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Old 11th December 2013, 11:42 AM   #1
PMA is offline PMA  Europe
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Default Small signal myths and real measurements

More and more posts in the forum deal with pseudo theories and unsupported claims about mysterious small signal distortions in circuits with negative feedback. I would like to post some of my measurements that prove that opposite is true in case of 'normal' design. I would like to encourage the others as well to post their results.

I am starting with a link-level preamplifier based on OPA627 and DRV134 and measurements related to 0dB = 146mV. I will appreciate your comments on 'small signal distortions'.
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Old 12th December 2013, 02:48 AM   #2
fas42 is offline fas42  Australia
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Pavel, would it be possible to do some IMD measurements on that component, where the level of the 2nd signal is down -20, -40, -60, etc, compared to the 1st - to satisfy my curiosity ...

Thanks,
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Old 12th December 2013, 03:11 AM   #3
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https://web.archive.org/web/20101203...m/tmt_1002.pdf

I find the intersting part starting ~ p15 - the ever popular external Class A bias of an op amp output - but with actual measurements
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Old 12th December 2013, 05:49 AM   #4
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Quote:
Originally Posted by fas42 View Post
Pavel, would it be possible to do some IMD measurements on that component, where the level of the 2nd signal is down -20, -40, -60, etc, compared to the 1st - to satisfy my curiosity ...

Thanks,
Frank, I appreciate your curiosity. Reducing amplitude of the 2nd signal only makes things easier for the amplifier.

1) 19+20kHz. All intermodualation products disappear and the only product you can see is the 2nd harmonic of 19kHz, i.e. 38kHz, about 96dB below 19kHz line level. Further reducing of 2nd signal amplitude brings no change so please allow me not to post images.

2) 1+20kHz. No intermodulation products available.

The conclusion is that smaller the signal components less work for the amplifier, in this circuit under test.
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Old 12th December 2013, 05:54 AM   #5
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Quote:
Originally Posted by jcx View Post
https://web.archive.org/web/20101203...m/tmt_1002.pdf

I find the intersting part starting ~ p15 - the ever popular external Class A bias of an op amp output - but with actual measurements
Thanks for the link, this is a good and interesting publication.

We may also replace rail resistor by self-biased JFET.
My question - in case of 1mA output stage idle current and 10k load the output pp stage would remain in class A up to 20Vp output voltage (never used in a link stage). Does it make any sense to use output stage supply resistor then? I understand it may make sense for 600 ohm load, but then I would go for a BUF634 or discrete buffer.
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Old 12th December 2013, 06:33 AM   #6
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Also interesting is no big influence of heavy load in case of small signals. Output impedance is 50 ohm, please compare effect of 10k and 50 ohm load. We can see the plot shifted down by 6 dB (50R : 50R divider). 0dB = 150mVrms approx., as shown in 1st post of the thread.
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Old 12th December 2013, 06:38 AM   #7
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Quote:
Originally Posted by PMA View Post
would like to post some of my measurements that prove that opposite is true in case of 'normal' design.
Test using sine waves?

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Old 12th December 2013, 06:50 AM   #8
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Would you prefer cosine?
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Old 12th December 2013, 07:03 AM   #9
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I would prefer real music. The tests with easy to reproduce periodic signals can really only prove something about periodic signals.

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Old 12th December 2013, 07:08 AM   #10
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"Would you prefer cosine? "


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