Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => AmpTools/Tech Tips => Topic started by: PRR on March 10, 2022, 08:53:33 pm
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Some people "get" impedance relations semi-intuitively. Others don't. Everybody struggles with some point.
This 1958 book tries to cover the single subject. It is a fairly classical treatment. It maybe goes too fast for many learners. But for $2.90 (original price) to free-on-the-Web, it is worth getting.
Impedance Matching by Alexander Schure, 1958
https://worldradiohistory.com/BOOKSHELF-ARH/Rider-Books/Impedance%20Matching%20-%20Alexander%20Schure.pdf
https://www.amazon.com/Impedance-Matching-Alexander-Schure/dp/0810406233
https://www.google.com/books/edition/Impedance_Matching/76BYAAAAYAAJ
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Many of the Rider books were good.
https://worldradiohistory.com/BOOKSHELF-ARH/Bookshelf_RIder.htm
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Thanks!
I’m a bit confused by the cathode follower discussion on p42-43
eg
The voltage gain of the circuit is always less than one because the grid circuit receives not only the input signal but also a 180° out-of-phase volt-.
age from the cathode resistor.
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I won't defend the book details.
But this point seems to hinge on "grid circuit", which of course is really the grid to cathode path. Put +10V at the grid, get +9V lift at cathode, the effective input the tube sees is only 1V.
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And if the Cathode is perfectly bypassed to ground, its voltage does not change. The Grid is biased negative of the Cathode, which makes a positive-going Grid-input reduce the negative Grid-to-Cathode voltage.
But in a cathode follower, the Cathode is not bypassed and also has a large resistance present.
For our same positive-going Grid-input, plate current through the large resistance is a big voltage moving the Cathode more-positive.
Which is the same as saying it is moving the Grid more-negative.
With the author's frame of reference, and referring all Grid-to-Cathode voltage to the Grid as though they're separate inputs to the same location, the positive-going Grid-input experiences a 180º out-of-phase voltage from the cathode resistor.
I prefer to think of it as you probably do: cathode follower has 100% negative feedback; know that the cathode voltage is offsetting some high-% of Grid input signal.