Groups > sci.electronics.design > #518582

Re: really fast buffers

From	Gerhard Hoffmann <ghf@hoffmann-hochfrequenz.de>
Newsgroups	sci.electronics.design
Subject	Re: really fast buffers
Date	2018-10-06 22:19 +0200
Message-ID	<g1sjm5Ffa4qU1@mid.individual.net> (permalink)
References	(9 earlier) <u8lfrdtffa2gqdvep73d514gd2qgp4q7j8@4ax.com> <787c7a2b-c1a9-4330-830b-8ab9a58d30d0@googlegroups.com> <XnsA972E4A74F46Cidtokenpost@69.16.179.22> <ApmdnWalI5KUniTGnZ2dnUU7-V2dnZ2d@supernews.com> <XnsA9739931E279Aidtokenpost@69.16.179.22>

Show all headers | View raw

Am 06.10.2018 um 21:03 schrieb Steve Wilson:
> Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:
> 
>> On 10/5/18 10:28 PM, Steve Wilson wrote:
>>> George Herold <gherold@teachspin.com> wrote:
>   
>>>> It oscillates because of a slow edge and no hysteresis?
>   
>>> It oscillates because of inductance in the base lead. Along with the
>>> base- emitter capacitance and capacitance from the emitter to ground,
>>> this forms a Colpitts oscillator, which is one of the most vigorous
>>> oscillators known. It can accept an extremely wide range of parameters
>>> and still oscillate.
>   
>>> The purpose of the resistor in the base lead is to dampen the Q of the
>>> tank so oscillations are no longer possible.
>     
>>>> Is it similar to cap. multiplier oscillations?
>   
>>> Could be. Any place where you have some inductance in the base and
>>> capacitance from the emitter to ground could form a Colpitts,
>>> especially where you have some inductance in the emitter lead.
>   
>> The key ingredient is negative input resistance.
> 
> That term always bothered me. Negative input resistance to what? If you
> look a the base input impedance, it is always positive. When the
> oscillations get going, the transistor is cut off most of the time. Where
> is the negative input impedance then?

Negative input impedance from base/gate to GND. It is not positive and
it sustains the oscillation. And it can be measured.

< 
https://www.flickr.com/photos/137684711@N07/34701106245/in/album-72157682404684680/ 
    >

This is S11 of a baseband amplifier with absurdly high gm to enforce
low voltage noise with IF3602 JFETs. Everything that is outside the
circle that goes through X=0 has a negative real part of the input
impedance. The exact values at the 3 cursors are given in the picture.

The green line is outside of that circle for most of the intended
frequency range, until the amplifier runs out of steam.

BTW I have found no way to cure that without compensating it with a
positive resistance in the input or cutting the feedback. None of this
is attractive in a low noise measurement amplifier.  :-(  :-(  :-(

Those network analyzers are so unforgiving. If you just ignore it,
you can happily measure around in circuits that have enough source
resistance. Just sometimes, it won't work. There are lots of
low noise amplifiers that share its fate.

BTW, I have built a variation of Winfield's 70 pV/rtHz amplifier from
AOE3. The non-differential version, since my source is never centered
around 0 Vdc. Works as promised. :-)

Yes, it could use even more 'lytics, but that was a test balloon for
a chopper amplifier. At 500 KHz the caps are no more that critical.

< 
https://www.flickr.com/photos/137684711@N07/45141749941/in/album-72157662535945536/ 
  >

  cheers,
Gerhard

Thread

really fast buffers John Larkin <jjlarkin@highland_snip_technology.com> - 2018-10-03 15:03 -0700
  Re: really fast buffers Lasse Langwadt Christensen <langwadt@fonz.dk> - 2018-10-03 15:23 -0700
    Re: really fast buffers Andy Bennet <andyb@andy.com> - 2018-10-04 08:20 +0100
      Re: really fast buffers Lasse Langwadt Christensen <langwadt@fonz.dk> - 2018-10-04 08:45 -0700
  Re: really fast buffers John Larkin <jjlarkin@highland_snip_technology.com> - 2018-10-03 15:52 -0700
    Re: really fast buffers George Herold <gherold@teachspin.com> - 2018-10-03 18:08 -0700
      Re: really fast buffers John Larkin <jjlarkin@highlandtechnology.com> - 2018-10-03 20:02 -0700
  Re: really fast buffers bitrex <user@example.net> - 2018-10-03 21:54 -0400
    Re: really fast buffers Steve Wilson <9fe142ac@gmail.com> - 2018-10-03 20:10 -0700
      Re: really fast buffers John Larkin <jjlarkin@highlandtechnology.com> - 2018-10-03 20:40 -0700
        Re: really fast buffers pcdhobbs@gmail.com - 2018-10-04 05:00 -0700
          Re: really fast buffers Steve Wilson <9fe142ac@gmail.com> - 2018-10-04 08:39 -0700
            Re: really fast buffers Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> - 2018-10-04 12:38 -0400
              Re: really fast buffers Steve Wilson <9fe142ac@gmail.com> - 2018-10-04 14:35 -0700
                Re: really fast buffers Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> - 2018-10-04 18:27 -0400
                Re: really fast buffers Steve Wilson <no@spam.com> - 2018-10-05 03:21 +0000
                Re: really fast buffers Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> - 2018-10-05 10:38 -0400
            Re: really fast buffers John Larkin <jjlarkin@highland_snip_technology.com> - 2018-10-04 11:50 -0700
              Re: really fast buffers Clifford Heath <no.spam@please.net> - 2018-10-05 09:26 +1000
                Re: really fast buffers John Larkin <jjlarkin@highland_snip_technology.com> - 2018-10-04 17:09 -0700
                Re: really fast buffers Steve Wilson <no@spam.com> - 2018-10-05 03:12 +0000
                Re: really fast buffers John Larkin <jjlarkin@highland_snip_technology.com> - 2018-10-05 14:18 -0700
                Re: really fast buffers Steve Wilson <no@spam.com> - 2018-10-05 23:42 +0000
                Re: really fast buffers Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> - 2018-10-06 14:41 -0400
              Re: really fast buffers Steve Wilson <no@spam.com> - 2018-10-05 03:09 +0000
                Re: really fast buffers Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> - 2018-10-05 10:40 -0400
                Re: really fast buffers Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> - 2018-10-05 11:06 -0400
                Re: really fast buffers John Larkin <jjlarkin@highlandtechnology.com> - 2018-10-05 19:16 -0700
                Re: really fast buffers John Larkin <jjlarkin@highland_snip_technology.com> - 2018-10-05 14:26 -0700
                Re: really fast buffers George Herold <gherold@teachspin.com> - 2018-10-05 18:36 -0700
                Re: really fast buffers John Larkin <jjlarkin@highlandtechnology.com> - 2018-10-05 19:22 -0700
                Re: really fast buffers Steve Wilson <no@spam.com> - 2018-10-06 02:28 +0000
                Re: really fast buffers <698839253X6D445TD@nospam.org> - 2018-10-06 06:03 +0000
                Re: really fast buffers Steve Wilson <9fe142ac@gmail.com> - 2018-10-05 23:41 -0700
                Re: really fast buffers <698839253X6D445TD@nospam.org> - 2018-10-06 08:12 +0000
                Re: really fast buffers Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> - 2018-10-06 14:46 -0400
                Re: really fast buffers Steve Wilson <no@spam.com> - 2018-10-06 19:03 +0000
                Re: really fast buffers Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> - 2018-10-06 15:32 -0400
                Re: really fast buffers Steve Wilson <no@spam.com> - 2018-10-06 20:08 +0000
                Re: really fast buffers Gerhard Hoffmann <ghf@hoffmann-hochfrequenz.de> - 2018-10-06 22:19 +0200
                Re: really fast buffers Steve Wilson <no@spam.com> - 2018-10-06 21:02 +0000
                Re: really fast buffers tabbypurr@gmail.com - 2018-10-06 16:57 -0700
                Re: really fast buffers krw@notreal.com - 2018-10-06 22:30 -0400
                Re: really fast buffers Gerhard Hoffmann <ghf@hoffmann-hochfrequenz.de> - 2018-10-07 04:30 +0200
                Re: really fast buffers <698839253X6D445TD@nospam.org> - 2018-10-07 07:15 +0000
                Re: really fast buffers John Larkin <jjlarkin@highlandtechnology.com> - 2018-10-07 08:24 -0700
                Re: really fast buffers bill.sloman@ieee.org - 2018-10-07 18:51 -0700
                Re: really fast buffers tabbypurr@gmail.com - 2018-10-08 08:16 -0700
                Re: really fast buffers Winfield Hill <hill@rowland.harvard.edu> - 2018-10-07 07:19 -0700
                Re: really fast buffers Robert Baer <robertbaer@localnet.com> - 2018-10-10 00:38 -0700
                Re: really fast buffers tabbypurr@gmail.com - 2018-10-10 03:02 -0700
                Re: really fast buffers bill.sloman@ieee.org - 2018-10-10 04:07 -0700
                Re: really fast buffers DecadentLinuxUserNumeroUno@decadence.org - 2018-10-10 11:28 +0000
                Re: really fast buffers Robert Baer <robertbaer@localnet.com> - 2018-10-11 22:06 -0700
                Re: really fast buffers bill.sloman@ieee.org - 2018-10-12 00:08 -0700
                Re: really fast buffers Robert Baer <robertbaer@localnet.com> - 2018-10-13 22:29 -0700
                Re: really fast buffers bill.sloman@ieee.org - 2018-10-14 00:30 -0700
                Re: really fast buffers Robert Baer <robertbaer@localnet.com> - 2018-10-15 19:38 -0700
                Re: really fast buffers bill.sloman@ieee.org - 2018-10-15 20:55 -0700
                Re: really fast buffers tabbypurr@gmail.com - 2018-10-14 04:31 -0700
                Re: really fast buffers bill.sloman@ieee.org - 2018-10-14 07:29 -0700
                Re: really fast buffers Robert Baer <robertbaer@localnet.com> - 2018-10-15 19:42 -0700
                Re: really fast buffers bill.sloman@ieee.org - 2018-10-15 20:57 -0700
                Re: really fast buffers tabbypurr@gmail.com - 2018-10-16 08:41 -0700
                Re: really fast buffers krw@notreal.com - 2018-10-16 17:38 -0400
                Re: really fast buffers bill.sloman@ieee.org - 2018-10-16 19:37 -0700
                Re: really fast buffers bill.sloman@ieee.org - 2018-10-16 19:40 -0700
                Re: really fast buffers Robert Baer <robertbaer@localnet.com> - 2018-10-15 19:40 -0700
                Re: really fast buffers Robert Baer <robertbaer@localnet.com> - 2018-10-11 22:04 -0700
                Re: really fast buffers tabbypurr@gmail.com - 2018-10-12 08:02 -0700
                Re: really fast buffers Gerhard Hoffmann <ghf@hoffmann-hochfrequenz.de> - 2018-10-10 14:06 +0200
                Re: really fast buffers John Larkin <jjlarkin@highlandtechnology.com> - 2018-10-10 08:31 -0700
                Re: really fast buffers Gerhard Hoffmann <ghf@hoffmann-hochfrequenz.de> - 2018-10-11 01:49 +0200
                Re: really fast buffers John Larkin <jjlarkin@highland_snip_technology.com> - 2018-10-10 17:41 -0700
                Re: really fast buffers "John Miles, KE5FX" <jmiles@gmail.com> - 2018-10-10 20:53 -0700
                Re: really fast buffers Jasen Betts <jasen@xnet.co.nz> - 2018-10-11 05:47 +0000
                Re: really fast buffers Gerhard Hoffmann <ghf@hoffmann-hochfrequenz.de> - 2018-10-11 12:35 +0200
                Re: really fast buffers George Herold <gherold@teachspin.com> - 2018-10-06 12:39 -0700
                Re: really fast buffers Steve Wilson <no@spam.com> - 2018-10-06 20:11 +0000
                Re: really fast buffers John Larkin <jjlarkin@highlandtechnology.com> - 2018-10-08 20:26 -0700
                Re: really fast buffers Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> - 2018-10-06 14:45 -0400
                Re: really fast buffers George Herold <gherold@teachspin.com> - 2018-10-06 12:46 -0700
                Re: really fast buffers John Larkin <jjlarkin@highlandtechnology.com> - 2018-10-08 21:00 -0700
                Re: really fast buffers Gerhard Hoffmann <ghf@hoffmann-hochfrequenz.de> - 2018-10-09 07:11 +0200
                Re: really fast buffers Chris Jones <lugnut808@spam.yahoo.com> - 2018-10-09 23:20 +1100
                Re: really fast buffers Jeroen Belleman <jeroen@nospam.please> - 2018-10-09 15:17 +0200
          Re: really fast buffers John Larkin <jjlarkin@highlandtechnology.com> - 2018-10-04 09:18 -0700
            Re: really fast buffers Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> - 2018-10-04 12:39 -0400
              Re: really fast buffers John Larkin <jjlarkin@highland_snip_technology.com> - 2018-10-04 11:51 -0700
        Re: really fast buffers Piotr Wyderski <peter.pan@neverland.mil> - 2018-10-08 13:19 +0200
          Re: really fast buffers John Larkin <jjlarkin@highlandtechnology.com> - 2018-10-08 08:52 -0700
  Re: really fast buffers whit3rd <whit3rd@gmail.com> - 2018-10-04 11:57 -0700
  Re: really fast buffers David Nadlinger <david@klickverbot.at> - 2018-10-04 23:05 +0100
    Re: really fast buffers John Larkin <jjlarkin@highland_snip_technology.com> - 2018-10-04 17:23 -0700
      Re: really fast buffers Gerhard Hoffmann <ghf@hoffmann-hochfrequenz.de> - 2018-10-05 11:26 +0200
        Re: really fast buffers Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> - 2018-10-05 10:44 -0400

csiph-web