In a message dated 9/9/2005 10:54:04 A.M. Central Standard Time,
nagato@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
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