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- Thread starter jjk
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berkeman

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Welcome to the PF.

What is the context of the question? In general, you need to solve the whole circuit together, to figure out what the composite time constant is. This is because the circuits load each other (when there is no buffer amp between them), which changes their individual behaviors.

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Thanks jjk or PVDF.

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berkeman

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Thank you to yourself? :uhh:Thanks jjk or PVDF.

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berkeman

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Yes, the PF software is having some issues at the moment. We'll clean up the multiple user names later (it's against the PF rules, but understandable at the moment).

On your question, write out the equations and solve them to see what the effect of the two series RC circuits is...

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berkeman

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Can you please post some of this work that you've been doing? Thanks.

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The circuitry I have been using is --RC--RC-- where the two RC circuits are in parallel, and the Z'=∑R_k/((1+(ωCR)^2 )) and Z" = -ω∑(CR^2)/((1+(ωCR)^2 )), however upon using this to model the data ie C1=50pF and C2=100pF, R1=R2=1Mohm and sweeping frequency 0.1-100 kHz it is an RC semicircle but the max. gives a tao = 0.1 ms (R2*C2), the experimental data on the other hand, upon subtracting the reference tao C1*R1 from the total tao I get 0.11 ms. I am only trying to determine C2 and I get the right C when subtracting C(total) from C1 but circuit analysis wise I don't see why this works?

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berkeman

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I'm sorry to be dense, but what do you mean by "the two RC circuits are in parallel"? Where are you injecting your signal, and where are you measuring the output signal. And what is the source impedance of your signal generator?The circuitry I have been using is --RC--RC-- where the two RC circuits are in parallel, and the Z'=∑R_k/((1+(ωCR)^2 )) and Z" = -ω∑(CR^2)/((1+(ωCR)^2 )), however upon using this to model the data ie C1=50pF and C2=100pF, R1=R2=1Mohm and sweeping frequency 0.1-100 kHz it is an RC semicircle but the max. gives a tao = 0.1 ms (R2*C2), the experimental data on the other hand, upon subtracting the reference tao C1*R1 from the total tao I get 0.11 ms. I am only trying to determine C2 and I get the right C when subtracting C(total) from C1 but circuit analysis wise I don't see why this works?

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