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Re: Running out of ideas: stability, bootstrap

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Am 29.05.2017 um 14:38 schrieb bitrex:

> I'm assuming the goal here is to build a low-noise source follower, with 
> the output buffered by an op amp.

No, a very low noise amplifier with 60 dB gain, 100 mHz to 1 MHz.
I'm aiming for 100pV/rtHz at least on the sweet spot.

> It looks like the non-inverting input of U1A is tied to a virtual 
> ground, and the inverting input is tied to the collector of Q3, I'm 
> guessing to try to use negative feedback to fix the collector voltage at 
> 4.57. But since Q3 is fed by current source Q2, the collector of Q3 
> (test point P3) is a high-impedance node and so it seems to me that the > only way the DC voltage there can be controlled is by U1A sensing the
> source voltage of the FET, through the R17/R23 network, and then around 
> the loop through the integrator (not shown) back to the gate of the FET, 
> if I'm understanding the circuit correctly.

The current source defines the bias current and the FET's gate voltage
is adjusted by an integrator until the cascode collector equals the
level of CENTER.


> The gain from the gate of the cascoded FET to the output of U1A isn't 
> well-defined; from "its perspective" it looks like a current source 
> loaded common-emitter amplifier feeding a transimpedance converter. It 
> will be very large. Plus the phase shift from U1A and the integrator to 
> the gate it sounds like a recipe for oscillation.

The signal gain from the gate to the output of U1A is determined by
the  gm of the FET array, which _is_ large and by the transimpedance
amplifier. (assuming open loop)

The current source and the cascode transistor do not enter the
equation other than second order effects like the ccs diverting
some signal current or that the BJT has probably a much higher
Early voltage and thus higher output impedance.

The integrator is also not in the signal path. It is as slow
as molasse. It takes a minute until the window comparator
thinks that the operating point has been reached. There may
be some influence in the mHz region, but I have no problem there.

> If I were dead-set on using a current source for the cascode collector 
> load I think it would be better to let one op-amp section handle a 
> feedback loop around the upper current source to set the DC bias at Q3's 
> collector, and let U1A and the integrator just handle setting the FET's 
> bias and the 0V DC level to the post amplifier. Delegate some 
> responsibility...

The current source is necessary to suppress the noise on VCC; without it
it takes a supply better than 2nV/rtHz (for 8 FETs and twice the
current). There also must be 5 diodes in series or the noise of
the current source emitter resistor starts playing a role.

Gerhard

who got a 54754A differential TDR plug in this morning, still a month
under service. That needs to be played with now.  :-)  :-)

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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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