DEVELOPMENT OF THE TETRODE
Interelectrode capacitance cannot be eliminated from vacuum tubes, but it can be reduced. The
easiest method found to reduce interelectrode capacitance is to split the capacitance between the grid and
plate (Cpg) into two capacitors connected in series. This is done by placing an extra grid, called the
SCREEN GRID, between the control grid and the plate. This is shown in figure 1-30.
Figure 1-30.Effect of the screen-grid on Interelectrode capacitance.
Remember from your study of capacitance that connecting capacitors in series reduces the total
capacitance to a value smaller than either of the capacitors. This is mathematically summed up as follows:
The addition of the screen grid has the effect of splitting Cpg into two capacitances (C1 and C2)
connected in series. Therefore, the total interelectrode capacitance between the control grid and the plate
is greatly reduced.
OPERATION OF THE BASIC TETRODE CIRCUIT
Figure 1-31 depicts a basic tetrode circuit. While the circuit may look complicated, it isnt. You are
already familiar with most of the circuit. Only three components have been added: the screen grid, the
screen grid dropping resistor, and the screen grid bypass capacitor (Csg).