Nobody really makes a Magic-Joerg-Block where the output saturation is adjustable (and preferably, accurate), so naturally, I made my own.
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Threshold between inputs is, of course, exponential (tanh more precisely, I suppose). The comparator output crosses threshold over 10s of mV difference, good enough for most purposes. Put an op-amp or comparator in if you'd like it sharper?
I find 31mV offset with fresh-out-of-the-bag transistors (say, that's about a V_TH). So unless you get lucky with a monolithic matched set, you'll need the offset trimmer, or a fancy op-amp setup, to do better.
Tim
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Deep Friar: a very philosophical monk.
Website: http://seventransistorlabs.com
The problem with bipolar limiters in general is that they slow down when you overdrive them, so you get a fair amount of AM-PM conversion. If your input is near ground, your diff pair will do its limiting by cutoff rather than saturation, which helps a lot.
IME FETs do a better job of limiting without a lot of AM-PM. Of course, I'm a big fan of the SA604A FM IF chip because it produces this amazingly useful logarithmic RSSI output for $2 or so, even though its AM-PM performance is fairly mediocre.
When I need a fast logic function, and TinyLogic is unsuitable, I favor discrete CML or ECL style over RTL/DTL/TTL style saturating stuff. :-) Of course one of the nice things about CML/ECL is the analog nature. Hence circuits like this...
I'm not doing radio here (as you might guess from the resistors, it'll go to... oh, probably a few MHz, which isn't bad, but a long way from FM BCB), so AM-PM isn't a problem.
To which kind of FET limiter are you referring? FETs will suck if used directly in a circuit like this, of course. The cascade-of-amps is time-tested, although if your FETs have a lot of Cds at low voltage, you'll see a lot of sloppiness.
I may investigate a log amp or something like that one day. Perhaps build the better part of a spectrum analyzer. Those damn things sure jacked up in price the last couple years.
Tim
--
Deep Friar: a very philosophical monk.
Website: http://seventransistorlabs.com
It looks as if a lot of your +90ppm/C temperature drift is in the temperature dependent current gain of the MMBTH81. You could get more current gain out of a PNP/NPN complementary Darlington, but IIRR they have a nasty tendency to oscillate. If you ran one with just 0.5mA in the MMVTH81 it would be slower, but still fairly quick (if it didn't oscillate).
The green LED version is unexpectedly horrible - did the MMVTH81 still have enough Vce left in your test circuit?
Quote "Setting the tones, bass mid and treble, volume for clean and crunch (+ saturation) of a single knob Tone for channel distortion with adjustable saturation and volume".
In the ham radio world you have clippers. I made a transistor clipper sometime in the late 70's, after I got my license.
Lets see now. I just quickly picked up the calculator and it appears the current sink for the high side NPN maybe over driving it a bit..
At a 50 ohm load I would say that is fine however, at a short circuit output, it looks like it forcing the high side output (NPN) to sit somewhere in the 200ma+ region.
Maybe you don't plan on output being low Z or shorted but I would be concerned with the livelihood of the NPN. who knows..
We recently designed something sort of similar, but not a ZCD. It's an amplifier with a photodiode input and two outputs that have x1 and x1000 gain ratios. The x1 is used to measure a main laser pulse, and the x1000 observes tiny pre- and post-pulse optical effects, within +-10 ns of the big pulse. We wound up using a number of cascaded, relatively low gain, clean-clamping amp stages, sort of like the Collins idea.
This was hard to test. The PD is an ideal floating signal source, but real signal generators and scopes have high frequency ground loops. With over 100 GHz of GBW, it really wants to oscillate. We wound up kluging a small floating, battery powered pulse generator to drive the input, fiberoptic coupled. We couldn't use the actual photodiode because we don't have a laser with enough pulse power to drive it.
Thanks for the Collins link.
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John Larkin Highland Technology Inc
www.highlandtechnology.com jlarkin at highlandtechnology dot com
It might have been cheaper to decouple the power supplies of each of the cascaded amplifier stages with a properly damped non-wound inductor/resistor/capacitor networks. Most amplifiers have very little common mode rejection at high frequencies, and an output stage with a releatively high output current can feed quite a lot of signal back into the input stage of the first amplifier in the chain through the power supply pins if you aren't very careful.
Regular decoupling capacitors crap out about 300MHz, so you can end up having to by-pass them with something less inductive. I didn;t have to go to the trouble of buying some 1nF porcelain capacitors, but I wasn't looking for your kind of GB product.
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