Groups | Search | Server Info | Keyboard shortcuts | Login | Register [http] [https] [nntp] [nntps]

Groups > sci.electronics.design > #462157

Re: Running out of ideas: stability, bootstrap

Show all headers | View raw

On 05/28/2017 11:11 AM, Gerhard Hoffmann wrote:
> 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

Which feedback loop? There are two that I see in the circuit as posted: 
the one driven by the FET source feeding the R17/R23 etc. RC network 
where U1A is acting as an inverting amplifier, and possibly some kind of 
integrator network off page used to generate the bias voltage for the 
FET gate.

> 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

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

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 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.

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...

Back to sci.electronics.design | Previous | Next — Previous in thread | Next in thread | Find similar | Unroll thread

Thread

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

csiph-web