1-46
TETRODE CHARACTERISTICS
Because the screen grid is in the electron stream, it will always draw some current. The current
drawn by the screen grid will be lost to the plate. This means that the transconductance of a tetrode, which
is based on the amount of plate current versus control-grid voltage, will be lower in tetrodes than in
triodes. The formula for transconductance of a triode,
must be adjusted for screen-grid current, and becomes
As you can see, the transconductance for a tetrode can never be as high as that of a triode of similar
construction.
While lowered transconductance in a tetrode is an undesirable characteristic, it is not the reason that
tetrodes have found little acceptance in electronics. The factor that severely limits the operation of
tetrodes is SECONDARY EMISSION.
Because the screen grid is positively charged, electrons traveling from the cathode to the plate are
accelerated. Electrons are accelerated to such an extent that they dislodge electrons from the plate when
they strike it. This is similar to the manner in which a high-velocity rifle bullet fired into a pile of sawdust
throws sawdust about. Some of these electrons are fired back into the tube, where they tend to accumulate
between the screen grid and the plate. This effect is most pronounced when the signal at the control grid is
going positive and conduction through the tube is increasing. The plate in this situation is going negative
in answer to the control-grid signal. This causes the electrons accumulating between the plate and screen
grid to be attracted to the screen grid. The current that is drawn by the screen grid is lost to the plate and
gain is decreased. Gain is also decreased in another way. The negative charge accumulated by secondary
emission causes some of the electrons (from the cathode) to be repelled from the plate, which further
reduces gain.
Another undesirable characteristic of tetrodes associated with secondary emission is that the outputs
are NOISY. What this means is that small sporadic signals appear on the main output signal, as shown in
figure 1-32. When electrons are knocked from the plate, they represent losses of plate current and
corresponding positive pulses on the output. Electrons falling back to the plate represent increases in plate
current and cause negative-going pulses to appear in the output.
