[AMPS] re:4-250A (AB2 in grid-driven amplifiers)

[Ian White, G3SEK]

yo3ctk wrote:
>Oh boy...I really hope I will not regret this.....
>
On the contrary, Mike, thank you for posting this.

>
>       Rich wrote:
>       <snip>
>       -  How many tubes does Eimac rate for Class AB2 grid-driven
>linear 
>       amplifier service?
>
>       For instance, EIMAC 4CX1500B.
[snip]


>
>       It is not my intention to argue with the major contributors to
>this list. Is just that I am slightly annoyed with words such as "never"
>in a scientific discourse. 

Thank you Mike - that's exactly the point!

Power amplifier design and performance is far too complex a subject for
absolute statements using that "N-word", or implying that there are no
exceptions. 

Life's too short to go into any subject in full breadth and depth within
a single posting to this list - we always have to generalize and leave
something out. Therefore it's very likely that, whatever we say,
somebody will come back with a counter-example where our generalization
is not true. The mature response is "Thanks - I hadn't thought of that",
and then we all gain some broader, deeper knowledge.

I don't have to tell anybody what commonly happens instead.


Returning to the subject...
>Maybe this example will emphasize the point
>that class AB1 and AB2 can be used with equally good results for
>grid-driven amplifiers, depending on the tube selected.
>
There are three components to this: the tube, the RF input circuit and
the grid bias supply. Even a tube that does not have an inherent kink in
its transfer characteristic at Vg=0 will still require grid swamping to
avoid a sudden change in input impedance (which will cause IMD in the
driver), and it also needs a low-resistance grid bias supply to avoid a
sudden shift in the tube's operating point when grid current starts to
flow. 

A reasonably small amount of grid current doesn't absolutely have to
cause IMD - not with the right tube, the right kind of grid circuit, 
and the right kind of grid bias supply, 


73 from Ian G3SEK          Editor, 'The VHF/UHF DX Book'
                          'In Practice' columnist for RadCom (RSGB)
                           www.ifwtech.demon.co.uk/g3sek

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