Bill, W7TI says:
>Something has always puzzled me about neutralizing, so I wonder if
>someone on this reflector could answer my question.
>
>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?
>I would think any coupling would be degenerative, not regenerative.
>
>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?
Bill,
If you have a triode with a tuned plate, and feedback through the plate
grid capacity, you get a pure capacitive input at resonance. Off
resonance, the input admittance of the tube is an impedance, and if the
plate circuit is inductive, that impedance has a negative resistance
portion. Putting the negative resistance across the input tank causes
oscillation if the negative resistance is big enough (or small enough -
depending which way you look at it) That's why an oscillator doesn't
actually oscillate at the resonant frequency of the tuned circuit,
although in practice, I don't think you could ever measure the
difference. It also depends on how you define resonance - from memory, I
think there are five different definitions of resonance you can use for
a parallel tuned circuit, giving five slightly different answers. The
one that is valid for series and parallel resonance is that the circuit
is resistive at resonance.
If you want the math derivation, I can copy it out of Terman's Radio
Engineering for you.
The basic equations are:-
Input resistance is minus Xcgp/A sin p, where Xcgp is the reactance of
the grid-plate capacity
A is the gain of the stage ie ratio of plate volts to grid volts, and p
is the angle by which the voltage across the load impedance leads the
equivalent voltage acting in the plate circuit. p is positive for
inductive loads.
Input capacity is Cgk plus Cgp(1 plus A cos p)
I wish e-mail supported mathematical symbols and Greek letters!!
Does this help?
73
Peter G3RZP
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