Question

inductors block high-frequency signals and conduct low-frequency signals, while capacitors do the reverse. A filter in which the signal passes through an inductor, or in which a capacitor provides a path to ground, presents less attenuation to low-frequency signals than high-frequency signals and is therefore a low-pass filter. If the signal passes through a capacitor, or has a path to ground through an inductor, then the filter presents less attenuation to high-frequency signals than low-frequency signals and therefore is a high-pass filter ....please explain this

Answer 1

@08surya ...why do they say a path in which capacitor is connected to ground allows low frequency signals..isnt that against the definition of capacitive reactance?

Answer 2

and similarly for an inductor connected to the ground?

Answer 3

@stgreen

Answer 4

ok lets begin with capacitors...you know capacitors have a "Time constant=RC"...means they need some time to get charged right?

Answer 5

yeah..63.2% of its initial value..right

Answer 6

Since frequency of DC is zero, when a DC signal is applied across a capacitor...it charges the capacitor in that particular time (which is usually very small) so after capacitor is charged, it has a constant voltage across it unless it starts discharging...right?

Answer 7

after the capacitor gets charged by DC signal, it has a constant voltage across it, and it doesn't draw anymore current from DC source..which means it appears to block DC after getting fully charged

Answer 8

got

Answer 9

ok now when AC signal is applied across a capacitor, it charges capacitor in positive cycle and discharges it in the negative cycle...it keeps on and on..so we say capacitors let AC signals pass through them

Answer 10

ok

Answer 11

so capacitors let high frequency signals pass through them and in a sense block low frequency signals...now you want to know about filters or what?

Answer 12

yeah filters..and abt that capacitor connected to ground

Answer 13

lets go through low pass filter first, see the figure attached, when a low frequency signal (DC) is applied as Vin...it charges the capacitor first (in it's time constant), then capacitor has no way to discharge, so whatever low frequency signal we apply appears across Vout

Answer 14

But when a high frequency signal is applied, it discharges the capacitor in the negative half cycle...and of course charges the capacitor in the next half cycle..means the high freq. signal passes through the capacitor (capacitor acts like a short-circuit)..mean Vout=0

Answer 15

Just remember two simple facts for capacitors:
1) they act like short-circuit for AC signal (high frequency signal)
2)they act like open-circuit for DC (or low frequency signal)
and why do they?See my first answer

Answer 16

Are you following?

Answer 17

ok crystal clr :)

Answer 18

great

Answer 19

Now lets see a high pass filter..

Answer 20

ready

Answer 21

See the attachment...when a DC (low frequency) signal is applied as Vin...will it pass through the capacitor after the capacitor gets charged?it's your turn tell me

Answer 22

no..because once its charged in a particular direction it stays that way

Answer 23

Right..so zero voltage appears at Vout...means low-frequency signal gets blocked

Answer 24

wait a sec...what happens to the drop across the capacitor...

Answer 25

ofcourse there's drop across capacitor..but we are looking at 'drop across R' which is Vout...see?

Answer 26

ok ok..sorry..

Answer 27

that's what i am saying