[Amps] A Meeting Ground

[Steve Thompson]

On Thursday 04 November 2004 20:14, Dennis12Amplify at aol.com wrote:
> Bill,
>
>  I have been reading your posts lately and want to commend you for  your
> 'sage' advice.
>
>  I also enjoy Rich's posts but lately he seems to be a bit  cranky.
>
>  I guess I would too if I kept having to fight the parasitic  battle.
>
>  I kept thinking about the phase angle thing and realized that for a
> grounded grid amp with .12pfd of plate to cathode capacitance to oscillate
> at  1000 megacycles, the uhf circuit gain at that frequency would have to
> be greater than unity at a point where there was either no phase shift, or
> -/+ 360  degrees of phase shift between cathode and anode.
>
>  With the feedback impedance of 1326 ohms at -90 degrees voltage  dividing
> with the input impedance of the tube (another vector but a low  impedance
> one), It would take a significant amount of stage gain to cause 
> oscillations to occur.

Sounds right to me. By 1000 MHz I suspect that the phase angle of the internal 
feedback might not be exactly -90 degrees - but I also doubt that it's zero.
>
>  And wouldn't there also be an additional capacitive voltage divider 
> between the plate to cathode capacitance and the grid to cathode
> capacitance  further reducing amplitude of the feedback signal?

Yes. Consider what happens in a hf amp if the cathode circuit provides a low 
impedance path to ground at vhf.
>
>  And at that frequency wouldn't we also have to consider the 'transit 
> time' for the signal on the cathode to cause a voltage  change on the anode
> for what would create an additional phase shift in  the amplifier stage
> where the output voltage would always lag the input voltage  by the transit
> time?
>
>  At approximately 11.8 inches per nanosecond, and at an applied  frequency
> of 1000MHZ every .1 inch of electron travel inside the tube would add 
> another 3 or 4 degrees of phase lag.
>
>  I've been 'retired' for over 3 years now so  my circuit analysis may be a
> little rusty, but I think I got it  right....

I'd say so - hence my earlier comment that lots of things affect gain >1 and 
phase =0

Steve