- #1

schaefera

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Hello, I'm trying to analyze the RC circuit... specifically a low-pass filter, so the set up is voltage source to resistor to capacitor, and the voltage source provides an oscillating voltage Vin, whereas I measure Vout across the capacitor.

I think I'm getting confused between two different possible analyses: first, comparing Vin and Vout and second, comparing the current and the voltage in the circuit. Please tell me where I'm going wrong.

If I consider the ratio Vout/Vin, I have the complex number: 1/(1+iwRC) which has the phase phi1= -arctan(wRC). This would imply that at very high w, the output voltage is 90 degrees behind the input voltage. But since at very high w the capacitor drops very little voltage, this circuit is essentially just a resistor dropping all the voltage. So why wouldn't we expect the output and input voltages to be IN phase in this case? Similarly, at very low w the capacitor drops most of the voltage and despite the fact that the phase angle approaches 0 in this limit. Since a purely capacitive circuit has a 90 degree phase shift, shouldn't we expect this shift to be present in that case?

I think it would be easier if I could draw a phasor diagram, but I can only draw a phasor diagram to compare Vout (which would be the sum of capacitive voltage and resistive voltage, the two being at right angles to each other) to the current (which is parallel to the resistive voltage)... I wouldn't know where to place Vin on such a diagram would I?

Again, I think I'm getting confused between looking at Vout/Vin and Vout/Total current. Please tell me where my errors are! Thank you!

I think I'm getting confused between two different possible analyses: first, comparing Vin and Vout and second, comparing the current and the voltage in the circuit. Please tell me where I'm going wrong.

If I consider the ratio Vout/Vin, I have the complex number: 1/(1+iwRC) which has the phase phi1= -arctan(wRC). This would imply that at very high w, the output voltage is 90 degrees behind the input voltage. But since at very high w the capacitor drops very little voltage, this circuit is essentially just a resistor dropping all the voltage. So why wouldn't we expect the output and input voltages to be IN phase in this case? Similarly, at very low w the capacitor drops most of the voltage and despite the fact that the phase angle approaches 0 in this limit. Since a purely capacitive circuit has a 90 degree phase shift, shouldn't we expect this shift to be present in that case?

I think it would be easier if I could draw a phasor diagram, but I can only draw a phasor diagram to compare Vout (which would be the sum of capacitive voltage and resistive voltage, the two being at right angles to each other) to the current (which is parallel to the resistive voltage)... I wouldn't know where to place Vin on such a diagram would I?

Again, I think I'm getting confused between looking at Vout/Vin and Vout/Total current. Please tell me where my errors are! Thank you!

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