Saturation NFB & Distortion

Uhhhh.... headroom?

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Les Cargill

The signal pickoff connector J2 is interesting. When I do a high-ratio scope pickoff on a serious HV pulser, I tend to see a little baseline noise, probably ground loops. You seem to have the scope grounded through a 4.7 ohm resistor, which makes up part of the 50 ohm source impedance. Is that what's going on? Does it improve the pickoff?

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John Larkin         Highland Technology, Inc 

lunatic fringe electronics

amp, but it will be driven outside its linear range into saturation a lot of the time. That's all well & good but for one thing: wrapping nfb round s aturating outputs doesn't work too well as it takes time for output devices to unsaturate, and the nfb effectively overreacts, adding distortion. Keep ing distortion low matters here. What tips would you recommend to keep unwa nted distortion minimised?

Maybe you've not read through the thread. That wouldn't do what's required.

I don't like forcing the scope ground to match the noisy instrument ground, so I isolate it a bit. But I'm not sure what's going on in the drawing, I'll have to check.

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 Thanks, 
    - Win

Seems like the 4.7 ohm resistor could increase the noise that the scope sees, by not forcing the scope ground to be the amp ground.

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John Larkin         Highland Technology, Inc 

lunatic fringe electronics

Low frequency ground loops cause magnetic pickup in both shield and centre conductor. Grounding the shield shorts that out to some degree, turning part of the pickup into differential mode.

Ground loops at 60 Hz have milliohm impedances, so that 4.7 ohm resistor looks pretty much like an open circuit to them.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs 
Principal Consultant 
ElectroOptical Innovations LLC / Hobbs ElectroOptics 
Optics, Electro-optics, Photonics, Analog Electronics 
Briarcliff Manor NY 10510 

http://electrooptical.net 
https://hobbs-eo.com

Likely that the big amp is grounded somewhere, and the scope is plugged into a grounded outlet somewhere else. The grounds eventually get together, but usually with lots of HF noise and lots of microhenries. A short coax hard connected to instrument ground on both ends can short out that ground inductance and reduce displayed noise. Sometimes a common-mode ferrite helps. A true isolated-input scope is great here.

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John Larkin         Highland Technology, Inc 

lunatic fringe electronics

I don't know why keeping it off the rails isn't inherently better than worrying about how it interacts with the rails to begin with.

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Les Cargill

Once again the required behaviour is that it rails. An amp that doesn't is no use at all.

NT

CFB opamps come out of saturated condition very rapidly.

--
 Thanks, 
    - Win

Yep. I've become a fan just from writing Spice models for Apex (Tucson). ...Jim Thompson

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cheers you 2, could be useful to know

NT

Fair enough. :)

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Les Cargill

Well just rail some upstream opamp into a load it's happy with. Distortion here and power later?

George H.

I still don't see why you don't just take some diodes and whatever and clip the input at slightly higher than the max input and clipped output.

Why ?

It's possible. But the clipping point would drift with temp, and the clipping be a bit nonlinear. I'd much prefer the clipping to occur in the output stage so it gets as near the rails as possible and so it's more psu variation tolerant.

NT