Poles and large signals ...

[Paolo Predonzani]

> source
> to GND. My filtered output signal would be the voltage across the current
> source (actually, with factor gm of the next stage). Now  the resulting "R" 
> of this
>  "RC" combination is the *sum* of both resistors ... and with one of these
> becoming very large (Ibias-Isignal near zero ...), the sum would be 
> dominated
> by this large resistor, forcing the pole of this stage down. Is this the 
> idea?

Yes. Keep in mind that the poles are re-calculated at EVERY simulation step.
This can be once, twice or 3 times per period, depending on the oversampling
factor.

> Now this whole approach should bring the same results for specific cases
> that we know or we can measure. We could set a filter to f=100Hz,
> and measure how far an input signal of 200Hz would be attenuated,
> dependend on input level. Of course we would measure the strengh of the
> fundamental only, at the output, to make sure that we just examine the
> effect of pole shift on the fundamental.
> I'd expect the fundamental being more attenuated in the case when the
> input is overdriven. *But* what is the reason for this:
> (1)The momentary pole migration, as described above?
> (2) The well-known *compression* effect of an overloaded differential pair 
> ??
> Or would both effects add to each other ???

Both effects are present.
Did you look at the pole plot? The test conditions are:
input: 262Hz 0.5V peak-peak
fcutoff: 4KHz
full resonance.
Now: the filter self-oscillates. Count the oscillations in one (output) period.
They are 11. What? They should be 4000/262 = 15 !!
I tested the circuit varying the input amplitude:
0.1V p-p  ->  14 oscillations
0.025V p-p  ->  15 oscillations
Of course the shape of the input wave is important too (a square wave 
overdrives the filter more than a sawtooth) but the results are similar.


> 
> 
> Thanks for listening - hope this is readable
> Any comments / correction welcome!
> 
> JH.
> 


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| Paolo Predonzani  |  email: predo at dist.dist.unige.it |
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