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Old 14th August 2004, 06:59 AM   #11
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Ya, in other words, CMRR doesn't apply here, but if you must, it's anti-rejected by the nature of the circuit. Hence = -gain............

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Old 14th August 2004, 12:10 PM   #12
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Quote:
Originally posted by runeight
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.

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".

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Old 14th August 2004, 01:47 PM   #13
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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".
Yep, forgot about this one. And when the cathode is used for NFB (as in the input stage of so many power amp designs) it is the CMRR that provides the NFB.
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Old 14th August 2004, 04:39 PM   #14
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Quote:
Originally posted by runeight


Yep, forgot about this one. And when the cathode is used for NFB (as in the input stage of so many power amp designs) it is the CMRR that provides the NFB.

Not completely correct

CMRR is the suppresion of common mode signals to a differential mode output.

The principle of feedback applied to the cathode is that the effective anode current modulation is the result of the difference between grid and cathode voltage, and gm

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Old 21st August 2004, 08:25 PM   #15
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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.

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