Re: Circuits that shouldn't work...?

On Sat, 31 Dec 2005 15:34:45 +0100, "Helmut Sennewald"
<helmutsennewald@xxxxxxxxxxx> wrote:

>"Kevin Aylward" <see_website@xxxxxxxxxxxxx> schrieb im Newsbeitrag
>news:Zmttf.42527$D47.14976@xxxxxxxxxxxxxxxxxxxxxxxxxxxx
>> Helmut Sennewald wrote:
>> > <onehappymadman@xxxxxxxxx> schrieb im Newsbeitrag
>> > news:1135984816.969794.45930@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
>> >> I remember someone once posted about a circuit that Spice said should
>> >> be unstable, yet when built, the circuit works perfectly fine.
>> >>
>> >> What circuit was this? What other circuits don't work in
>> >> simulations, but work perfectly fine in real life???
>> >
>> > Hello,
>> >
>> > we still wait for the circuits. I remember that somebody
>> > (sorry I forgot his name) want bring the circuits
>> > next week. This was many months ago. Let's hope he reads
>> > this thread and bring the circuits now. I admit that
>> > there may be circuits where SPICE fails because the
>> > models are not good enough or SPICE doesn't take noise
>> > into account.
>> >
>> > I know of one circuit where SPICE calculates very precisely
>> > a mathematical correct solution for a noiseless circuit,
>> > but in real life the output of this circuit would run to
>> > the rails because of noise at the inputs. It' a very very
>> > simple circuit. You wouldn't believe it when you haven't seen it.
>> >
>>
>> Ho hum, *any* high gain amp with zero offset will give zero output in
>> spice with zero input yet its real life embodiment will rail out. So,
>> whats new?
>>
>> GIGO.
>>
>> Kevin Aylward
>
>
>Hello Kevin,
>
>My circuit is an amplifier(E-source) in an inverting circuit,
>but by mistake the resistors are connected to the positive input.
>LTspice and any other SPICE will find a solution which
>is mathematically correct, but you will be unable to reproduce
>that with any real circuit. The calculated ouput voltage is
>-1.0002V with the wrong circuit and -0.9998V with the correct
>inverting circuit.
>
>Again, the math done by SPICE is correct in both cases!
>Everybody would say the inputs are wrongly connected,
>but SPICE is so clever and finds a solution you never
>have seen in any book! Patent pending! :)
>
>
>Have fun and a happy new year.
>
>Best regards,
>Helmut
>
>
> R2 R1
> in ___ N001 ___
>V1 ---<___>----o-----<___>------o----out
>(1V) | | (-1.0002V)
> | --- |
> ---|+ |--------
> ---|- |-- v=10000
> _|_ --- _|_
> - E1 -
>
>
>
>This would be the correct circuit.
>
> R2 R1
> in ___ N001 ___
>V1 ---<___>----o-----<___>------o----out
>(1V) | | (-0.9998V)
> | --- |
> ---|- |--------
> ---|+ |-- v=10000
> _|_ --- _|_
> - E1 -
>
[snip]

But you haven't proven anything. The ideal E-source (infinite input
Z, zero output Z, no phase shift, no offset voltage, noiseless)
doesn't exist in the real world.

...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona Voice:(480)460-2350 | |
| E-mail Address at Website Fax:(480)460-2142 | Brass Rat |
| http://www.analog-innovations.com | 1962 |

"Winners never quit, quitters never win", Jack Bradley Budnik ~1956
.