Ring of two current source

On 14 May 2015 03:02:29 -0700, Winfield Hill Gave us:

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DecadentLinuxUserNumeroUno
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About 0.35 pF. It's fairly independent of voltage, which is nice to make linear ramps.

CEL doesn't say on

NEC, or whoever they are now, is killing off all their phemts. Sad.

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John Larkin         Highland Technology, Inc 
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John Larkin

NE3509 is a phemt, not a mosfet. I measured about 0.35 pF drain capacitance.

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John Larkin         Highland Technology, Inc 
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John Larkin

I have a light over my kitchen table with more bandwidth than that.

The issue isn't bandwidth, it's linearity.

And the fact that it's really hard to probe a ramp on a 22 pF cap without disturbing the ramp.

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John Larkin         Highland Technology, Inc 
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John Larkin

Right, it's the reset that makes the opamp integrator tricky.

And the opamp bias current maybe.

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John Larkin         Highland Technology, Inc 
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John Larkin

Well, it works. In that app, I can give up the ends of the ramp; I just need a very linear central region, 2 volts of slope maybe.

Ramp starts are always nasty. Curvature and ringing.

Right, a transistor would have a big TC, which would have to be compensated somehow.

Small schottky diodes have low or zero TCs if run at the right current.

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John Larkin         Highland Technology, Inc 
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John Larkin

Hmm, interesting.

I usually find that sort of thing disappointing, because of the amount of input error required to get high slew rates, which gets passed on to the output as a step. (Maybe another opportunity for a biased cascode front end.) ;)

The bootstrap approach seems better intuitively, because the input error will be constant and will just look like a slight change in the reference voltage.

Op amps make lousy cascodes, for the same reason.

Cheers

Phil Hobbs

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Phil Hobbs

On Thu, 14 May 2015 08:17:03 -0700, John Larkin Gave us:

You are an abject idiot. a four bit light computer will run rings around anything you ever saw or imagined.

Reply to
DecadentLinuxUserNumeroUno

On Thu, 14 May 2015 08:17:03 -0700, John Larkin Gave us:

Perhaps that is what a differential probe is for.

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DecadentLinuxUserNumeroUno

I called CEL, and they say that it's still a good part. Digikey seems to be deleting lots of parts, maybe because they don't sell enough.

CEL will email me if it EOLs, so we can buy a reel. They're fairly cheap.

It's really a nice part. It doesn't need much gate swing to go on/off, and it enhances nicely.

I also like the Avago enhancement phemts, even easier to drive. Avago seems to keep stuff in production forever.

You can estimate capacitances from the s-parameters. Sort of.

For example, to get gate capacitance, I scan down the table for the angle of S11, and note the frequency in GHz where it hits 45 degrees. The reciprocal of that is an estimate of Cg. That's a useful quick screen.

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John Larkin         Highland Technology, Inc 
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John Larkin

In a delay generator, the ramp drives a comparator, with the other comparator input driven by a DAC. In that case, you can do software curvature fixes. In the extreme case, generate a polynomial to calibrate every channel.

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John Larkin         Highland Technology, Inc 
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You're obviously in 50% over your head. That has _eleven_ transistors, as well as four diodes. (duck and run)

Cheers

Phil Hobbs

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Phil Hobbs

Chuckle,... I noticed that too, but I wasn't going to say anything.

(It's lucky if my circuits have one transistor. :^)

George H.

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George Herold

Yeah well, it was the *deluxe* model! ;)

Tim

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Tim Williams

I've thought it would be fun to teach an electronics course that involved designing and analyzing circuits that used just a few of the same parts. Seven would work, but not all transistors.

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John Larkin         Highland Technology, Inc 
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Heh, you could get pretty far if those resistor arrays you're so fond of aren't counted as cheating. Tons of isolated resistors per "part", plus handy ratios.

Although most ICs would probably be against the spirit of such a course, I'd argue strongly to include TL431. "Alright kids, remember how I said Vbe sucks? Today it doesn't!..."

A related subject is the "long tail" of Legos. Back in the day, they used to be predominantly generic blocks and plates and caps and ... Nowadays, a huge fraction of kits and assortments is special-use pieces for theme kits. Takes all the fun out of it.

Tim

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Tim Williams

Den fredag den 15. maj 2015 kl. 22.47.07 UTC+2 skrev Tim Williams:

yeh, the kits are mostly like any other toy the few parts it's made of it just happens to be mostly held together with knobs had one of these when I was a kid:

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what's left of it is still in a box somewhere

-Lasse

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Lasse Langwadt Christensen

Nice!

I didn't realize until last year that Mom still has the "space jet fighter" thingy I had made. Of all the things I (and my brothers) had made, I think that was the most enduring. I "flew" that thing around so much!

Also, the last Lego project I ever made, which was a double acting steam engine (I just had to, after seeing an example online -- that must've been

2002-ish). For being made out of stacked plastic, it works amazingly well!

Tim

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Tim Williams

Who said Vbe sucks? Without it, most bipolar logic wouldn't work. Bipolar current sources would be problematic, as well.

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krw

I have some scribbles of all the resistor values and all the divider ratios that you can make with a 4-resistor pack. Thing is, you'd better get it right the first time, because you can't ECO individual resistors in a quad-pack.

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I've got the divider set somewhere...

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John Larkin         Highland Technology, Inc 
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