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Re: Running out of ideas: stability, bootstrap

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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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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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