Figure 1-25. - Shunt grid-leak biasing.

In view B of the figure, the first positive alternation of a series of ac alternations, E_in is applied to the circuit. The positive-going voltage causes the left-hand plate of C _c to go positive. The left-hand plate must lose electrons to go positive. These electrons leave the left-hand plate of C_c and travel to the input source where they will be coupled to ground. From ground, current flows through R_g causing a negative (bottom) to positive (top) voltage drop across R_g. 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 R_g 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 C_c. This will cause the right-hand plate to go negative. (Electrostatic repulsion from the right-hand plate of C_c will force electrons from the left-hand plate of C_c, 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 right-hand plate of C_c. This is the biasing charge cycle . You may wonder why the charge current went through the tube rather than through R_g. 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 R _g.

When the first negative alternation is applied to the circuit (view D), the left-hand plate of C_c must go negative. To do this, electrons are drawn from the right-hand plate. The electrons travel from the right-hand plate of C_c, through R_g causing a voltage drop negative (top) to positive (bottom), from the bottom of R_g, through the source, to the left-hand plate of C_c. C_c will discharge for the duration of the negative alternation.

BUT C_c can only discharge through R_g, 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. C_c can therefore charge through the low resistance of rgk to its maximum negative value during the positive half-cycle. Because C_c discharges through R_g (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, C_c still retains part of the negative charge it gained during the positive alternation. When the next positive alternation starts, the right-hand plate of C_c will be more negative than when the first positive alternation started.

During the next cycle, the same process will be repeated, with C_c charging on the positive alternation and discharging a lesser amount during the negative alternation. Therefore, at the end of the second cycle, C_c will have an even larger negative charge than it did after the first cycle. You might think that the charge on C_c 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 C_cforces 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 R_g. At this point, charge and discharge of C_c will equal one another and the grid will remain at some negative, steady voltage. What has happened in this circuit is that C_c and R_g, 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 R_g and C_c. 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, C_g, the left-hand plate must lose electrons to go positive with the input. Electrons will leave the left-hand plate and flow through R_g, causing a negative (left-hand side) to positive (right-hand side) voltage drop. From the right-hand side of R_g, the electrons will flow to the right-hand plate of C_g. The positive voltage developed at the right-hand side of R_g will be coupled to the grid. As the grid goes positive, it will draw current, causing C_g to start to charge through the low resistance path of the tube. During the negative alternation of the input, C_g will discharge through the high resistance path of R_g. 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.

Figure 1-26. - Series grid-leak biasing.

In summary, grid-leak bias causes the grid to draw current when the input signal goes positive. This grid current (which is a negative charge) is stored by the coupling capacitor (C_c,) which will keep the grid at some negative potential. It is this potential that biases the tube.

Q.21 What type of bias requires constant current flow through the cathode circuit of a triode?
Q.22 When a circuit uses cathode biasing, the input signal can cause variations in the biasing level How is this problem eliminated?
Q.23 In a circuit using grid-leak biasing, the coupling capacitor (C_c) charges through a low resistance path. What resistance is used in this charge path?
Q.24 Grid-leak biasing in effect rectifies the input ac signal. What feature of the circuit is used to accomplish this rectification?