Theoretical Gain and Signal-to-Noise Ratio of the Grounded-Grid Amplifier at Ultra-High Frequencies

With the grounded-grid or cathode-input-amplifier circuit the input signal is applied between the grid and cathode of the tube and the output signal is obtained between the grid and plate of the tube. It is shown that, to obtain maximum gain, the input circuit should be designed to impedance-match the equivalent generator resistance to R
t
/M and the output circuit should be designed to impedance-match the final load resistance to R
p
M where R
t
is the shunt resistance between cathode and grid due to transit-time loading and Rp is the effective plate resistance of the tube and M = 1+[(u+1)R
t
/R
p
]. Under these conditions the ratio of the voltage across the final load to the generator voltage (i.e., the gain of the complete stage) is G = [(u+1) R
t
/R
1
R
1
/R
p
]/2(1+M). Using the acorn tube 955 the noise factors of a receiver, using the cathode-input circuit and one using the more usual grid-input circuit, are compared showing that for this specific case the noise factor of the cathode-input circuit is almost 3.5 decibels smaller than that for the grid-input circuit, at 100 megacycles. At higher frequencies however the two noise factors approach the same values.