On Thursday 04 November 2004 20:14, Dennis12Amplify@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
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