capacitor response

[ISSAC NEWTON]

what happens to the phase of a sine wave when passed through a high pass filter circuit and a low pass filter circuit.does capacitor always produce a 90 degree phase shift for a sine wave at all frequencies.please tell me the response at different frequencies of signals.can we see the phase shift produced by an inductor to current in a cro , or only voltages can be seen in it? please comprehend.

[TonyMcC]

If you are looking at simple filters using either a capacitor in series with the load to allow high frequency signals to pass or in parallel with the load to shunt high frequencies you will always get phase shift. The amount of phase shift will change with frquency but will never be as great as 90 degrees. If you consider a capacitor in isolation, i.e theoretically in a circuit with no resistance then any sine wave voltage will produce a sine wave of current with the current waveform leading the voltage waveform by 90 degrees. It is similar to a pure inductor except the current waveform lags the voltage by 90 degrees. A memory aid is the word CIVIL which helps you to remember "For a capacitor current leads voltage, voltage leads current in an inductor". Using a cro to see current waveforms can be done by inserting a small resistor of known value (say 0.1 ohm) and looking at the voltage waveform across the resistor. Calculate current using Ohm's law. If the resistor is small compared to the reactance of the inductor it will have little effect on the circuit action.

[ISSAC NEWTON]

If you are looking at simple filters using either a capacitor in series with the load to allow high frequency signals to pass or in parallel with the load to shunt high frequencies you will always get phase shift. The amount of phase shift will change with frquency but will never be as great as 90 degrees. If you consider a capacitor in isolation, i.e theoretically in a circuit with no resistance then any sine wave voltage will produce a sine wave of current with the current waveform leading the voltage waveform by 90 degrees. It is similar to a pure inductor except the current waveform lags the voltage by 90 degrees. A memory aid is the word CIVIL which helps you to remember "For a capacitor current leads voltage, voltage leads current in an inductor". Using a cro to see current waveforms can be done by inserting a small resistor of known value (say 0.1 ohm) and looking at the voltage waveform across the resistor. Calculate current using Ohm's law. If the resistor is small compared to the reactance of the inductor it will have little effect on the circuit action.

 

THANK YOU FOR YOUR REPLY.NOW TELL ME WILL THERE BE A PHASE SHIFT IN THE INPUT SINE WAVE IN A LOW AND HIGH PASS FILTER WHEN THE GAIN=1?I GOT NO PHASE SHIFT WHEN I SIMULATED IT(@ GAIN=1).IF NO SHIFT,WHY? I WOULD LIKE TO KNOW THE REASON BEHIND IT.PHYSICAL INTERPRETATIONS ARE WELCOME RATHER THAN MATHEMATICAL REASONING.THANK YOU.

[TonyMcC]

I am a bit puzzled by your question. Let's consider the high pass filter consisting of a capacitor in series with your load. There must be some voltage drop across the capacitor. This means that the gain cannot be 1. This suggests your simulation has a capacitor with zero reactance - i.e no capacitor at all ( a straight piece of wire connecting the signal to the load). Hence no phase shift or loss of signal.

If you make a low pass filter using an inductor instead of a capacitor the same argument applies. The voltage drop across the inductor will ensure a gain of less than 1. Your simulation is for an inductor with zero reactance - i.e. no inductance at all (i.e a straight piece of wire). Hence no phase shift.

In the case of the high pass filter you could theoretically use a capacitor with infinite capacitance!( Xc=1/(2*pi*f*C) ohms.

I hope I understand your problem and that this helps.

Edited January 25, 2011 by TonyMcC