# AoE RC chapter 2, page 56, voltage divider using C1 and R2

Discussion in 'Homework Help' started by ninjaman, Jul 28, 2016.

1. ### ninjaman Thread Starter Member

May 18, 2013
306
1
Hello,
I am trying to read through the lab manual to the AoE 3rd ed. I am on chapter 2, RC circuits. It is showing a voltage divider setup with capacitor in R1 position and resistor in R2 position. It mentions integrators and differentiators. There is a part about error, this is the part that I find confusing as I do not know what they are talking about. They mention dVin/dt = dVcap/dt, this is apparently good but causes the Vout to move. Then says that the Vout should move above ground as it is required to give an output?
I find some parts of the AoE to be a bit confusing, is this normal. I have read that other people find it a bit of a confusing read. I have some other books but none that seem to cover as much as AoE, but then the others seem to be more to the point.

2. ### JoeJester AAC Fanatic!

Apr 26, 2005
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can you post the schematic? or a picture of the page.

Last edited: Jul 28, 2016
3. ### ninjaman Thread Starter Member

May 18, 2013
306
1
I think I could do a scan of the page in the book but the picture is of a voltage supply (Vin+) at the top, then a resistor going down to a capacitor (in series) and then ground. In between the capacitor and resistor is the output. It looks like what I would call the example of a voltage divider but with a capacitor in place of a resistor. There are two pictures, one with the capacitor on top, the other on the bottom. There are no values as it is talking about how the integrator and differentiator work. It mentions this error thing that I don't understand. I don't think that understanding calculus is going to help either as it is something to do with the capacitor causing a rise in voltage at the output. No maths is mentioned really. This rise in voltage is apparently not welcome, though required as the output wouldn't be an output without a difference in voltage above ground. I think that the point they are trying to get across (that I am missing) is that the voltage across a capacitor causes something that is unwanted compared to a current source. There is mention that the RC values should be chosen to meet Vout << Vin. They mention that the circuit should "murder" the incoming signal, so the output should be significantly different to the input. I am just confused as to what this "error" is.

JoeJester, that NEETS file that you have, is that better than AoE? if there is anything about RC circuits and would do a better job than the AoE lab manual that would be great!

4. ### Jony130 AAC Fanatic!

Feb 17, 2009
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The capacitor is a example of an "ideal differentiator". Without the bottom resistor the capacitor current is proportional to I = C* dVin/dt.
So the capacitor alone (without resistor) is a "ideal" input voltage to current differentiator. But if we add a resistor we have a error, because now the capacitor voltage is no longer equal to Vin. Now the capacitor voltage is Vc = (Vin - Vr). So the capacitor do not differentiate Vin only. So now we no longer have a "ideal differentiator".

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