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Old 10th May 2013, 05:37 PM   #11
dadod is online now dadod  Croatia
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Originally Posted by manso View Post
Damir, you should try to use a zobel filter at the output buffer instead of the output resistor. There are several disadvantages with that resistor. Another way is compensation by using a RC network from the output to the input of the buffer. This will probably give the best results.
Thanks manso,
Could you explain what disadvantages with output resistor? Similar resistor is normaly used to prevent an OPA to oscilate with capacitive load.
I tried different ways to compensate and used compensation worked well in the simulation.
I will try your zobel and RC suggestion.
Damir
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Old 10th May 2013, 05:53 PM   #12
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Loss of bandwith, slewrate and output impedance is increased.
I dont know why Opamp manufacturers dont suggest the use of zobel networks, these have the disadvantage that the zobel should be tuned to cancel the effect of the capacitive load, the problem being the use of different loads requiring different zobel characteristics for optimal stability.
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Old 10th May 2013, 05:58 PM   #13
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Originally Posted by manso View Post
Damir, you should try to use a zobel filter at the output buffer instead of the output resistor. ......... Another way is compensation by using a RC network from the output to the input of the buffer. This will probably give the best results.
Here is gain simulation with your suggestion and indeed it works, but with a bit higher distortion at 20kHz. Could be that my values are not the best.
Thanks Damir
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File Type: jpg GainWire-mk2-e-elvee-dado-buffer-2-gain.jpg (235.3 KB, 680 views)
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Old 10th May 2013, 06:24 PM   #14
matze is offline matze  Germany
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C2 and R19 are here to stabilize the amp, as it should work with long cable, means capacitans. I simulate up to 1nF.
One would expect that R19 already does sufficiently isolate the first stage from buffer and capacitive load. Nevertheless, you include C2 and additionally change it from 47p to 10p in the version with cascoded buffer. Why is C2 necessary?

Matthias
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Old 10th May 2013, 06:32 PM   #15
matze is offline matze  Germany
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Here is gain simulation with your suggestion and indeed it works, but with a bit higher distortion at 20kHz. Could be that my values are not the best.
Thanks Damir
I would not disconnect compensation of the cascodes.

What are these resonances at about 20MHz and 100MHz? Do they already appear without the buffer?

Matthias
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Old 10th May 2013, 09:20 PM   #16
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Hi Damir,
the usual performance from you ....
Kind regards,
Matthias
The Shoe Is The Sign - The Beginning - YouTube
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Old 10th May 2013, 09:40 PM   #17
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??
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Old 10th May 2013, 09:48 PM   #18
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Originally Posted by matze View Post
One would expect that R19 already does sufficiently isolate the first stage from buffer and capacitive load. Nevertheless, you include C2 and additionally change it from 47p to 10p in the version with cascoded buffer. Why is C2 necessary?

Matthias
Hi Matthias,
Here is the gain plot without output buffer. First one is without C2 and second with C2.
BR damir
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File Type: jpg GainWire-mk2-e-elvee-dado-gain-noC2.jpg (239.5 KB, 615 views)
File Type: jpg GainWire-mk2-e-elvee-dado-gain.jpg (241.6 KB, 580 views)
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Old 11th May 2013, 10:36 AM   #19
matze is offline matze  Germany
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Holy grail! And Cheers!
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Old 11th May 2013, 10:42 AM   #20
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Here is gain simulation with your suggestion and indeed it works, but with a bit higher distortion at 20kHz. Could be that my values are not the best.
Thanks Damir
Interesting, some performance is lost. Can it be made stable if you remove the zobel and only the RC in place ??
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