Low-pass filter with C but no R

C

carbonzit

Active Member
This is something that's bugged me for some time now.

I hasten to point out that IANAEE (I am not an electronics expert); I know enough to get into trouble, and maybe a little more. Hence my puzzlement on this point.

In another thread there's a discussion of why, when a guy added a series resistor to the input of an amplifier, it degraded the sound quality by what sounded like attenuating high frequencies, the classic effect of a low-pass filter. The theory advanced was that by adding the series resistor, it formed a low-pass filter with an already-existing capacitor at the input of the amp.

This does not make sense to me. Let me explain why.

This shows the classic low-pass filter with both a C and R, but also simply a C:



We know the C-R network forms a low-pass filter. But doesn't a C by itself also form one? It's simply the degenerate case where R=0, right? It will still function to attenuate high frequencies. Look at power supplies that have filter capacitors but no series resistors (some have them, some don't). There's your R-less LPF.

Just to be clear, I am not suggesting that one doesn't ever need an R to make a LPF; it's certainly needed to create the proper response curve, rolloff and cutoff frequency. But if that amplifier already had a C across its input, I don't see how adding an R would suddenly create a LPF where none existed before. It would certainly change the response of the filter, but my guess is that it would have less effect, overall, than the C already has.

So what am I missing here? I'll let the experts here respond.
 
Sort by date Sort by votes
carbonzit said:
So what am I missing here?
Click to expand...
To create a low-pass filter there is always some R, even if it's not explicitly shown.
If R were truly zero than the LP rolloff frequency would be infinity--clearly not a real-world possibility.
So a real signal source will always have some finite impedance, even if it is small.

A power supply likely will have a low output impedance at low frequencies, but the impedance generally goes up with frequency.
The output capacitance then keep the impedance low at the higher frequencies.
 
Upvote 0 Downvote
Nigel Goodwin said:
The 'r' is the source impedance, so the configurations are identical.
Click to expand...
So are you saying that even if you omit the R, it'll still be present as whatever the source impedance (output impedance of the source) is? That makes sense; there's always some R, even if not explicitly placed as part of the LPF.
crutschow said:
To create a low-pass filter there is always some R, even if it's not explicitly shown.
If R were truly zero than the LP rolloff frequency would be infinity--clearly not a real-world possibility.
Click to expand...
So even in a power supply with filter capacitors but no resistors, there's some R (the resistance of wires, diodes, etc.).

So what I showed as "C" with no "R" is a valid LPF, yes?

Would this be a fair representation of a LPF with C but no R?

 
Last edited:
Upvote 0 Downvote
OK, so this discussion has gone pretty much the way I thought it might. But something still bothers me:

When we draw a schematic for a circuit, we generally don't show things like the resistance of wires, or the mutual capacitance of adjacent conductors or the self-inductance of wires, right? (To be clear, I'm not talking RF here! Those things become actual components at those frequencies.) Even though we recognize that every component has some L, C and R in it.

So when I show a LPF with just C and no R, that's exactly what I mean: whatever R there is in wires or other components should be completely swamped by the other components, since R is essentially zero, correct? So in other words, we can have a LPF that has (virtually) no R.

If we had to show all those existing but usually ignored things, our schematics would look like hell and be pretty indecipherable.
 
Upvote 0 Downvote
In simulations it is not uncommon to add parasitic components to achieve results that more closely resemble reality. If the purpose of the schematic is to layout a PCB then the parasitics are not represented, but that does not mean they are not there.
 
Upvote 0 Downvote
carbonzit said:
If we had to show all those existing but usually ignored things, our schematics would look like hell and be pretty indecipherable.
Click to expand...
That is true. Every schematic is a simplification, and it only shows part of the picture.

Good circuit design includes making sure that everything that is important is shown in the circuit diagram, but it's also very difficult to know if something is important.

Most circuit designers know that it's a really good idea to decouple every IC, but the reasons for that, the inductance of the power tracks and the ESR of the reservoir capacitors, are almost never shown on a circuit diagram. In effect, every decoupling capacitor is a low-pass filter.

At audio frequencies, it's the resistance is likely to be the source impedances (not shown on circuit diagrams) rather than the wire impedances (also not shown on circuit diagrams).

As for adding a resistor causing a reduction in high frequencies, there are good reasons to have some capacitance on the input to an audio amplifier. Audio amplifiers shouldn't have much gain beyond 20 kHz, to reduce the chance of them oscillating at ultrasonic frequencies. If there is significant impedance on the input, then a low value resistor will cause a reduction in high frequency response.
 
Upvote 0 Downvote
Diver300 said:
As for adding a resistor causing a reduction in high frequencies, there are good reasons to have some capacitance on the input to an audio amplifier.
Click to expand...
Not to quibble too much, but couldn't that capacitance be elsewhere in the circuit, in some stage after the input stage? But yeah, I totally get the need to establish rolloff.
 
Upvote 0 Downvote
carbonzit said:
Not to quibble too much, but couldn't that capacitance be elsewhere in the circuit, in some stage after the input stage? But yeah, I totally get the need to establish rolloff.
Click to expand...
There is a vast difference between "lumped" capacitance where capacitances in parallel can be added together and distributed capacitance which distributes the capacitance along a length dimension or over an area or a volume.
 
Upvote 0 Downvote
You must log in or register to reply here.

Similar threads

M
  • Solved
GE motor with no wiring diagram.
Replies
14
Views
1K
atferrari
K
  • Question
Low loss power OR-ing with MOSFETs & Op Amps
Replies
6
Views
1K
Karklebapper
K
F
  • Question
50uH LISN needed but not for EMC test
Replies
3
Views
1K
Flyback
F
  • Question
No contact AC voltage sensing
Replies
15
Views
4K
lilimike
S
  • Question
Low power consumption relays
2
Replies
23
Views
4K
Tony Stewart
Menu
Log in
Register
Cookies are required to use this site. You must accept them to continue using the site. Learn more…