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
(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
BIRDS BARKING?
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,
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,
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.
Ya, I wasn't very clear. I mean either you filter what needs it, or use a circuit that doesn't care about ripple.
Tim
- 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.
fdegrove said: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.
Tim
Re: BIRDS BARKING?
Frank
The thread started with common cathode amps
cheers
fdegrove said: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
Hi,
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:
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.
Cheers,
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:
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.
Cheers,
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.
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.
runeight said:
Hi
Nice discussion
Common in "my" terminology stems from the fact that input and output have the cathode in common, which is what runeight calls grounded cathode.
Such a stage does not have CMRR but ofcourse has PSRR.
Any stage with (intended) differential inputs has CMRR as well as PSRR.
Now back to the grounded cathode:
One may argue that such a stage has CMRR because the cathode and the grid both can be considered as "differential" inputs though most of the times one of the inputs is "fixed".
cheers
Yes, I see what you're saying, but I think your comment above is correct enough under most circumstances.
That is, if the plate to cathode voltage is very large compared to the AC signal applied to the cathode, then the affect of changing the cathode voltage on the operating point because the plate voltage is changing is generally a second order effect. If this is so, then to first order applying a signal to the cathode is the same as applying a signal to the grid with the inverse effect and so the grid-cathode form a sufficient differential input for CMRR to be relevant.
No?
That is, if the plate to cathode voltage is very large compared to the AC signal applied to the cathode, then the affect of changing the cathode voltage on the operating point because the plate voltage is changing is generally a second order effect. If this is so, then to first order applying a signal to the cathode is the same as applying a signal to the grid with the inverse effect and so the grid-cathode form a sufficient differential input for CMRR to be relevant.
No?
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.