Its said that as frequency increases, Xc decreases. If Xc is the opposition to AC current, then how does a capacitor roll off the high end if at higher frequencies the Xc is actually lower ?? What am I missing here ? thanks. :thumb:
Capacitive reactance
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Larrchild
Well-known member
If the capacitor is in parallel with the path from source to load ,and its reactance drops at higher freqencies then it will shunt more highs than lows, no?
CJ
Well-known member
Recall the formula for Inductive Reactance?
XL = 2 pi f L
It's just a simple equation that says when frequency goes up, resistance goes up.
Capacitive reactance is the exact same formula, just substitute C for L, and put a 1 over it.
This means that the resistance will go down as the frequency gets higher, because the denominator gets bigger with higher frequency, which means the number gets smaller.
So just use coils and caps as resistors, remembering their relationship of ohms vs frequency, and you can analyze all sorts of combinations.
XL = 2 pi f L
It's just a simple equation that says when frequency goes up, resistance goes up.
Capacitive reactance is the exact same formula, just substitute C for L, and put a 1 over it.
This means that the resistance will go down as the frequency gets higher, because the denominator gets bigger with higher frequency, which means the number gets smaller.
So just use coils and caps as resistors, remembering their relationship of ohms vs frequency, and you can analyze all sorts of combinations.
NewYorkDave
Well-known member
Theory discussions should really go into the Drawing Board, unless you want to see them vanish quickly beneath pages of SSL Clone threads :wink:
To help yourself understand, draw a capacitor in series with a resistor which are connected in series to an AC source.
Now imagine relatively what voltages will be present over either the resistor or the cap at either;
a) very low frequencies
b) very high frequencies
No need for calculations as this is just to display a point.
Capacitors will present a high resistance at low frequencies and vice versa.
Now imagine relatively what voltages will be present over either the resistor or the cap at either;
a) very low frequencies
b) very high frequencies
No need for calculations as this is just to display a point.
Capacitors will present a high resistance at low frequencies and vice versa.
[quote author="CJ"]Recall the formula for Inductive Reactance?
XL = 2 pi f L
It's just a simple equation that says when frequency goes up, resistance goes up.
Capacitive reactance is the exact same formula, just substitute C for L, and put a 1 over it.
This means that the resistance will go down as the frequency gets higher, because the denominator gets bigger with higher frequency, which means the number gets smaller.
So just use coils and caps as resistors, remembering their relationship of ohms vs frequency, and you can analyze all sorts of combinations.[/quote]
Fair point Dave. I accidently posted it in the lab.
If Resistance goes DOWN ans freq. increases though, wouldnt this allow more high freq. through. Ive tried the formula and it indeed works out, im just struggling thinking the resistance should increase with higher freq if it is to be rolled off.
XL = 2 pi f L
It's just a simple equation that says when frequency goes up, resistance goes up.
Capacitive reactance is the exact same formula, just substitute C for L, and put a 1 over it.
This means that the resistance will go down as the frequency gets higher, because the denominator gets bigger with higher frequency, which means the number gets smaller.
So just use coils and caps as resistors, remembering their relationship of ohms vs frequency, and you can analyze all sorts of combinations.[/quote]
Fair point Dave. I accidently posted it in the lab.
If Resistance goes DOWN ans freq. increases though, wouldnt this allow more high freq. through. Ive tried the formula and it indeed works out, im just struggling thinking the resistance should increase with higher freq if it is to be rolled off.
CJ
Well-known member
Ahh, I see your point.
It depends on how the cap is used. If it is used as a short to ground, then as the higher the frequency goes, the less resistance it will have to ground, where it dies.
But if the cap is used as a series resistor to the output, then this less resistance will let more signal thru, nbot short it to ground.
It depends on how the cap is used. If it is used as a short to ground, then as the higher the frequency goes, the less resistance it will have to ground, where it dies.
But if the cap is used as a series resistor to the output, then this less resistance will let more signal thru, nbot short it to ground.
[quote author="CJ"]Ahh, I see your point.
It depends on how the cap is used. If it is used as a short to ground, then as the higher the frequency goes, the less resistance it will have to ground, where it dies.
But if the cap is used as a series resistor to the output, then this less resistance will let more signal thru, nbot short it to ground.[/quote]
AS ABOVE, thats what I thought, more signal through...hmmmm how does that cause low end roll off ?
Ok, lets say the said capacitance where talking about is grid/plate capacitance x miller effect. A high capcitance (X miller) means lower resistance to high frequencies as established. We have the miller cap. in parallel with the Grid resistor.
Now, given that its not shunted to ground, how does a LOWER resistance to high frequencies actually roll them off ?
Can it be something to do with the fact that the grid resistor and tube capacitance form a RC network? Surely the capcitance couldnt roll off the low end all by itself?
Sorry to harp on this, Im researching and researching and still struggling... :shock:
It depends on how the cap is used. If it is used as a short to ground, then as the higher the frequency goes, the less resistance it will have to ground, where it dies.
But if the cap is used as a series resistor to the output, then this less resistance will let more signal thru, nbot short it to ground.[/quote]
AS ABOVE, thats what I thought, more signal through...hmmmm how does that cause low end roll off ?
Ok, lets say the said capacitance where talking about is grid/plate capacitance x miller effect. A high capcitance (X miller) means lower resistance to high frequencies as established. We have the miller cap. in parallel with the Grid resistor.
Now, given that its not shunted to ground, how does a LOWER resistance to high frequencies actually roll them off ?
Can it be something to do with the fact that the grid resistor and tube capacitance form a RC network? Surely the capcitance couldnt roll off the low end all by itself?
Sorry to harp on this, Im researching and researching and still struggling... :shock:
hmmm thinking...
MAYBE, seeing the grid is at ground potential (I.e connected to ground), the higher frequencies coupled back from plate to grid will be rolled off because of the lower resistance (at high freq.) path to a ground potential ?? :shock:
MAYBE, seeing the grid is at ground potential (I.e connected to ground), the higher frequencies coupled back from plate to grid will be rolled off because of the lower resistance (at high freq.) path to a ground potential ?? :shock:
Larrchild
Well-known member
In series configuration, a component that has less resistance at higher freq's will pass more of them than lower freq's and effect a high freq boost.
In parallel or shunt configuration, a component that has less resistance at high frequencies will short out more highs than lows and effect a high freq rolloff.
In parallel or shunt configuration, a component that has less resistance at high frequencies will short out more highs than lows and effect a high freq rolloff.
[quote author="Larrchild"]In series configuration, a component that has less resistance at higher freq's will pass more of them than lower freq's and effect a high freq boost.
In parallel or shunt configuration, a component that has less resistance at high frequencies will short out more highs than lows and effect a high freq rolloff.[/quote]
Larrchild,
Is your last paragraph basically confirming my post above? Seeing the grid resistor is in parallel with the tube capcitance in the example discussed.
In parallel or shunt configuration, a component that has less resistance at high frequencies will short out more highs than lows and effect a high freq rolloff.[/quote]
Larrchild,
Is your last paragraph basically confirming my post above? Seeing the grid resistor is in parallel with the tube capcitance in the example discussed.
Larrchild
Well-known member
i was answering that.
