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Positive feedback loop strangely converge.

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 ?

-- Thanks, Fred.

******************** Version 4 SHEET 1 1260 1144 WIRE 48 384 48 448 WIRE 208 384 208 416 WIRE 208 384 48 384 WIRE 208 496 208 624 WIRE 208 624 416 624 WIRE 624 608 624 528 WIRE 624 528 560 528 WIRE 480 528 416 528 WIRE 416 528 416 624 WIRE 416 624 576 624 WIRE 768 944 768 528 WIRE 768 528 624 528 WIRE 768 944 912 944 WIRE 416 672 48 672 WIRE 208 960 208 1056 WIRE 208 736 208 624 WIRE 256 944 768 944 WIRE 416 672 416 896 WIRE 416 896 256 896 WIRE 48 704 48 672 WIRE 48 784 48 800 WIRE 416 672 576 672 WIRE 208 880 208 816 WIRE 48 576 48 528 FLAG 624 688 0 FLAG 208 1056 0 FLAG 48 800 0 FLAG 912 944 out IOPIN 912 944 Out FLAG 48 576 0 SYMBOL lib\\sym\\voltage 48 432 R0 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 SYMATTR InstName V1 SYMATTR Value 10 SYMBOL lib\\sym\\res 224 512 R180 WINDOW 0 36 76 Left 0 WINDOW 3 36 40 Left 0 SYMATTR InstName R2 SYMATTR Value 100K SYMBOL lib\\sym\\e2 624 592 R0 SYMATTR InstName E2 SYMATTR Value 1e6 SYMBOL lib\\sym\\res 576 512 R90 WINDOW 0 0 56 VBottom 0 WINDOW 3 32 56 VTop 0 SYMATTR InstName R5 SYMATTR Value 100K SYMBOL lib\\sym\\voltage 48 688 R0 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 SYMATTR InstName V2 SYMATTR Value 5V SYMBOL lib\\sym\\res 192 720 R0 SYMATTR InstName R9 SYMATTR Value 10K SYMBOL lib\\sym\\e 208 864 M0 SYMATTR InstName E_pb SYMATTR Value 1E6 TEXT 14 1128 Left 0 !;op TEXT 760 1040 Left 0 !.nodeset v(out)=0 TEXT 368 1032 Left 0 !.dc V1 0 10 0.1
Reply to
Fred Bartoli
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"Fred Bartoli" a écrit dans le message de news:416cf528$0$6495$ snipped-for-privacy@news.free.fr...

how

should

but

Just forgot to mention that this is not an LTspice pb and should be reproducible with other spices. At least it is with the other one I have and that's why I tried LTspice as a doucble check.

--
Thanks,
Fred.
Reply to
Fred Bartoli

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?

Kevin Aylward snipped-for-privacy@anasoft.co.uk

formatting link
SuperSpice, a very affordable Mixed-Mode Windows Simulator with Schematic Capture, Waveform Display, FFT's and Filter Design.

Reply to
Kevin Aylward

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.

--
Thanks,
Fred.
Reply to
Fred Bartoli

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 snipped-for-privacy@anasoft.co.uk

formatting link
SuperSpice, a very affordable Mixed-Mode Windows Simulator with Schematic Capture, Waveform Display, FFT's and Filter Design.

Reply to
Kevin Aylward

"Fred Bartoli" schrieb im Newsbeitrag news:416cf528$0$6495$ snipped-for-privacy@news.free.fr...

how

should

but

Hello Fred, I was involved in a discussion about a similar "problem" with a simple inverting opamp circuit a year ago. If you use an E-device in an inverting amplifier configuartion, then you can exchange the + and - input and LTSPICE and PSPICE still converge.

There is really a mathematical solution which fulfills Ohm's law even when you exachange the + and - input of an opamp. It's just noise that guarantees that you never can experience that in a real circuit.

The only safe way to discover such a wrong connection is a .TRAN simulation after you have added e.g. 100p parallel to the 100k feedback resistor in your circuit.

Best Regards, Helmut

----- Original Message ----- From: "Helmut Sennewald" Newsgroups: de.sci.electronics Sent: Sunday, November 09, 2003 2:14 PM Subject: Re: Spannung subtrahieren mit OP

ist

egal

Reply to
Helmut Sennewald

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