[Amps] RF parts and parasitics!!

[jeff millar]

Bill, W6WRT wrote:
> REPLY:
>
> While I generally agree with W8JI's conclusions, he makes a serious mistake
> about halfway through the article when he says " When grid current is absent,
> the grid-to-cathode impedance is nearly an open circuit". In a grounded-grid
> linear amplifier (the kind under discussion) this is simply not true. 
>   
I think the statement about grid impedance is true because it refers to 
a principle of _tube_ operation. The grid does indeed have a high 
impedance with _respect_ to the cathode without grid 
current...regardless of amplifier circuit. On the other hand, an 
_amplifier_ may have a grounded grid, but that's a statement about grid 
impedance with respect to amplifier input. The grid still has a high 
impedance with respect to cathode on those frequencies where the tube's 
grid to socket internal wiring inductance has high impedance. (which is 
where the parasitics occur)
> His article is at:  http://www.w8ji.com/vhf_stability.htm
>
> Comments welcome.
>
> 73, Bill W6WRT
>