# Forum: Analog Circuits Cutoff frequency for high/low pass filters

 Author: Benny (Guest) Posted on: 2013-04-14 01:23

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Hey guys,

Could you shed a little light on this problem here? I'm trying to
calculate the half power frequency for these 2 circuits, and I'm not
sure if my reasoning/math is right:

For the former one, I did Fc=1/(2πR*C) where R=(R1*R2)/(R1+R2)+R3. Is
this correct? Is this all?

Thank you

 Author: Helmut S. (helmuts) Posted on: 2013-04-14 09:11

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Hello Benny,

You have to look from the capacitor into the circuit when you calculate
the corner frequency or the time constant. Treat the voltage source as a
short (0Ohm) for the calculation, because it has zero internal
resistance.

a) Lowpass Filter

fc = 1/(2*pi*R*C)

R = 1/(1/R1 + 1/R2 + 1/R3)

b) Highpass Filter

fc = 1/(2*pi*R*C)

R = R1 + 1/(1/R2 + 1/R3)

By the way you can simulate this circuit with LTspice to check the
results.

Best regards,
Helmut

 Author: Benny (Guest) Posted on: 2013-04-14 15:53

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Thanks a lot Helmut

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