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Old 28th December 2012, 03:55 AM   #1321
RNMarsh is offline RNMarsh  United States
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Composite amps -compensation ---> www.pdk101.com/Design_related/OPA/OPA_lowPower_HighGain_compensation.pdf

Thx-RNMarsh
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Old 28th December 2012, 04:34 AM   #1322
davada is offline davada  Canada
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Hi Rick.

This is a dead link.
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Old 28th December 2012, 03:47 PM   #1323
vicnic is offline vicnic  Latvia
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Hi,
What do you think about photo resistive optoisolators? Before some days, I thought, that these are one of the better things for VCRs. Now I have different opinion. Vactrol VTL5C7 measuring result (light diode current 6.5mA, voltage across photo resistor 40mV p-p, resistance app 200ohm, 10kohm resistor between 1kHz sine source and photo resistor):
http://content7-foto.inbox.lv/albums...TL5C7dist2.jpg
Also I put them in different regimes and use another one, but results are very similar.

Victor.
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Old 28th December 2012, 04:00 PM   #1324
1audio is offline 1audio  United States
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If you look at the distortion vs. level you will see that it increases rapidly with voltage. For an oscillator that has a target distortion at -120 dB or lower the -80 dB you show at 40 mV needs to be attenuated by another 40 dB which really limits the amount of gain range possible. The Silonex LDR's are supposed to have less distortion. I will have some early next year and I'll see what I can find out about their distortion.
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Old 28th December 2012, 04:10 PM   #1325
vicnic is offline vicnic  Latvia
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If you look at the distortion vs. level you will see that it increases rapidly with voltage. For an oscillator that has a target distortion at -120 dB or lower the -80 dB you show at 40 mV needs to be attenuated by another 40 dB which really limits the amount of gain range possible. The Silonex LDR's are supposed to have less distortion. I will have some early next year and I'll see what I can find out about their distortion.
You are right, but as I previously wrote, FET in similar conditions shows more better results:
http://content8-foto.inbox.lv/albums...nt/FET40mV.jpg
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Old 28th December 2012, 07:22 PM   #1326
davada is offline davada  Canada
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You are right, but as I previously wrote, FET in similar conditions shows more better results:
http://content8-foto.inbox.lv/albums...nt/FET40mV.jpg
Hi vicnic,

Have you considered trying a thermal approach.

I know the LT part for RMS detection is no longer available and hard to get but a pair of RDTs in an insulated chamber might work. I doubt fast settling would be achievable.

Cheers,
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Old 28th December 2012, 09:16 PM   #1327
vicnic is offline vicnic  Latvia
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Originally Posted by davada View Post
Hi vicnic,

Have you considered trying a thermal approach.

I know the LT part for RMS detection is no longer available and hard to get but a pair of RDTs in an insulated chamber might work. I doubt fast settling would be achievable.

Cheers,
Hi, David
I made some experiments around thermal in past, but not got good results. Almost one thing I remember - pain . Thermal inertia, that I can not change.

Victor
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Old 29th December 2012, 05:47 AM   #1328
1audio is offline 1audio  United States
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There is a positive temperature coefficient thermistor in an evacuated bulb that is very easily had in many values. Its called an incandescent light. The thermal inertia is essential to not add even order distortion as it changes with the waveform.

There are still issues of overall amplitude stability which my idea was to add a dc current to tweak the overall resistance to the target value that is controlled by a precision level detector.
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Old 29th December 2012, 12:40 PM   #1329
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Quote:
Originally Posted by davada View Post
Hi vicnic,

Have you considered trying a thermal approach.

I know the LT part for RMS detection is no longer available and hard to get but a pair of RDTs in an insulated chamber might work. I doubt fast settling would be achievable.

Cheers,
True RMS detectors are plentiful? I tried a Tellabs precision wirewound thermistor designed to compensate KT/q in log amps in a chamber. Settling was hopeless.
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Old 29th December 2012, 01:08 PM   #1330
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Originally Posted by scott wurcer View Post
True RMS detectors are plentiful?
I think he was referring to the now obsolete LT1088 == Linear's "thermopile on a chip". Jim Williams did an ap-note on True RMS and mentioned the device. Here's a link to the datasheet: http://cds.linear.com/docs/Datasheet/lt1088.pdf
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