> So if we ignore the input and output impedance the time constante is R1
and
> R2 in parallell* C ?
> regards
>         Luis

No!
If you ignore the input and output impedances the answer is meaningless or
wrong :-(.

If you assume that Ri and Ro are both very low then what you say is true.
This is not ignoring them but making assumptions about them (which may or
may not be true).

In real circuits the usual rough approximation that you can make is that Rin
is zero low (output of a buffer etc) and Rout is infinite or high (input of
an amplifier etc). In such a case the effective components are R1 and C and
the circuit acts like a simple 1 pole RC low pass filter.

In practice R2 is usually added as shown to "impedance match". If Rout is
actually low then adding R2 makes the capacitor 'see' R2 rather than a near
short circuit. The effect on signal levels is as per my previous formula (id
i didn't make an error somewhere)(anyone checked?)



        RM

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