Path: csiph.com!fu-berlin.de!uni-berlin.de!individual.net!not-for-mail
From: Gerhard Hoffmann <ghf@hoffmann-hochfrequenz.de>
Newsgroups: sci.electronics.design
Subject: Running out of ideas: stability, bootstrap
Date: Sun, 28 May 2017 17:11:52 +0200
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Xref: csiph.com sci.electronics.design:462075

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