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Re: Running out of ideas: stability, bootstrap

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On 05/28/2017 11:11 AM, Gerhard Hoffmann wrote:
> Hi, all,
> 
> we had a bank holiday here, nicely positioned close to
> the weekend and I took the free time to play with my
> multi-IF3601 low noise amplifier.
> 
> That was not so funny because it features a negative
> input impedance over most frequencies, so with
> a suitable inductance on the input it makes a stable
> oscillator.
> 
> Some observations:
> 
> - Zin is negative also with the feedback loop cut
> 
> - Choice op op amp does not matter: ADA4898 vs. THS4022
> 
> - There is no coupling via the bias system; base bias
>    used to have a 2nd current source that shared the
>    BAV99 string; that could produce startup problems (solved)
> 
> - type of cascode transistor did not make much of a difference,
>    using an Infineon BFQ19S instead of FZT851/ZXT690B looked
>    slightly better, but not good enough.
> 
> - Leaving the cascode transistor out remedied the situation,
>    but I need the cascode transistor to introduce the bootstrap
>    to eliminate Cgd of the paralleled FETs.
> 
> - The THS4022 suppresses the Miller effect quite nicely at
>    least to 100 KHz, but the unmultiplied Cgate-drain is still
>    easily a nF of more.
> 
> - The obvious solution of gate stoppers does not make sense
>    because of the noise. A resistor in the base lead of the
>    cascode makes things worse; 6R8 between the drains and
>    the cascode emitter made no difference. (could not use
>    more without messing up the DC operating point, it ahlso
>    would interfere with the bootstrap)
> 
> - ferrite beads in the gates do exactly nothing.
> 
> circuit snippet is at
> < 
> https://www.flickr.com/photos/137684711@N07/34904768906/in/album-72157682404684680/ 
>  >
> 
>    Currently, there are 4 FETs in parallel.
> 
> Does the combined expertise of the group have any idea on
> how to stabilize the cascode or an alternative way to
> introduce the bootstrap?
> 
> Best regards,
> 
> Gerhard

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 mindset is catchy apparently...

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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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