Re: The myth of triode NFB

Posted by kurt s on January 19, 2007 at 08:41:30

In Reply to: The myth of triode NFB posted by Henry Pasternack on January 19, 2007 at 08:20:02:

>>It is a myth. Or, rather, an artifact of a model. Other models explain triode operation just as well, and more simply, without negative feedback.
I have participated in strenuous, protracted, agonizing debates on this subject and have looked at all the arguments very carefully before coming to this conclusion.<<

Well I hadn't heard that one before, and I would not know exactly why either is the case by the lack of in-depth explanation here.

I once did point out, though, that there is inherent NFB in a plate loaded common cathode triode stage (with cathode resistor bypassed). This is because of the nature of the triode's plate curves. As the grid is made more positive, the plate current goes up, naturally. But in a triode when you follow the plate curves, as the plate current increases with a given delta-Vgk, the plate current must decrease because of the effect of the delta-Vpk. This occurs by the nature of the "vertical" curves of triodes, and is not there in the horizontal curves of pentodes, where delta-Vpk has essentially no effect on plate current.

I also point out, that in the case of the common cathode triode stage, this NFB circuit caused by the vertical plate curves is NOT being produced by sampling an output and reinserting it back to an error-correcting input like all established NFB models. It is merely an artifact of the triode's own plate curves working against its plate load. Is this the source of a "pure" form of NFB inherent in this type of circuit? I don't know for sure.

Kurt

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Follow Ups Full Thread

Topic - NFB - when it's good, it's good, when it's bad, it's bad - kurt s 20:13:43 01/18/07 (24)

HERESY! Burn you witch, BURN IN HELL!!!! -nt - TomÃ¡s de Torquemada 13:55:04 01/24/07 (0)

Lynn Olson discusses the issue of inherent triode feedback - nerdorama 12:45:19 01/19/07 (0)

Re: NFB - when it's good, it's good, when it's bad, it's bad - lipmanl 10:21:31 01/19/07 (0)

Yep, IMHO only pentode amps need/benefit - Thomas Martens 08:38:46 01/19/07 (1)

Re: Yep, IMHO only pentode amps need/benefit - Triode_Kingdom 22:01:54 01/19/07 (0)

Re: NFB - when it's good, it's good, when it's bad, it's bad - Anthony Dockrill 01:24:09 01/19/07 (3)

It may have nothing to do with triode-pentode issue. - sser2 12:39:12 01/19/07 (1)

No, not as simple as merely OPT issues/variables... - Thomas Martens 16:43:30 01/19/07 (0)

Re: NFB - when it's good, it's good, when it's bad, it's bad - kurt s 08:05:33 01/19/07 (0)

Re: NFB - when it's good, it's good, when it's bad, it's bad - PÃ¤r 00:12:44 01/19/07 (13)

Oops! I need a refresher course on tube physics. - kurt s 08:01:19 01/19/07 (12)

The myth of triode NFB - Henry Pasternack 08:20:02 01/19/07 (11)

Pentode positive feedback - Mark Kelly 15:15:38 01/19/07 (0)

The myth of triode NFB - an article that perpetuates this myth - kurt s 09:26:15 01/19/07 (6)

Re: The myth of triode NFB - an article that perpetuates this myth - Henry Pasternack 11:39:26 01/19/07 (5)

Re: The myth of triode NFB - an article that perpetuates this myth - Naz 21:57:16 01/23/07 (1)

Beautifully stated, Naz (nt) - BBeck 08:55:48 01/24/07 (0)

Nice job! - Russ57 12:22:14 01/19/07 (0)

You make a lot of sense, Henry. - kurt s 11:45:28 01/19/07 (1)

Kurt and Henry: Nice Tutorial! - Thomas Martens 16:55:13 01/19/07 (0)

Re: The myth of triode NFB - PÃ¤r 09:24:26 01/19/07 (0)

Re: The myth of triode NFB - kurt s 08:41:30 01/19/07 (0)

Now you make me started thinking.. - cheap-Jack 08:27:55 01/19/07 (0)

Interesting! - Ray Moth 22:44:28 01/18/07 (0)

Follow Ups

Re: The myth of triode NFB - kurt s 08:41:30 01/19/07 (0)

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