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- Thread starter Specul8tr
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BobMaximus
pfm Member
Why is it better than what?
The first RC shorts AC (noise) from the power rail to 0v. The second R therfore receives less noise from the power rail.
The first RC shorts AC (noise) from the power rail to 0v. The second R therfore receives less noise from the power rail.
Specul8tr
pfm Member
Thanks for your reply. What I meant was, why is RCR better than RC? Why the use of a second resistor which surely adds more Johnson noise into the circuit. Why not just use RC?
BobMaximus
pfm Member
OK, good question. I had a think about this while I was in the bath. RC ought to work better than RCR. I'd never thought about it before, I just "assumed" it was correct.
Someone else will have to give a definitive answer!
Someone else will have to give a definitive answer!
martin clark
pinko bodger
Ah , you may be thinking of when this is used to bias a current source (either one- or two-transistor kind.)
in simple source, a tail resistor is used - usu quite a large value. 22K or more - to set a small current through a voltage reference.)
Splitting this R into two and taking a cap from the midpoint to the rail at the reference end means the current stays the same, but the filtering effect means the PSRR for the source overall is greatly improved, because variation in the supply rail aren't appearing across the voltage reference.
E.g. see fig 1 & 2 at the very end of this article, and the discussion of it:
http://www.waltjung.org/PDFs/AX_Letters_0907.pdf
in simple source, a tail resistor is used - usu quite a large value. 22K or more - to set a small current through a voltage reference.)
Splitting this R into two and taking a cap from the midpoint to the rail at the reference end means the current stays the same, but the filtering effect means the PSRR for the source overall is greatly improved, because variation in the supply rail aren't appearing across the voltage reference.
E.g. see fig 1 & 2 at the very end of this article, and the discussion of it:
http://www.waltjung.org/PDFs/AX_Letters_0907.pdf
Arkless Electronics
Trade: Amp design and repairs.
The RC is the noise reduction part. The extra resistor has no noise reduction properties and is part of the current source or LTP tail resistor. The identical circuit in the Walt Jung article is something different as the cap goes to the V+ rail and is there as a bootstrap...
martin clark
pinko bodger
it is for the the constant current source; configured as as sink, it would return to 0v on that diagram (i.e the whole thing the 'other way up'); it's then still a bootstrap cap, as Jez points out...
Anyway - that's about the only place I've can think of using a cap to 'split' a given R value.
Anyway - that's about the only place I've can think of using a cap to 'split' a given R value.
BobMaximus
pfm Member
I have now realised why I've got mixed up here. The following was my original line of thought. However, R2b is needed, because we don't want to end up filtering out the actual signal!
Here was that stupid thought:
But I think the OP is asking why 3 resistors are being used (rather than 2). Unless I have misunderstood something? For example, Neil Jadman’s NJ322:
http://www.oxfordcitybranch.org.uk/pink/NJ322.jpg
This filtering arrangement seems to be used a lot with PFM projects. Why not use 2 resistors (combine R2a + R2b to make R2) and connect the filter capacitor to the mid point of R2 / R3?
Here was that stupid thought:
But I think the OP is asking why 3 resistors are being used (rather than 2). Unless I have misunderstood something? For example, Neil Jadman’s NJ322:
http://www.oxfordcitybranch.org.uk/pink/NJ322.jpg
This filtering arrangement seems to be used a lot with PFM projects. Why not use 2 resistors (combine R2a + R2b to make R2) and connect the filter capacitor to the mid point of R2 / R3?
martin clark
pinko bodger
Because that would also 'short' the wanted input audio input signal to 0v... along with the unwanted noise component.This is also why R2/R3 are usually large values - the two values in parallel (usually) effectively dominate or define the following amplifier stage input impedance.
Input biasing arrangements like that (for single-supply stages like that, whether transistor input, or opamps) always need three resistors, but there are two ways you can arrange it:
1 - as you diagram shows, or
2 - two resistors to split the rail, cap at midpoint to reference (0v); resistor from this midpoint junction, to the input device input pin, and the input signal fed-in at its junction with that input device (i.e. opamp input pin or transistor base). There's little to choose between them - except as part of a whole design. (1 is probably the better choice overall, imo)
if you want an example of 1 implemented as a diy mod, it's exactly what my suggested mod 7 for Naim CD3.5 and CD5 players does - adding the equivalent of R2a and C11 in your diagram, which is then shared by two pairs of R2/R3, one for each channel, which directly biases the following opamp in each channel.
(it's a heck of a lot cheaper & more effective than adding a hicap...)
Input biasing arrangements like that (for single-supply stages like that, whether transistor input, or opamps) always need three resistors, but there are two ways you can arrange it:
1 - as you diagram shows, or
2 - two resistors to split the rail, cap at midpoint to reference (0v); resistor from this midpoint junction, to the input device input pin, and the input signal fed-in at its junction with that input device (i.e. opamp input pin or transistor base). There's little to choose between them - except as part of a whole design. (1 is probably the better choice overall, imo)
if you want an example of 1 implemented as a diy mod, it's exactly what my suggested mod 7 for Naim CD3.5 and CD5 players does - adding the equivalent of R2a and C11 in your diagram, which is then shared by two pairs of R2/R3, one for each channel, which directly biases the following opamp in each channel.
(it's a heck of a lot cheaper & more effective than adding a hicap...)
Specul8tr
pfm Member
BobMaximus, I was referring to the 2 resistor scenario.
The point of the cap is to short anything that is not DC (ie. AC) to ground and so making this midpoint the entry point for your signal (AC) intrinsically means this signal also gets shorted to ground, making this a perfect scenario for any forthcoming Spice Girls ‘Audio’ material. (Other cr*p bands available..)
The point of the cap is to short anything that is not DC (ie. AC) to ground and so making this midpoint the entry point for your signal (AC) intrinsically means this signal also gets shorted to ground, making this a perfect scenario for any forthcoming Spice Girls ‘Audio’ material. (Other cr*p bands available..)