When I was trying to see the effect of load at the output of an RC low-pass filer I found myself in some sort of confusion. Below is an RC low-pass filter circuit with another R as a load and its Thevenin equivalent:
LTspice simulation in time domain shows that these circuits are giving same outputs at any frequency. Therefore I thought the cut-off frequency of the Original circuit must be the same as its Thevenin equivalent.
To verify it I first plotted the frequency response of the Original circuit as below:
As you see in the above plot, the Vout starts from -6dB and there is no -3dB in the entire plot for a cut-off frequency.
Then I plotted the frequency response of the Thevenized equivalent as below:
Above plot shows the -3dB point corresponds to 2.3Hz cut-off frequancy.
And finally I used the cut-off formula fc = 1/(2 * pi* R * C) for the Thevenized equivalent circuit and found fc = 31.8Hz.
What is going on wrong here?
All three ways give different results. Which one is the right one?