[Amps] *** SPAM *** Re: Parasitics & Filament Sag

[Peter Chadwick]

Rich said:
>> The 'conventional' parasitic approach is, if I understand the  
> various arguments, to lower the circuit gain at VHF by reducing the  
> plate load impedance. I don't see how a resistance wire suppressor  
> does that, since a resistance and inductance in series have an  
> increasing impedance, ...

To see it, one converts the R/L suppressor's series R to Rp, its  
parallel-equivalent resistance -- which is what the anode sees at its  
VHF self-resonance.  .  This conversion is not a simple task.<
The conversion is very simple. For a series X and R, as X goes up, Rparallel (Rp) goes up, and Xparallel goes up as the square of the increase in X
i.e. Rp = Rs {1+(Xs*2)/(Rs*2)] and Xp = Rs*2 + Xs*2
where *2 means squared - since not everyone's email necessarily reproduces these symbols.
So as both Rp and Xp  increase with frequency, so does Zp - or Yp if you like.
So the VHF gain increases, being gm[(Zp.Ra)/Zp+Ra], where Ra is the ac plate resistance. Like a peaking choke in the old style video amplifier.

>What is the reactance of C-feedback in an 8877 at 500MHz?<
What, as they say, has this to do with the price of fish? The feedback capacitance doesn't matter if the stage is inherently degenerated by virtue of the grid being tuned HF of the plate circuit. At 500MHz, this may not be possible with an 8877(because of the grid structure size) , but at 50MHz, that's another matter. In grounded cathode, a problem with unneutralised feedback capacitance is either instability at the operating frequency because the input impedance goes negative, or massive detuning and damping because the grid input resistance is positive and Miller effect causes detuning. Feedback capacitance is used and augmented externally in phantastrons and the like......


73

Peter G3RZP