Limiter

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.

Cheers

Phil Hobbs

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

Use my LED current sources!

They glow in the dark, too.

r in

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?

The emitter resistor tempco spec is 100 PPM.

I think it's pretty good. A junction drop has a voltage change of about 4000 PPM/K.

What is the purpose? You'll just add a lot of distortion this way.

There are a couple of opamps that have explicit clamp inputs. HFA1130 (fast and a tad quirky) and the slower but more accurate AD8036/8037.

Sure they do. For example in the audio world:

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.

Am 16.03.2013 16:04, schrieb John Larkin:

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Or TI OPA698/699. Work nicely here in a slope amplifier in the Oliver Collins style.

On some OpAmps one can clamp the compensation pin. That limits the drive to the output followers.

regards, Gerhard

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..

Jamie

"John Larkin" wrote in message news: snipped-for-privacy@4ax.com...

Ten bucks, you're kidding right?

Tim

and

I hadn't seen Collins' work before. Interesting.

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.

Me? Kid?

We pay $1.40 for the HFA1130.

--

John Larkin Highland Technology Inc

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

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Why? 15ppm/C resistors are widely available, if not quite as cheaply as 100ppm/C parts.

Sure, but you've cancelled that out.

-- Bill Sloman, Sydeny

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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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Precisely. Well, approximately. Plus, it glows in the dark.

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The oscillation was caused by the external test equipment, as noted. The board doesn't oscillate itself when driven from the floating photodiode.

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Precisely enough that your crummy emitter resistor or the temperature dependence of the PNP base current are enough to explain the residual tempco.

If you'd made them a bit better, you might have a better idea of how precisely the LED cancells the PNP's Vbe. And it would still glow in the dark.

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Well, do it better and tell us how things work out.