It does make a big difference!
By convention' all impedances are measured from the electrode to ground
The grid impedance is actually very high and mostly capacitive, as long as
you don't draw grid current. With grid current it is about equal to the grid
voltage divided by the current measured-more or less.
In GG, the input impedance is approximately the load resistance divided by
the tube's transconductance, very approximately and it's mostly resistive:
that's why it absorbs power from the driver.
The element common to both the input and the output is considered to be
grounded- the grid in our case- so any measurement referred to it is
actually referred to ground, not the grid. So, the grid to cathode impedance
is a non existing entity.
Alex 4Z5KS
-----Original Message-----
From: amps-bounces@contesting.com [mailto:amps-bounces@contesting.com] On
Behalf Of TexasRF@aol.com
Sent: Thursday, August 27, 2009 4:52 AM
To: dezrat1242@yahoo.com; amps@contesting.com
Subject: Re: [Amps] RF parts and parasitics!!
When the grid is grounded for cathode driven operation is there a
difference between cathode to grid impedance and cathode to ground
impedance?
Doesn't seem there would be.
73,
K5GW
In a message dated 8/26/2009 7:59:38 P.M. Central Daylight Time,
dezrat1242@yahoo.com writes:
ORIGINAL MESSAGE:
On Wed, 26 Aug 2009 20:09:40 -0400, "Carl" <km1h@jeremy.mv.com> wrote:
>
>I fail to see the phrase "very high" anywhere in that article.
>
>Perhaps you can assist these tired old eyes.
REPLY:
My tired old eyes read his statement "When grid current is absent, the
grid-to-cathode impedance is nearly an open circuit" and paraphrased that
as
"very high". Did I err?
73, Bill W6WRT
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