# bode plots for low pass filter circuits

Joined Jul 8, 2004
1
I am in 2nd year uni doing electronics as a part of my physics course, I have done an experiment with low pass filters for two circuits (A&B) for the results see attached file
as a part of my homework I need to create a bode plot for
Amplitude of voltage transfer function v's Frequency
Phase of voltage transfer function v's Frequency for both circuits
However I have no idea where to start!
and definately no idea on how to find the values needed for the phase

If anyone can help I would be soooo greatfull

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#### mozikluv

Joined Jan 22, 2004
1,435
hi,

is your low pass filter a single pole or two pole topology? what's the Q? what's your standard reference point for your roll-off? what's your input voltage? what's the group delay?

the amplitude response of a low pass filter is flat near DC up to a point it starts to roll-off. the usual reference point for your roll-off is the point where the amplitude has gone down by 3dB which is about 71% of the original amplitude. the passband of the filter is the region where from near DC to the point where the amplitude is 3dB down. while the stopband of the filter is where the range of frequencies from 3dB point up to infinity.

the amplitude of the filter at 10x the 3dB frequency is attenuated for a total of 20dB for a one pole filter while for a 2 pole it's 40dB. now at higher frequencies your amplitude will continue to roll-off in a linear manner and the slope of the line is -20dB/decade for a one pole & -40dB/decade for 2 pole.

the change in phase of a single pole is 45* at 3dB and ends 90* at infinity, while a 2 pole is 90* at 3dB and ends at 180* at infinity.

this is the least i can do for your bode plot.