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groundedgridnotes.txt
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groundedgridnotes.txt
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GROUNDED GRID VACUUM TUBE RF AMPLIFIER - MAJOR COMPONENTS
FILAMENT TRANSFORMER - the secondary winding connects directly to the filament of the vacuum tube and is center tapped to prevent the applied AC from modulating the emitted wave
TRIODE VACUUM TUBE - the control grid is connected to ground. The cathode is connected to a radio frequency choke and then a B- (bias) power supply. The plate is connected to a radio frequency choke which in turn connects to a B+ (high voltage) power supply.
PI NETWORK - connects to the plate of the triode vacuum tube through a blocking capacitor. The plate and load tuning controls of a pi network tuned circuit allows for efficient transfer of power to the antenna at the resonant frequency.
The input signal is applied to the cathode.
Harmonics produced in an early stage of a transmitter may be reduced in a later stage by using tuned circuit coupling between stages.
If an RF power amplifier stage is 50% efficient, the amount of power required by the stage will be twice the output power rating. Input power to the final stage can be calculated with the formula:
P = E I (where P = power E = voltage I = current)
> A grounded-grid amplifier runs with the grid at ground potential. The cathode is above RF ground and serves as the input. A DC bias is applied to the cathode via an RF choke. Positive voltage (B+) is supplied to the plate via an RF choke. The plate is the output, a blocking capacitor passes the RF out to the matching network. A transformer provides AC filament voltage. The heater (in an indirectly-heated cathode tube) is bypassed to ground so radiofrequency does not stray out on the filament supply lines. [ If a tube is directly-heated (no separate cathode), filament voltage is brought through a filament choke. The side of the choke connected to the transformer is bypassed to ground with two capacitors. ]
%\begin{frame}{RF amps}{}
%include graphic simple CW transmitter
%Inductive coupling may be used between oscillator and RF power amp. This forms another tuned circuit to further reduce undesired frequencies (harmonics)
%Note uses of RF chokes to keep RF out of the power supply
%Neutralization may be required on some vacuum tube amps to prevent parasitic oscillation due to capacitances inside the tube. Couple a small amount of the output back to the input 180 degrees out of phase (negative feedback)
%Neutralization is indicated by minimum change of grid current while output circuit is tuned.
%\end{frame}