While the amplifierf is in its linear region the voltage accross the
Rx will be the output voltage divided by the opamp gain (at that
frequency). So if the output is 1 volt and the gain is 1000 then the
voltage will be 1mV (plus any constant offset).
However, if the amplifier output saturates then the voltage accross Rx
will increase & can be easily calculated by fixing the output end of
the 100k resistor to the saturation voltage.
So to work out the voltage accross Rx youneed to know the opamp open
loop gain and the operating conditions (linear or non linear).

Richard P

On 26/08/06, Mark Rages <markrages@gmail.com> wrote:
> Hi,
>
> The circuit I've attached is a staightforward op-amp amplifier with a
> gain of -10.
>
> For reasons outside the scope of this question*, I needed to add the
> resistor I've called Ra.
>
> In the traditional "virtual ground" method of analysis, Ra doesn't
> appear in the gain equation.  I tried Ra=10k in my circuit, and indeed
> it continued to work unchanged.  But I'm pretty sure if I set Ra=0 the
> circuit would stop working!
>
> To analyze this, I guess I will need a non-ideal op-amp model.  What
> is the important parameter to include?  Finite gain?  Offset voltage?
>
> Regards,
> Mark
> markrages@gmail
>
> * Because someone will want to know, it's to limit the voltage swing
> at an analog switch IC (not pictured).
> --
> You think that it is a secret, but it never has been one.
>  - fortune cookie
>
>
> --
> http://www.piclist.com PIC/SX FAQ & list archive
> View/change your membership options at
> http://mailman.mit.edu/mailman/listinfo/piclist
>
>
>
>
-- 
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist