[Amps] Cathode impedance

[TexasRF@aol.com]

In a message dated 9/9/2005 10:54:04 A.M. Central Standard Time,  
nagato at arnet.com.ar writes:

Is any rule of thumb to estimate input impedance for a tube/s in  GG
configuration?.

Best regards
Guillermo -  LU8EYW.
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My old ITT Handbook shows GG input impedance to be cathode voltage/cathode  
current. The voltage here is the rms rf voltage (not dc bias) and the current 
is  the sum of plate, grid and screen currents.
 
You can see from this that a low gain tube will require more drive power;  
therefore more voltage, resulting in a higher input impedance.
 
The handbook did not say anything about resonating the input circuit but it  
makes sense to do that as the capacitance between the cathode and grounded 
grid  is substantial. As frequency is raised the input shunt Xc can become quite 
low.  The typical circuit uses the input shunt C as part of the matching 
network. At  higher frequencies the major part of designing the network can end up 
being how  to deal with the input C; the actual impedance matching being 
secondary in  design effort. 
 
73/k5gw