[AMRadio] Re: Average Age etc..

ne1s ne1s at neandertech.com
Tue Jul 24 12:43:04 EDT 2007

Geoff/W5OMR writes:
> Because When Inductive Reactance (X(l)) equals Capacitive Reactance (X(c)) 
> you get Resonance!  At least in triodes.  Pentodes need some extra work, 
> due to screen voltages/current.  

As far as I know, the condition of resonance: XsubL = XsubC ; holds for the 
LC resonant circuit, regardless of the active element it may be connected 
to, be it a pentode, tride, bipolar transistor, FET, tunnel diode, or 
whatever. Of course when the LC is connected to other "stuff," the parasitic 
reactances of the whole mess need to be considered and will contribute to 
the "total" XsubL and XsubC which determines the resonant point. 

> That brings up a question... (and I'm 
> asking because I just don't know pentodes/tetrodes) should the screen 
> circuit be resonant at the operating frequency, as well? 

If we are considering a pentode/tetrode amplifier, the screen should be 
grounded for RF (i.e., well-bypassed at the operating frequency). Exceptions 
are certain oscillator circuits (modified Pierce, for example) where the 
screen is actually used as the plate of the triode formed by the cathode - 
control grid - screen grid, or when the screen may be used for signal 
injection in a mixer circuit. 


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