Drawing a Frequency-Magnitude (Bode) Plot: Thomas's Questions

[thomas49th]

I need to draw a frequency- magnitude (bode) plot of this circuit:

Firstly I need to find the RC but I don't have a clue.

It's meant to be 1ms, and I guess that's from 10k * 100nF

BUT why is the 20K not involved?

Thanks
Thomas

 

[gneill]

Looks to me like you've got a voltage divider comprising two impedances. One is the serial combination of a 100nF capacitor and a 20K resistor, and the other a 100nF capacitor and 10K resistor. You should be able to derive the transfer function on that basis.

 

[thomas49th]

I can easily derive the transfer function... but how do I find the RC value

 

[gneill]

I can easily derive the transfer function... but how do I find the RC value

Well, what do you mean by the "RC value"? The circuit is a combination of two filters.

 

[thomas49th]

in the transfer function there will be some jwRC. What is the value of RC? What resistors and caps contribute to the RC?

 

[thomas49th]

Hang on a sec. I've had a brainwave. I define the RC. I set R = 10K and C = 100nF. I've got myself confused...

Thanks for the help!

 

[berkeman]

Hang on a sec. I've had a brainwave. I define the RC. I set R = 10K and C = 100nF. I've got myself confused...

Thanks for the help!

Er, no.

You may be able to simplify that circuit into two RC responses, one HPF and one LPF, if the two RC values are far enough apart. That may be what you are referring to here.

Otherwise, you use the transfer function that you derived to draw the Bode plot...

 

[berkeman]

Er, no.

You may be able to simplify that circuit into two RC responses, one HPF and one LPF, if the two RC values are far enough apart. That may be what you are referring to here.

Otherwise, you use the transfer function that you derived to draw the Bode plot...

EDIT -- in the case of this circuit, the component values are too close together to use the trick of separating the LPF and HPF behaviors...