Module 6 - Introduction to Electronic Emission, Tubes, and Power Supplies

Navy Electricity and Electronics Training Series (NEETS)
Chapter 1:  Pages 1-31 through 1-40

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, Ecc, 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 180º inversion.) Thus, increasing

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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 Ck through Rk, to the top plate of Ck. 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 Rk and Ck, Ck will act to eliminate this voltage differential.

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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 180º 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.

Grid-Leak 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

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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

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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 flat-topped 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.

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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 (Cc,) which will keep the grid at some negative potential. It is this potential that biases the tube.

Q21.    What type of bias requires constant current flow through the cathode circuit of a triode?

Q22.    When a circuit uses cathode biasing, the input signal can cause variations in the biasing level

How is this problem eliminated?

Q23.    In a circuit using grid-leak biasing, the coupling capacitor (Cc) charges through a low resistance path. What resistance is used in this charge path?

Q24.    Grid-leak biasing in effect rectifies the input ac signal. What feature of the circuit is used to accomplish this rectification?

OPERATING ClassificationS of TUBE AmplifierS

While the discussion of amplifiers will be covered in detail in later NEETS modules, some discussion of the classes of operation of an amplifier is needed at this point. This is because their operation class is directly determined by the bias voltage of the tube.

The classification of amplifiers by operation is based on the percentage of the time that the tube conducts when an input signal is applied. Under this system amplifiers may be divided into four main classes: A, AB, B, and C.

Class a Operation

An amplifier biased into Class a operation, is one in which conduction through the tube occurs throughout the duration of the input signal. Such an amplifier is shown in figure 1-27, view A. This is the same type of circuit with which you are already familiar. Notice when you compare the input to the output that the tube is always conducting, and that the entire input signal is reproduced at the output.

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Figure 1-27. - Classes of amplifier operation.

Class AB Operation

The Class AB amplifier is one in which the tube conducts for more than half, but less than the entire input cycle.

View B of figure 1-27 depicts an amplifier biased into Class AB operation. Notice that in this application, grid bias has been increased to -9 volts. We will assume that the tube reaches cutoff when the voltage on the grid is -10 volts. Under these conditions, when the input reaches -10 volts, the tube will cut off and stay cut off until the input goes above -10 volts. The tube conducts during the entire duration of the positive alternation and part of the negative alternation. If you remember back in the discussion of distortion, we pointed out that this represents distortion. In some amplifiers, faithful reproduction of the input is not an important requirement. Class AB amplifiers are used only where this distortion can be tolerated.

Class B Operation

A Class B biased amplifier is one in which the tube will conduct for only half of the input signal duration. This is done by simply biasing the amplifier at cutoff. View C of figure 1-27 depicts a class B biased amplifier.

As you can see, the tube conducts on the positive alternations. As soon as the input signal voltage reaches 0 volts, the tube cuts off. The tube will remain cut off until the input signal voltage climbs above zero volts on the next positive alternation. Because the tube conducts during the entire positive alternation, but not on the negative alternation, the tube conducts for only half the input cycle duration.

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Class C

Class C amplifiers are biased below cutoff, so that the tube will conduct for less than half of the input signal cycle duration. View D of figure 1-27 depicts a Class C amplifier. Notice that the tube is biased one-half volt below cutoff. The tube will only conduct on that part of the positive alternation that is above +.5 volts. Therefore, the tube conducts for less than one-half cycle of the input. Again, this class can be applied only where severe distortion can be tolerated.

TUBE CONSTANTS

In the discussion of triodes, we only considered the effects of the external circuit on the passage of current through the tube. The behavior of the electron stream in a conducting tube is also influenced by the physical structure of the tube. The effects that the physical structure of a tube has on the tube's operation are collectively called TUBE CONSTANTS. Four of the most important of these tube constants are: TRANSIENT TIME, Interelectrode Capacitance, TRANSCONDUCTANCE, and Amplification FACTOR.

TRANSIT TIME

Unlike electron flow in a conductor, electrons in a vacuum tube do not move at the speed of light. Their velocity is determined by the potential difference between the plate and the cathode. The amount of time the electrons take to travel from the cathode to the plate is called TRANSIT TIME. As a result of this time difference, the appearance of a signal at the end of a tube is not followed instantaneously by a change in current flow in the tube. Under normal conditions, the effect of this small time lag between the input signal and a change in tube current is unnoticed. However, at frequencies such as those used in radar equipment, this is not the case. Transit time at these frequencies has a very marked effect on tube operation. It is a major factor that limits the use of a given tube at higher frequencies.

Q25.    Match each amplifier characteristic listed below with its class of amplification.

a.    Current flows through the tube for one-half cycle.

b.    Current flows through the tube for less than one-half cycle.

c.   Current flows through the tube for the entire cycle.

MU and TRANSCONDUCTANCE

In your study of triodes so far, you have seen that the output of a triode circuit is developed across the tube. The output is caused by the voltage dropped across RL due to current flow from tube conduction. In all the demonstrations of gain, we assumed that RL was held constant and current through the tube was varied. In this manner we achieved a voltage gain. If the resistance of RL is changed by the designer, the gain of a triode circuit can be either increased or decreased. This is fairly easy to understand. Assume that a circuit is composed of a triode with a plate-load resistor of 100 kohms. If a +2 volt signal causes 2 additional milliamperes to conduct through the tube, the voltage drop across RL (the output) will be:

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 Thus, the gain of the circuit is 100. If the plate-load resistor is reduced to 50 kohms and the input is kept at +2 volts, the gain will be reduced to:

As you can see, voltage gain depends on both the tube characteristics and the external circuit design. The voltage gain is a measure of circuit efficiency, not tube efficiency.

The actual characteristics of a tube are measured by two factors: mu(µ) or Amplification FACTOR; and TRANSCONDUCTANCE or gm. The amplification factor (represented as µ) of a tube is equal to the ratio of a change in plate voltage to the change in grid voltage required to cause the same change in plate current. This is expressed mathematically as

While this may sound complicated, it really isn't. Look at figure 1-28. Here you see in view a a triode with a +1 volt input signal. At this grid voltage, current through the tube is at 1 milliampere. If the input voltage is raised to +3 volts, current through the tube increases to 2 milliamperes. The change in Eg (¨Eg) is then 2 volts. This is shown in view B. Suppose that the grid voltage is returned to +1 volt, and the plate voltage is increased until the ammeter in view C reads 2 milliamperes of plate current. At this point plate voltage is measured. Plate voltage had to be increased by 100 volts (350-250) to get the same change in plate current (1 mA). The change in plate voltages (Ep) is then 100 volts. The amplification factor (µ) of just the tube is then equal to

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Figure 1-28. - Obtaining gain and transconductance.

As you can see, mu is a measure of the ability of a tube to amplify. By comparing the mu of two different types of tubes, you can get an idea of their efficiency. For example, assume you have two different tubes, one with a mu of 50, and the other with a mu of 100. If you place each tube in a circuit whose input varies by 2 volts, you can expect the following changes in plate voltage.

Tube 1:

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