> From: wrt@eskimo.com (Bill Turner)
> To: amps@contesting.com
> Reply-to: wrt@eskimo.com
> Subject: [AMPS] Question about neutralizing
To: <amps@contesting.com>
> Date: Thu, 19 Jun 97 23:44:23 +0000
Hi Bill,
> With a conventional triode in grounded cathode configuration, the
> plate voltage is 180 degrees out of phase with the grid, yet the books
> say the cause of instability is capacitive coupling from the plate to
> the grid. How can this be a cause of instability if they are 180 out?
If system reactances were zero, then what you say would be true.
The phase shift is not 180 degrees at RF, the feedback capacitance
puts phase lead in the system (remember it's a series C, driving a
finite resistive load). The amount phase lead is modified by the
impedance of the grid to cathode resistance, and the anode to
cathode resistance. These resistances change with drive level and so
you not only have phase shift, you have phase shift that varies with
drive level!!!
The phase is also changed as you change the resonant frequency of the
tank circuits in the grid and anode systems. That's why the dip
become nonsymmetrical! On one side of the dip the phase of the
feedback will add to grid drive, on the other side the phase shift
will subtract from grid drive. You can plainly see this as you tune a
un-neutralized PA, or a PA that employs intentional "RF Feedback".
That's why I am not enamored with tunable PA's that have RF
feedback from the output to the input in an attempt to decrease
distortion. For every tuning condition where distortion is reduced,
there is one where distortion is enhanced.
> I would think any coupling would be degenerative, not regenerative.
It actually shifts between either condition in most circuits, and
can shift as drive is varied!!
> Even if you allow for phase shift from the reactance of the plate-grid
> capacitance, the result should still be degenerative within the 0-90
> degree range and would only become regenerative if you went beyond 90
> degrees. In an RC circuit the maximum possible phase shift is 90, so
> what's going on? Any ideas?
Nothing, you just overlooked the tank's effect. There are other
effects, but most are so small we can ignore them at HF.
73, Tom W8JI
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