, Photonics, Analog Electronics 160 North State Road #203 Briarcliff Manor NY 10510 USA +1 845 480 2058 hobbs at electrooptical dot net http://electrooptical.net

mv,

If Vi is the voltage due to the sensed current and Iv is the current applied to the diff pair due to the drain voltage, the differential signal current, Isig, is...

Isig = Iv*TANH(Vi/(2*kT/q))

So a significant TC, if the diff pair sees any heating at all.

So, given the fist-full of parts, why not just buy an off-the-shelf cheapy multiplier like a 1496 (or modern equivalent) and lose the TC? ...Jim Thompson

| James E.Thompson | mens | | Analog Innovations | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona 85048 Skype: Contacts Only | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at http://www.analog-innovations.com | 1962 | I love to cook with wine. Sometimes I even put it in the food.

mv,

OK, why multiply when adding is easier?

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This behaves like a classic foldback current limiter. Since SOAR curves are generally not true constant-power curves (less power is allowed at high voltages in most mosfets) we can pull down the X1 endpoint a bit to accommodate that.

May as well add the cap to account for thermal taus (different time lines on the SOAR graph) and toss in heatsink temperature, too.

John Larkin Highland Technology, Inc jlarkin at highlandtechnology dot com http://www.highlandtechnology.com Precision electronic instrumentation Picosecond-resolution Digital Delay and Pulse generators Custom laser drivers and controllers Photonics and fiberoptic TTL data links VME thermocouple, LVDT, synchro acquisition and simulation

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