On Fri, 20 Jun 1997 12:10:00 +0100, Peter Chadwick
<Peter.Chadwick@gpsemi.com> wrote:
>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
>
<><><><><><><><><><><><><><><><><><><><><><><><><><><><>
Yes, it makes sense that the oscillation would be somewhat off the
true resonant frequency of the tank circuit. This sounds like the
missing link to the puzzle. Thanks!
73, Bill W7TI
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