Figure 1-18.Typical grid waveforms.
Because the bias voltage is more negative than the signal voltage is positive, the resultant voltage
(bias plus signal), Eg, is ALWAYS negative. The signal, in this case, makes the grid voltage go either
MORE or LESS NEGATIVE, (-9 to -3) but cannot drive it positive.
Under these circumstances, the negative grid always repels electrons from the space charge. The grid
cannot draw current. Any problems associated with grid current are eliminated, because grid current
cannot flow to a negative grid.
You have probably already realized that the negative bias also reduces plate current flow. (Negative
charge on grid-less plate current, right?) The trick here is for the circuit designer to choose a bias and an
input signal that, when added together, do not allow the grid to become positive nor to become negative
enough to stop plate current.
Tube biasing is very important. You will learn much more about it shortly. From this brief
introduction, you should have learned that grid bias
is a steady, direct voltage that in most cases makes the grid negative with respect to the cathode;
is in series with the signal voltage between grid and cathode;