1-33The capacitor, then, can be said to regulate the current flow through the bias resistor. This action isconsidered as BYPASSING or eliminating the effect of the ac input signal in the cathode. For allpractical purposes, you can assume that ac flows through the capacitor to ground. But, remember, ac onlyappears to flow across a capacitor. In reality the ac signal is shunted around the capacitor.There are two disadvantages associated with cathode biasing. To maintain bias voltage continuously,current must flow through the tube, and plate voltage will never be able to reach the maximum value ofthe source voltage. This, in turn, limits the maximum positive output for a negative input signal(remember the 180º inversion). In addition, maximum plate voltage is decreased by the amount ofcathode-biasing voltage. What this means is that you can't get something for nothing. If the cathode isbiased at +20 volts, this voltage must be subtracted from the plate voltage. As an example, consider atriode with a 10,000 ohm plate resistor and a +300 volts dc source voltage. If a current of 2 milliamperesflows through the tube under quiescent conditions, 20 volts are dropped across the plate-load resistor. Themaximum plate voltage is then 300 volts - 20 volts = 280 volts dc. Now, consider the 20-volt droppedacross the cathode resistor. Plate voltage becomes 280 volts - 20 volts = 260 volts. To understand this alittle more thoroughly, look at figure 1-24. In view A, the source voltage is 300 volts dc. There are twoways that this voltage can be looked at; either the plate is at +300 volts and the cathode is at 0 volts(ground), or the plate is at +150 volts and the cathode is at -150 volts. In electronics, it is commonpractice to assume that the plate is at +300 volts while the cathode is at 0 volts. To simplify thisdiscussion, we will assume that the plate is at +150 volts, and the cathode is at -150 volts. The potentialdifference between the plate and the cathode is 300 volts. If a plate-load resistor is installed, as shown inview B, 20 volts are dropped by R_{L}. The potential difference between the plate and the cathode is now280 volts. In view C, R_{k} has now been placed in the same circuit. R_{k} drops 20 volts. Therefore, the effectof cathode biasing is to reduce the maximum positive signal that the circuit can produce. In this case, themaximum positive signal has been reduced by 20 volts. Despite these disadvantages, cathode biasing hastwo main advantages. It is simple and economical.Figure 1-24.—Loss due to cathode biasing.Grid-Leak BiasingThe second type of self-biasing to be discussed is GRID-LEAK BIAS. As the name implies, biasvoltage is developed in the grid leg portion of the circuit. Bias voltage in this type of biasing is derived byallowing the positive input signal to draw grid current through a circuit made up of a resistor and acapacitor. There are two types of grid-leak bias commonly in use: SHUNT TYPE and SERIES TYPE.Because shunt type grid-leak biasing is the simplest, we will discuss it first. Figure 1-25 depicts asimplified triode circuit using the shunt-type grid-leak biasing. Before we begin the explanation of shunt