1-34
grid-leak biasing, there is one thing you should bear in mind. Because the bias is derived from the
positive input signal through capacitive action, the input signal must go through several positive
alternations before the final operating bias voltage is achieved. We will explain why this is so in the
following discussion.
View A of figure 1-25 shows the circuit under quiescent conditions. You will notice that the circuit
is similar to the one we used to explain the action of a triode. The only additions are the grid resistor, Rg
coupling capacitor, Cc, and resistance rgk. Resistance rgk doesn’t exist as a physical component, but it is
used to represent the internal tube resistance between the triode’s cathode and grid. Electrically, rgk is
quite small, about 500 ohms. Under quiescent conditions, some conduction occurs through the tube. Some
electrons will strike the wires of the grid, and a small amount of GRID CURRENT will flow through Rg
to ground. This will cause the right-hand plate of Cc to go slightly negative. This slight negative charge
will, in turn, keep the grid of the tube slightly negative. This limits the number of electrons that strike the
grid wires.
Figure 1-25.—Shunt grid-leak biasing.
In view B of the figure, the first positive alternation of a series of ac alternations, Ein is applied to the
circuit. The positive-going voltage causes the left-hand plate of Cc to go positive. The left-hand plate must
lose electrons to go positive. These electrons leave the left-hand plate of Cc and travel to the input source
where they will be coupled to ground. From ground, current flows through Rg causing a negative (bottom)
to positive (top) voltage drop across Rg. In effect, the ac signal has been coupled across the capacitor.
Because of this, capacitors are said to pass the ac signal while blocking dc. (In reality, the ac signal is
coupled around the capacitor.) In view C of the figure, the positive-going voltage at the top of Rg will be
coupled to the grid causing the grid to go positive. The positively charged grid will attract electrons from
the electron stream in the tube. Grid current will flow from the grid to the right-hand plate of Cc. This will
cause the right-hand plate to go negative. (Electrostatic repulsion from the right-hand plate of Cc will
force electrons from the left-hand plate of Cc, causing it to go positive.) The electrons will flow through
the signal source, to ground, from ground to the cathode, from the cathode to the grid, and finally to the
