TYPES OF BIASING
There are two main classes of biasing -
FIXED and SELF. In a tube circuit that uses fixed bias, the grid-bias voltage is supplied from a power source external tothe circuit. You are already familiar with battery bias, which is one form of fixed bias.When fixed bias is used in a circuit, it can be represented as either a battery (fig.1-21, view A), or as a conductor connected to -E3ecc (fig. 1-21, view B). Fixedbias is rarely used in electronics today. Therefore, we will not discuss it further.
Figure 1-21. - Fixed bias: A. Battery B. Conductor
In circuits using self-bias, the bias voltage is developed across a resistor in the cathode or grid circuit by tube current. There are two main methods of self-bias: cathode biasing and grid-leak biasing.
In circuits using cathode bias, the cathode is made to go positive relative to the grid. The effect of this is the same as making the grid negative relative to the cathode. Because the biasing resistor is in the cathode leg of the circuit, the method is called CATHODE BIASING or CATHODE BIAS. A triode circuit using cathode bias is shown in figure 1-22.
Figure 1-22. - Cathode bias.
The only difference between the illustrated circuit and the one used to demonstrate triode operation is the elimination of the battery, E cc, and the addition of circuit components Rk, the cathode-biasing resistor; Ck, the cathode ac-bypass capacitor; and a grid resistor (whose purpose will be explained later).
When the tube conducts, current flows from the battery through Rk to the cathode, through the tube to the plate, and through RL to the positive terminal of the battery. The current flowing through Rk will cause a voltage drop across Rk. The bottom of Rk goes negative while the top goes positive. This positive voltage at the top of Rk makes the cathode positive relative to the grid.
You may wonder what purpose Ck serves in this circuit. Ck serves as an AC BYPASS. Without Ck, the bias voltage will vary with ac input signals. This is particularly troublesome in the higher frequencies like those found in radio receivers. Rk, the cathode-biasing resistor, is used to develop the biasing voltage on the cathode.
The input signal will be developed across Rg. You will read more about the circuit component later in this chapter. Cathode-biasing voltage is developed in the following manner.
As we mentioned earlier, the bias voltage will vary with the input unless Ck, the cathode bypass capacitor, is used.
To understand how the bias voltage will vary with an ac input signal, disregard Ck for the moment and refer to figure 1-22 again.
Notice that under quiescent conditions, the voltage drop at the top of Rk is +10 volts. Now let's apply the positive-going signal illustrated to the left of the tube. When the positive signal is applied, conduction through the tube will increase. The only trouble is that current through Rk will also increase. This will increase the voltage drop across Rk, and the cathode voltage will now be greater than +10 volts. Remember, at this time the plate is going negative due to increased conduction through the tube. The combination of the negative-going plate and the positive-going cathode will decrease the electrostatic attraction across the tube and lower the conduction of the tube. This will reduce the gain of the tube.
When the negative-going signal is applied, conduction through the tube decreases. Current through Rk decreases and the voltage drop across Rk decreases. This causes the cathode to go more negative, which tends to increase conduction through the tube. A negative-going signal is amplified by decreasing plate current and allowing the plate to go positive (remember the 180° inversion.) Thus, increasing conduction on the negative half-cycle decreases the gain of that half-cycle. The overall effect of allowing cathode biasing to follow the input signal is to decrease the gain of the circuit with ac inputs.
This problem can be overcome by installing Ck. The purpose of Ck is to maintain the cathode bias voltage at a constant level. In common usage, the action of Ck is referred to as "bypassing the ac signal to ground."
The action of Ck will be explained using figure 1-23. View A shows the circuit under quiescent conditions. With some conduction through the tube, the cathode and the tops of Rk and Ck are at +10 volts.
Figure 1-23. - Effect of the bypass capacitor.
In view B, the positive-going signal is applied to the grid. This causes increased conduction through the tube, which attempts to drive the cathode to +20 volts. But notice that the top of Ck is still at +10 volts (remember capacitors oppose a change in voltage). The top plate of Ck is, in effect, 10 volts negative in relation to the top of Rk. The only way that Ck can follow the signal on the top of Rk (+20 volts) is to charge through the tube back to the source, from the source to the lower plate of Ck. When Ck charges through the tube, it acts as the source of current for the cathode. This causes the cathode to remain at +10 volts while the capacitor is charging.
View C of the figure shows the same signal. Under these conditions, conduction through Rk will decrease. This will cause a decrease in current flow through Rk. Decreased current means decreased voltage drop. The top of Rk will try to go to +5 volts. Ck must now go more negative to follow the top of Rk. To do this, current must flow from C k through Rk, to the top plate of C k. This discharging of Ck will increase current flow through Rk and increase the voltage drop across Rk, forcing the top to go more positive. Remember, the voltage drop is due to current flow through the resistor. (The resistor could care less if the current is caused by conduction or capacitor action.) Thus, the cathode stays at +10 volts throughout the capacitor-charge cycle.
There is one point that we should make. Ck and Rk are in parallel. You learned from previous study that voltage in a parallel circuit is constant. Thus, it would seem impossible to have the top of Rk at one voltage while the top plate of Ck is at another. Remember, in electronics nothing happens instantaneously. There is always some time lag that may be measured in millionths or billionths of seconds. The action of Ck and Rk that was just described takes place within this time lag. To clarify the explanation, the voltages used at the components Rk and Ck were exaggerated. Long before a 10-volt differential could exist between the tops of R k and Ck, Ck will act to eliminate this voltage differential.
The capacitor, then, can be said to regulate the current flow through the bias resistor. This action is considered as BYPASSING or eliminating the effect of the ac input signal in the cathode. For all practical purposes, you can assume that ac flows through the capacitor to ground. But, remember, ac only appears 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 of the source voltage. This, in turn, limits the maximum positive output for a negative input signal (remember the 180° inversion). In addition, maximum plate voltage is decreased by the amount of cathode-biasing voltage. What this means is that you can't get something for nothing. If the cathode is biased at +20 volts, this voltage must be subtracted from the plate voltage. As an example, consider a triode with a 10,000 ohm plate resistor and a +300 volts dc source voltage. If a current of 2 milliamperes flows through the tube under quiescent conditions, 20 volts are dropped across the plate-load resistor. The maximum plate voltage is then 300 volts - 20 volts = 280 volts dc. Now, consider the 20-volt dropped across the cathode resistor. Plate voltage becomes 280 volts - 20 volts = 260 volts. To understand this a little more thoroughly, look at figure 1-24. In view A, the source voltage is 300 volts dc. There are two ways 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 common practice to assume that the plate is at +300 volts while the cathode is at 0 volts. To simplify this discussion, we will assume that the plate is at +150 volts, and the cathode is at -150 volts. The potential difference between the plate and the cathode is 300 volts. If a plate-load resistor is installed, as shown in view B, 20 volts are dropped by RL. The potential difference between the plate and the cathode is now 280 volts. In view C, Rk has now been placed in the same circuit. Rk drops 20 volts. Therefore, the effect of cathode biasing is to reduce the maximum positive signal that the circuit can produce. In this case, the maximum positive signal has been reduced by 20 volts. Despite these disadvantages, cathode biasing has two main advantages. It is simple and economical.
Figure 1-24. - Loss due to cathode biasing.
The second type of self-biasing to be discussed is GRID-LEAK BIAS. As the name implies, bias voltage is developed in the grid leg portion of the circuit. Bias voltage in this type of biasing is derived by allowing the positive input signal to draw grid current through a circuit made up of a resistor and a capacitor. 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 a simplified triode circuit using the shunt-type grid-leak biasing. Before we begin the explanation of shunt 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, C c, 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.