Groups > sci.electronics.design > #462075

Running out of ideas: stability, bootstrap

From	Gerhard Hoffmann <ghf@hoffmann-hochfrequenz.de>
Newsgroups	sci.electronics.design
Subject	Running out of ideas: stability, bootstrap
Date	2017-05-28 17:11 +0200
Message-ID	<ep07lpF4vr5U1@mid.individual.net> (permalink)

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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 also
   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

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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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