I am new to electronics and I have a little trouble applying the basics I learnt so far. My problem is basically that I don't understand why we consider the second circuit below as the Thevenin equivalent, even though the behaviour of the transfer function is different.
Here you can see the parallel RC circuit.
The supposed Thevenin equivalent.
Blue is the parallel RC-Circuit and green the Thevenin equivalent. I think the behaviour of the two circuits is apparent. Also, if we derive the transfer function for the circuits, we get two different solutions.
Transfer function for parallel RC-Circuit:
Transfer function for thevenin equivalent: