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From: liz@poppyrecords.invalid.invalid (Liz Tuddenham)
Newsgroups: sci.electronics.design
Subject: Re: Grounded grid VHF front-end
Date: Sun, 10 Nov 2024 19:33:39 +0000
Organization: Poppy Records
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john larkin <JL@gct.com> wrote:

> On Sun, 10 Nov 2024 16:03:25 +0000, liz@poppyrecords.invalid.invalid
> (Liz Tuddenham) wrote:
[...]

> >The point I was making about grounded-grid operation is that the input
> >impedance of the valve is very nearly the characteristic impedance of
> >the co-ax (voltage ratio 3:2 for a triode-strapped EF91 drawing 6mA from
> >a 200V HT line).   A Pi network or a 3:2 winding on a ferrite core could
> >be used to match them
> 
> A tuned circuit into the grid has voltage gain,

It's about power noise and power gain, not voltage noise and voltage
gain., a tuned circuit doesn't have power gain.  At lower impedances the
noise voltage and the signal voltage are reduced equally so the S/N
power ratio isn't altered.  Matching the feeder and aerial impedance to
the circuit impedance is what matters, that wsy you get maximum signal
power transfer.

Grounded grid circuits were common above 150 Mc/s and gave good S/N
ratios, the reason they weren't much used below that frequency was
because specialised valves in conventional circuits could do the job
adequately.  I am trying to push a general purpose valve to do the job
of a specialised one.  (Like a BC109 being used as a VHF oscillator - it
can work if you get it right!)

-- 
~ Liz Tuddenham ~
(Remove the ".invalid"s and add ".co.uk" to reply)
www.poppyrecords.co.uk