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Old 3rd November 2003, 11:57 PM   #1
JoeBob is offline JoeBob  Canada
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Default Can one differentiate a signal using only passive components?

Can one differentiate a signal using only passive components? I'm guessing it would be possible through the use of capacitors, but I'm not sure when they effct the phase, I mean is it all frequencies?

Also, can it be done without decreasing the signal strength? I ask because usual opamps don't have rails high enough for my circuit...
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Old 4th November 2003, 02:20 AM   #2
JoeBob is offline JoeBob  Canada
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I know there's a way, I just don't know what it is, I'm sure it's really simple and everyone else just takes it as common knowledge...
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Old 4th November 2003, 12:49 PM   #3
JoeBob is offline JoeBob  Canada
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I hate to bump this up again, but I saw schematics on the web concerning a passive differentiator, a cap, and a resistor were used with an inductor to limit the lf rolloff, I tried without the inductor, for low frequency rolloff isn't a problem for me since I kept it real low (or shouldn't I have) and I saw no phase change...
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Old 4th November 2003, 01:36 PM   #4
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limit LF rolloff? that seems a little magical don't you think? i mean wouldn't that imply the passive circuit was making power? and besides, differentiating would supress low frequency response as the act of differentiating would give higher frequencies less attenuation or more gain. sin(w t) -> w cos(w t), remember, so as frequency increases, so does gain!

otherwise are you just talking about RC or RLC networks? what is it going to do?
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Old 4th November 2003, 01:51 PM   #5
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Quote:
but I'm not sure when they effct the phase
Quote:
Also, can it be done without decreasing the signal strength?
A differentiator will always have a phase-shift of +90 degrees. Its output will also increase proportionally with frequency.

So what do you expect from a differentiator ? What do you want to use it for ?

Regards

Charles

P.S.: A simple capacitor IS already a differentiator if your input variable is voltage across the capacitor and the output variable is current through said capacitor !
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Old 4th November 2003, 03:35 PM   #6
JoeBob is offline JoeBob  Canada
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I re-read the article and and L was to retain phase characteristics even throughout the rolloff, sorry for that I misunderstodd the article as well.

What I want it for is because I'm measuring the velocity of a woofer moving and I want to convert it to acceleration to be re-introduced into the feedback path of the amplifier. There is obviously a feedback resistor and I thought that a simple cap in series with a resistor shunted to ground after, then fed into the feedback resistor might do the job. But when I probed the voltage from the cap/resistor (without wiring it up to the feedback yet) I saw no phase change, so there was obviously something I didn't understand.

I obviously don't care if there is rolloff, I'll just use a large enough capacitor so that the rolloff is low enough to not affect the circuit. I can't use a standard opamp because it would clip with the signals I would be sending to it. And if it can be done simply with passive parts I'm always happier with that.

Thanks for the input though.
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