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Re: Running out of ideas: stability, bootstrap

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Am 28.05.2017 um 18:09 schrieb bitrex:


> Did you check out some of the ideas guys had on the thread "Fast buffer 
> idea?"
> 
> As JL might say, this cascode bootstrapping arrangement looks way too 
> complicated and uses too many parts. I don't completely understand it. 
> Why not drive the boostrapping transistor from its base?

The bootstrapping is not yet included in the board; only the
cascode as a precondition.  The cascode also gave quite a rise
in bandwidth when I used the ADA4898 and better Miller killing
abilities at up to 160 mA.

I first wanted to drive the base
from a leftover FET opamp used as a follower; OTOH the same
voltage is available at the sources of the IF3601; a 0.1 Ohm
source resistor looks impressively low impedance, but at this
point the signal has already travelled through the cascode
and the THS4022 op amp, that introduces unwelcome dependencies.

The 0.1 Ohms look small and everything seems common source,
but the feedback through the op amp makes it behave like a high
impedance current source from the FET's point of view. The
source follows the gate quite closely after all.

The drain looks into ~ 0 Ohms of the cascode emitter, and if
the signal fed back from the op amp has the right phase, it
looks capacitively for the FET. That alone could provide
the negative input impedance (like in a standard VHF VCO,
aka capacitivly loaded follower).

Only the fact that Zin is still negative when I cut the
feedback makes me assume that the cascode transistor is the
culprit.

The 0.1 Ohm source resistors btw were necessary with 8
parallel FETs, or they would cost 10 pV/rtHz in simulation
when you are just at +/- 100 pV/rtHz.


> :-( the mindset is catchy apparently...

Cheers, Gerhard

(got a nice Tempranillo)

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Thread

Running out of ideas: stability, bootstrap Gerhard Hoffmann <ghf@hoffmann-hochfrequenz.de> - 2017-05-28 17:11 +0200
  Re: Running out of ideas: stability, bootstrap bitrex <bitrex@de.lete.earthlink.net> - 2017-05-28 12:09 -0400
    Re: Running out of ideas: stability, bootstrap Gerhard Hoffmann <ghf@hoffmann-hochfrequenz.de> - 2017-05-28 23:01 +0200
      Re: Running out of ideas: stability, bootstrap Jim Thompson <To-Email-Use-The-Envelope-Icon@On-My-Web-Site.com> - 2017-05-28 14:30 -0700
        Re: Running out of ideas: stability, bootstrap Gerhard Hoffmann <ghf@hoffmann-hochfrequenz.de> - 2017-05-28 23:56 +0200
          Re: Running out of ideas: stability, bootstrap Jim Thompson <To-Email-Use-The-Envelope-Icon@On-My-Web-Site.com> - 2017-05-28 15:08 -0700
  Re: Running out of ideas: stability, bootstrap John Larkin <jjlarkin@highlandtechnology.com> - 2017-05-28 09:46 -0700
    Re: Running out of ideas: stability, bootstrap Gerhard Hoffmann <ghf@hoffmann-hochfrequenz.de> - 2017-05-28 22:09 +0200
      Re: Running out of ideas: stability, bootstrap Joerg <news@analogconsultants.com> - 2017-05-29 07:13 -0700
        Re: Running out of ideas: stability, bootstrap dagmargoodboat@yahoo.com - 2017-05-29 20:56 -0700
          Re: Running out of ideas: stability, bootstrap Winfield Hill <hill@rowland.harvard.edu> - 2017-05-30 03:55 -0700
            Re: Running out of ideas: stability, bootstrap Gerhard Hoffmann <ghf@hoffmann-hochfrequenz.de> - 2017-05-30 15:53 +0200
              Re: Running out of ideas: stability, bootstrap dagmargoodboat@yahoo.com - 2017-05-30 10:42 -0700
  Re: Running out of ideas: stability, bootstrap bitrex <bitrex@de.lete.earthlink.net> - 2017-05-29 08:38 -0400
    Re: Running out of ideas: stability, bootstrap bitrex <bitrex@de.lete.earthlink.net> - 2017-05-29 08:40 -0400
    Re: Running out of ideas: stability, bootstrap Gerhard Hoffmann <ghf@hoffmann-hochfrequenz.de> - 2017-05-30 16:51 +0200
  Re: Running out of ideas: stability, bootstrap Winfield Hill <hill@rowland.harvard.edu> - 2017-05-29 07:36 -0700
    Re: Running out of ideas: stability, bootstrap bitrex <bitrex@de.lete.earthlink.net> - 2017-05-29 11:20 -0400
      Re: Running out of ideas: stability, bootstrap Jan Panteltje <pNa0nStpealmtje@yahoo.com> - 2017-05-29 15:36 +0000
        Re: Running out of ideas: stability, bootstrap bitrex <bitrex@de.lete.earthlink.net> - 2017-05-29 11:44 -0400
          Re: Running out of ideas: stability, bootstrap bitrex <bitrex@de.lete.earthlink.net> - 2017-05-29 11:53 -0400
          Re: Running out of ideas: stability, bootstrap Jan Panteltje <pNa0nStpealmtje@yahoo.com> - 2017-05-29 17:01 +0000
        Re: Running out of ideas: stability, bootstrap Winfield Hill <hill@rowland.harvard.edu> - 2017-05-29 08:46 -0700
          Re: Running out of ideas: stability, bootstrap Jan Panteltje <pNa0nStpealmtje@yahoo.com> - 2017-05-29 16:56 +0000
      Re: Running out of ideas: stability, bootstrap dagmargoodboat@yahoo.com - 2017-05-29 21:12 -0700

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