C_notes_ trying for a simple, intuitive view of RC filters

C_notes_ trying for a simple, intuitive view of RC filters...

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Class 2 Follow-up: C Notes 1 Class 2 Recapitulation: C Notes REV 0 1 ; July 6, 2007 Contents 1 C’s differ from R’s, of course 1 2 Lowpass Filter 2 2.1 A lowpass attenuates high frequencies:. ................. 2 2.2 A lowpass passes low frequencies . ................... 2 3 High-pass 3 3.1 High-pass does transfer quick changes, from input to output. ...... 3 a) A quick edge passe s.......................... 3 b) A quick square wave passes. ..................... 4 c) A quick sinusoid also passes. 4 3.2 . ..butthe high-pass does not pass low frequencies . ........... 5 1 C’s differ from R’s, of course This note tries some very-simple arguments, in the hope that they may help you to strengthen your intuitive grip on the behavior of capacitors: not the sort of grip provided by X C = - j ω ˙ C , but the sort of grip that might be provided by a mental picture of the way voltages change on the two plates of the cap, as an input is applied. Resistors live in the present: Figure 1: Hippy Resistor: no sense of time Capacitors, in contrast, are fundamentally conservative. The voltage across a cap can’t change abruptly, and RC circuits may be much influenced by what happened a while ago 2 : Figure 2: Proustian Capacitor: . . . believes that the past
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C_notes_ trying for a simple, intuitive view of RC filters...

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