Re: Positive feedback loop strangely converge.

From: Kevin Aylward (salesEXTRACT_at_anasoft.co.uk)
Date: 10/13/04 

Date: Wed, 13 Oct 2004 20:27:06 GMT

Fred Bartoli wrote:
> "Kevin Aylward" <salesEXTRACT@anasoft.co.uk> a écrit dans le message
> de news:Icdbd.72726$BI5.15451@fe2.news.blueyonder.co.uk...
>> Fred Bartoli wrote:
>>> It's not the first time I encounter this problem and I don't
>>> understand how the solver can converge.
>>>
>>> I've made an LTspice file enclosed at the end.
>>>
>>> This is part of a CM reduction feedback loop. The pb is that the
>>> loop should and do converge with E_pb with a -1E6 gain and
>>> *shouldn't* for +1E6 gain but it strangely does, whatever I try.
>>>
>>> Anyone have a clue about this ?
>>
>> You haven't supplied enough info. There is no general reason why
>> positive feedback loops don't converge. Indeed, they usually do. Why
>> do you think they shouldn't?
>
> Hi Kev,
> Ok, I've badly worded my thought. It's not that it shouldn't converge
> but that it shoud not converge to the values it does.
>
> Of course my first reaction was that I've made a mistake somewhere
> since this kind of stuff is pretty obvious, but I've carefully
> checked and rechecked to not avail. I'm still convinced I overlooked
> something, but what ?
>
> What I meant is that a positive feedback loop, having an overall
> positive 10^12 loop gain and some incoming signal should clamp to the
> supply rails or go to PB's gigavolts values (DC operating point).
>
> ISTR that you have LT spice installed. If so you should have a look
> and try to play with E_pb gain sign. It's pretty obvious from the
> schematics.
>
> In that case, the operating point is the same with positive loop gain
> and negative loop gain of the same absolute value (10^12).
>
>
> Strange indeed.

This can happen. In fact I had it happen the other day:-). I also had a
feedback amp with its inputs swapped.

In many circuits, if the input is effectively zero, the output can be
zero irrespective of whether the feedback is positive or negative.
0*x=0, independent of x. The zero point may actually be a non-zero with
respect to ground.

I'll have a look to see if this is the case here.

Kevin Aylward
salesEXTRACT@anasoft.co.uk
http://www.anasoft.co.uk
SuperSpice, a very affordable Mixed-Mode
Windows Simulator with Schematic Capture,
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