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Old 12th August 2004, 08:43 AM   #1
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Default cmrr of common cathode

Hi,

How do you specify (calculate) the CMRR of a common cathode stage (that is non-differential) ? In Valve Amplifiers by Morgan Jones, PSRR of a common cathode stage is given as 20dB (RL = 47k, ra = 4.95k), but there is no calculation.

Thanks in advance.

MB
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Old 12th August 2004, 09:33 AM   #2
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Default Re: cmrr of common cathode

Quote:
Originally posted by metebalci
Hi,

How do you specify (calculate) the CMRR of a common cathode stage (that is non-differential) ? In Valve Amplifiers by Morgan Jones, PSRR of a common cathode stage is given as 20dB (RL = 47k, ra = 4.95k), but there is no calculation.

Thanks in advance.

MB

Hi

CMRR does not exist, do you mean power supply rejection ratio ?

That is roughly equal to effective anode impedance devided by anode resistor

cheers
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Old 12th August 2004, 11:58 AM   #3
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OK. Then I misunderstood. Thanks...
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Old 12th August 2004, 06:55 PM   #4
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CMRR = -gain

(The term is rejection, so it'd have to be negative... meaning not only is it not rejected, it is amplified...)

PSRR is from 3 to 10, i.e., around 9 to 20dB. Think of the tube as a resistor (remember to factor in NFB if the cathode resistor is unbypassed!) and consider the B+ hum/plate resistor/tube voltage divider, Vo = ripple * Rp / (RL+Rp). Oh- don't forget Rp also includes the following stage's grid leak - it is in parallel (AC-wise) with the tube. So Rp in the above = Ra * Rg / (Ra+Rg) where Ra is anode resistance. ( )

Being pentodes have very high plate resistance, their PSRR is poor (maybe -6dB) and special measures have to be taken in the design to account for such, if ripple is great enough to matter. But that's also another design consideration that shouldn't even be a matter.

Tim
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Old 13th August 2004, 12:07 AM   #5
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Default BIRDS BARKING?

Hi,

Quote:
CMRR does not exist, do you mean power supply rejection ratio ?
Huh???
Well, sure enough it can't exist for a non-differential input but once you have a balanced source CMRR properties are one of the most important reasons why you'd use balanced operation in the first place.


Quote:
(The term is rejection, so it'd have to be negative... meaning not only is it not rejected, it is amplified...)
Guess you'd better reject this definition as well, then?

Either way, PSRR and CMRR are two totally different subjects that have little to do with each other in the way these work.

Quote:
But that's also another design consideration that shouldn't even be a matter.
Meaning what exactly? That ripple should be negligibly small from the start or that it's one of those design parameters you don't worry about??

I suppose I must have missed something here or maybe it was just the heat....

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Old 13th August 2004, 01:41 AM   #6
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Default Re: frankS BARKING?

- I mean CMRR in that, since the signal is entirely common mode, and it follows, although opposite in phase, to the output, amplified by whatever the gain figure is. But it's an anti-CMRR amplifier, so it's a negative dB number.

Quote:
Originally posted by fdegrove
Meaning what exactly? That ripple should be negligibly small from the start or that it's one of those design parameters you don't worry about??
Ya, I wasn't very clear. I mean either you filter what needs it, or use a circuit that doesn't care about ripple.

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Old 13th August 2004, 01:49 AM   #7
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Hi,

Quote:
- I mean CMRR in that, since the signal is entirely common mode, and it follows, although opposite in phase, to the output, amplified by whatever the gain figure is. But it's an anti-CMRR amplifier, so it's a negative dB number.
Errr....Other than you and me...Anyone else understands this?

Tim, the main thing is: there's no common mode rejection to speak of, period.

99 words to follow if you persue this line of thought... promise.


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Old 13th August 2004, 09:08 AM   #8
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Default Re: BIRDS BARKING?

Quote:
Originally posted by fdegrove
Hi,



Huh???
Well, sure enough it can't exist for a non-differential input but once you have a balanced source CMRR properties are one of the most important reasons why you'd use balanced operation in the first place.

Guess you'd better reject this definition as well, then?

Either way, PSRR and CMRR are two totally different subjects that have little to do with each other in the way these work.

Meaning what exactly? That ripple should be negligibly small from the start or that it's one of those design parameters you don't worry about??

I suppose I must have missed something here or maybe it was just the heat....

Cheers,
Frank

The thread started with common cathode amps

cheers
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Old 13th August 2004, 07:45 PM   #9
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Hi,

Quote:
The thread started with common cathode amps
Sorry but it says "common cathode stage".
I'm still at a loss why you'd say that such a topology does not have CMR...

In fact you state:

Quote:
CMRR does not exist, do you mean power supply rejection ratio ?
Even if you mean to say it does not exist for a common cathode stage I'd still beg to differ unless it would be qualified by the statement that a single ended CMS doesn't have any CMR which should be obvious anyway.

Sorry to be a PITA but I'd rather have it cleared up so any further confusion can be discarded.

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Old 14th August 2004, 12:39 AM   #10
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I have noticed an occasional confusion in terminology for some tube topologies. i'm not saying which is necessarily correct, but maybe that's the problem here.

i think the original question is for a "common cathode" topology. i have seen the term "common cathode" applied to what I would call a "grounded cathode", the latter being the typical gain stage with an Rk and a Ck. this guy has only a PSRR.

a "common cathode" to me is two devices with their cathodes connected together, and separate plate and grid connections. which, if both inputs are used, is a differential amplifier with CMRR and PSRR.

if only one input is used and the other grounded, then this configuration is sometimes a phase splitter or just a non-inverting amplifier. the topology has no CMRR, but it has PSRR

if one grid is used for the input and the other for NFB, then the CMRR is what gives the NFB.

don't know if this has been the confusion, just a thought.
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