[AMPS] More amplifier experiments

[Tom Rauch]

> Date:          Fri, 29 May 1998 15:45:53 +0100
> From:          "Geurts, Gerard" <ggeurts@amp.com>

> >  I have never been clear on that point since it appears that a minute
> > amount of inductance is compensated for by the L that is selected.
> > 
> Indeed, why would this resistor have to be non-inductive?

Think about what we want. We want the resistor to be fully in the 
circuit at the frequency of instability, and fully out of the circuit 
at the operating frequency.

Now draw the circuit on paper and look at it. If the resistor has 
series inductance, the series inductance decreases the current 
through the resistor at increasing frequency and increases SYSTEM Q 
at higher frequencies.  

If the resistor had series capacitance, the opposite would be true. 
The resistor would more rapidly shift into the path with increasing 
frequency and more rapidly shift out at lower frequencies.

> Some people use resistive inductors to improve their surpressors, so why
> not use inductive resistors?

Well, they "think" they improve the suppressors. What they do is 
increase series resistance at all frequencies from dc up, that's why 
ten meter performance drops.

Anything done with a resistive wire can be accomplished with a change 
in lumped resistance.

> From a circuit analysis point of view a
> non-inductive (I know, they don't exist) resistor in parallel with an
> inductor is the same as an inductive resistor in parallel with a
> slightly bigger inductor.

Not at all. The reactive part increases compared to resistance. 
Upper frequency Q, when that component is installed in series with 
the lead from the anode to tank capacitor, increases.

> The same goes for the series resistor we put in the high voltage line to
> limit the current. I see references to non-inductive resistors for this
> application all the time, but this resistor is switched in series with
> the plate RFC. Again, from a circuit analysis point of view a
> non-inductive resistor in series with an inductor (the plate RFC) is the
> same as an inductive resistor in series with a slightly smaller RFC. The
> only thing we need to make sure is that the resistor can handle the
> energy dumped in it during a major 'event'.

That's correct. The only concern for that component is the ability 
to withstand the momentary impact of a fault. Inductance matters 
little, because it is small compared to the rest of the 
impedance...and actually any inductance would help.

The most common energy absorbing resistors are non-inductive, but 
that is side effect of having high voltage ratings rather than a 
goal. RCD manufactures pulse rated wire wound resistors with wide 
turns spacing that are inductive, and work fine in fault 
protection applications.

> The extreme case of this would be if you wind the RFC using resistance
> wire. This would integrate the RFC and limiting resistor in one
> component, reducing component count, and would also lower the Q of the
> RFC making it easier to produce one without resonances. Does anybody
> know a source of suitably insulated resistance wire so I can give this a
> try?

You wouldn't want to do this, because choke dissipation would 
greatly increase. The choke stores and returns energy to the anode 
system throughout the RF cycle, and resistance in series with that 
energy storage is a bad idea, especially on the lower end of the 
choke's operating frequency range.

73, Tom W8JI
w8ji.tom@MCIONE.com

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