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Re: The myth of triode NFB - an article that perpetuates this myth

207.180.182.58

Posted by Henry Pasternack on January 19, 2007 at 11:39:26

In Reply to: The myth of triode NFB - an article that perpetuates this myth posted by kurt s on January 19, 2007 at 09:26:15:

The Stockman paper proposes the existence of a fictitious grid inside the triode and then goes on to show that by imagining this grid connected to the plate, you can express the basic triode model in terms of negative feedback. It is commonly cited by people as "proof" that the negative feedback inside triodes is concrete and real.

You can just as easily propose the existence of a fictitious grid inside an ordinary resistor and then go on to derive its behavior in terms of negative feedback as well. Yet, I don't see many people arguing that resistors are negative feedback devices.

The problem with the triode NFB model is that it's just a model, not a fundamental property of the triode. People who haven't really thought all of this through (or who don't have the knowledge to do so) find the inherent NFB idea satisfying. Having looked at the subject in depth, I don't see much in the way of value or substance in the idea.

If you think of the triode as a pentode with the screen grid connected to the plate, then by definition there is negative feedback. But if you think of the triode as a diode with a control grid, then there is no negative feedback -- unless you believe the diode has negative feedback as well.

When discussing (or arguing) this subject, I always get down to the diode question because the control grid is really irrelevant to the NFB model. It's the relationship between the plate and the space charge that comprises the supposed negative feedback loop. I will point out that a diode is really the same as a triode with the control grid connected to the cathode. Most triode NFB proponents insist that a diode doesn't have internal NFB, that it is a different case entirely. This is where the breakdown of reality begins.

This is one of those ideas that is just simple enough to have widespread popular appeal, and just complicated enough to be beyond the understand of most people who believe in it. The idea that the two outputs of a split-load phase inverter have different output impedances is another such idea. These are excellent topics for teaching, because there are easy traps to fall into, and identifying those traps and avoiding them is instructive. More often than not, sadly, trying to engage these subjects just leads to an argument. People don't like to work hard for their knowledge.

Patrick Turner of Australian tube DIY fame is a regular poster on rec.audio.tubes and an ardent proponent of the triode NFB concept. While many people hold him in high esteem, from my technical debates with him I know him to be the worst kind of snob, the self-professed anti-intellectual who takes pride in his limits and insists on imposing them on others.

I just had to say that.

If you look at Google groups for "Diodes, triodes, and negative feedback" on rec.audio.tubes, you will find more of my thoughts on the subject.

-Henry

-Henry

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