1-35
right-hand plate of Cc. This is the biasing charge cycle. You may wonder why the charge current went
through the tube rather than through Rg. When the grid goes positive in response to the positive-going
input signal, electrostatic attraction between the grid and cathode increases. This, in turn, reduces the
resistance (rgk) between the grid and cathode. Current always follows the path of least resistance. Thus,
the capacitor charge path is through the tube and not through Rg.
When the first negative alternation is applied to the circuit (view D), the left-hand plate of Cc must
go negative. To do this, electrons are drawn from the right-hand plate. The electrons travel from the
right-hand plate of Cc, through Rg causing a voltage drop negative (top) to positive (bottom), from the
bottom of Rg, through the source, to the left-hand plate of Cc. Cc will discharge for the duration of the
negative alternation. BUT Cc can only discharge through Rg, which is a high-resistance path,
compared to the charge path. Remember from your study of capacitors that RC time constants and the
rate of discharge increase with the size of R. Cc can therefore charge through the low resistance of rgk to
its maximum negative value during the positive half-cycle. Because Cc discharges through Rg (the high
resistance path), it cannot completely discharge during the duration of the negative half-cycle. As a result,
at the completion of the negative alternation, Cc still retains part of the negative charge it gained during
the positive alternation. When the next positive alternation starts, the right-hand plate of Cc will be more
negative than when the first positive alternation started.
During the next cycle, the same process will be repeated, with Cc charging on the positive alternation
and discharging a lesser amount during the negative alternation. Therefore, at the end of the second cycle,
Cc will have an even larger negative charge than it did after the first cycle. You might think that the
charge on Cc will continue to increase until the tube is forced into cutoff. This is not the case. As the
negative charge on the right-hand plate of Cc forces the grid more negative, electrostatic attraction
between the grid and cathode decreases. This, in effect, increases the resistance (rgk) between the cathode
and the grid, until rgk becomes, in effect, the same size as Rg. At this point, charge and discharge of Cc
will equal one another and the grid will remain at some negative, steady voltage. What has happened in
this circuit is that Cc and Rg, through the use of unequal charge and discharge paths, have acted to change
the ac input to a negative dc voltage. The extent of the bias on the grid will depend on three things: the
amplitude of the input, the frequency of the input, and the size of Rg and Cc. This type of biasing has the
advantage of being directly related to the amplitude of the input signal. If the amplitude increases, biasing
increases in step with it. The main limiting factor is the amount of distortion that you may be willing to
tolerate. Distortion occurs during the positive alternation when the grid draws current. Current drawn
from the electron stream by the grid never reaches the plate; therefore the negative-going output is not a
faithful reproduction of the input, while the positive-going output (during the negative input cycle) will be
a faithful reproduction of the input. This is similar to the situation shown in the flattopped portion of the
output signal in figure 1-20.
The SERIES GRID-LEAK BIAS circuit shown in figure 1-26 operates similarly to the shunt grid-
leak circuit. When the first positive alternation is applied to the left-hand plate of the grid capacitor, Cg,
the left-hand plate must lose electrons to go positive with the input. Electrons will leave the left-hand
plate and flow through Rg, causing a negative (left-hand side) to positive (right-hand side) voltage drop.
From the right-hand side of Rg, the electrons will flow to the right-hand plate of Cg. The positive voltage
developed at the right-hand side of Rg will be coupled to the grid. As the grid goes positive, it will draw
current, causing Cg to start to charge through the low resistance path of the tube. During the negative
alternation of the input, Cg will discharge through the high resistance path of Rg. Once again it will not be
completely discharged at the end of the negative alternation, and the capacitor will continue on its way
toward charge equilibrium.
