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Running out of ideas: stability, bootstrap

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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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#462175

On 05/29/2017 11:44 AM, bitrex wrote:
> On 05/29/2017 11:36 AM, Jan Panteltje wrote:
> 
>> I looked at it twice and declined to comment as I
>> 1) have no idea what it does.
>> 2) have no idea what it is supposed to do.
>> 3) have no idea why make a peeseebee for something that one has not 
>> tested.
>> 4) looks overly complicatiaotiantiated to me, brain abort.
>>
> 
> Yeah, this is why I don't feel too bad if my suggestion wasn't on the mark.
> 
> I know I'm guilty of this myself sometimes, but while people are very 
> familiar with their own designs I don't think everyone else can 
> immediately intuit what's going on in a circuit that has say, more than 
> a couple transistors or op amps. A few sentences of explanation of 
> what's _supposed_ to happen would help OP a lot before jumping right 
> into what the problems are with...whatever it is.
> 
> My impression was that it's a cascode source follower with U1A acting as 
> a post amplifier, then feeding an integrator which then feeds the FET 
> gate resistor. Holding the collector of the top cascode transistor at 
> virtual ground by tying it to the op amp inverting input is clever, but 
> it also seems sketchy as the only way U1A has to hold it there is around 
> the loop to the FET gate. Transimpedance amp-connected op amp -> 
> integrator is at least 270 degrees of phase shift at high frequency 
> right off the bat. Seems sketch.

Unless one is willing to grunge through all the math to make sure 
everything works out then IMO feedback loops around stuff need to be 
super-tight and encompass the bare minimum of stages. Just wrapping 
stuff around stuff and assuming it will all work out because the ideal 
op amp equations say this voltage must be equal to this voltage in a 
negative feedback configuration is asking for trouble - even if the 
feedback loop is only "designed" to operate at DC.

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#462182

On a sunny day (Mon, 29 May 2017 11:44:20 -0400) it happened bitrex
<bitrex@de.lete.earthlink.net> wrote in <olXWA.100487$df3.20124@fx30.iad>:

>On 05/29/2017 11:36 AM, Jan Panteltje wrote:
>
>> I looked at it twice and declined to comment as I
>> 1) have no idea what it does.
>> 2) have no idea what it is supposed to do.
>> 3) have no idea why make a peeseebee for something that one has not tested.
>> 4) looks overly complicatiaotiantiated to me, brain abort.
>> 
>
>Yeah, this is why I don't feel too bad if my suggestion wasn't on the mark.
>
>I know I'm guilty of this myself sometimes, but while people are very 
>familiar with their own designs I don't think everyone else can 
>immediately intuit what's going on in a circuit that has say, more than 
>a couple transistors or op amps. A few sentences of explanation of 
>what's _supposed_ to happen would help OP a lot before jumping right 
>into what the problems are with...whatever it is.
>
>My impression was that it's a cascode source follower with U1A acting as 
>a post amplifier, then feeding an integrator which then feeds the FET 
>gate resistor. Holding the collector of the top cascode transistor at 
>virtual ground by tying it to the op amp inverting input is clever, but 
>it also seems sketchy as the only way U1A has to hold it there is around 
>the loop to the FET gate. Transimpedance amp-connected op amp -> 
>integrator is at least 270 degrees of phase shift at high frequency 
>right off the bat. Seems sketch.

Yes, and there is more we need to know, drive impedance, requirements,
I posted long ago here about my (accidently discovered) better alternative
vidicon preamp (better than the common bootstrap circuit).
It all depends...

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#462172

Jan Panteltje wrote...
>
>4) looks overly complicatiaotiantiated to me, brain abort.

 Bleep! Bleep!  Jan Panteltje brain abort!  Bleep!


-- 
 Thanks,
    - Win

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#462181

On a sunny day (29 May 2017 08:46:13 -0700) it happened Winfield Hill
<hill@rowland.harvard.edu> wrote in <oghfo5012sr@drn.newsguy.com>:

>Jan Panteltje wrote...
>>
>>4) looks overly complicatiaotiantiated to me, brain abort.
>
> Bleep! Bleep!  Jan Panteltje brain abort!  Bleep!

The real Art is Simple City, ehh simplicity.
:-)

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#462255

On Monday, May 29, 2017 at 11:20:49 AM UTC-4, bitrex wrote:
> On 05/29/2017 10:36 AM, Winfield Hill wrote:
> > Gerhard Hoffmann wrote...
> >>
> >> Some observations:
> >>
> >> - Zin is negative also with the feedback loop cut
> > 
> >   I think you could safely add a little resistance
> >   in Q3's emitter.  BTW, while studying the Zin
> >   scene, you can make changes that would damage
> >   the noise performance, just saying.
> > 
> > 
> 
> If you have a chance could you confirm and/or deny that there might be a 
> problem with the feedback connection from Q3's collector to U1A? It 
> looks that way to me or else I'm not understanding correctly how this 
> circuit is supposed to work, which is entirely possible given it's still 
> before noon ;-)

Q2 is a current-source load.

U1A keeps the FET current constant by feeding back cascode Q3's collector
node, producing the output voltage in the process (not shown).
G = R20/R(drain) = 1,000 if I'm not mistaken.  That's rather ambitious
for one stage.

Weirdness at Q3's base will affect the loop, & has to be ruled out. Q3
might also oscillate. I don't see that it should, but sometimes weird
things happen (e.g., negative input impedance).

Cheers,
James Arthur

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