Re: gEDA-user: ideal op amps in gschem

[Dan McMahill <dan@xxxxxxxxxxxx>]

Larry Doolittle wrote:
> Guys -
>
> On Tue, Jan 24, 2006 at 07:00:16PM -0800, Steve Meier wrote:
>
> > I once was listening to a EE Professor who was giving a lecture on the
> > "Ideal" opamp. As I recall, he stated that the "Ideal" opamp should have
> > a large imput impedence and infinite gain..... Which he went on to
> > further define as "approximetly 400."
>
>
> Putting on my "engineering theorist" hat ...
>
> There are conceptual problems with an amplifier that has
> infinite gain at finite frequencies, or even finite gain
> at all frequencies.
>
> An ideal op-amp has large, finite gain-bandwidth-product,
> which I could further define as "approximately 1 GHz".
> Note that this treatment does involve infinite gain at DC,
> which is OK.


so just for fun, now that we're on the topic of silly op-amp tricks....
Here's a circuit which I came across inside some commercially sold 
equipment.  The unit was not cheap (in terms of cost) and was not for 
the consumer market.

This is a little circuit with only an op-amp and 3 passives.  Can't get 
more simple than that right?  Turns out that the designers of it had 
some troubles understanding it.

The answers (to an accurate enough degree for engineering calculations) 
can be had in about 3 lines of hand analysis but I think it illustrates 
nicely how one must be careful with those "ideal" op-amps.  It's also a 
good case of where one shouldn't substitute a simulator (with a model 
which may not capture important details) for thinking!

Have fun.

-Dan


p.s.  hint:  The pin on the schematic is the only connection (other than 
power and ground) to the rest of the circuit that this little gem was 
found in.

Attachment: opamp.pdf
Description: Adobe PDF document