[Amps] A Meeting Ground

[Dennis12Amplify at aol.com]

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.
 
 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?
 
 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....
 
Regards,
 
Dennis O.