Module 6 - Introduction to Electronic Emission, Tubes, and Power
Pages i - ix,
1-1 to 1-10,
1-11 to 1-20,
1-21 to 1-30,
1-31 to 1-40,
1-41 to 1-50,
1-51 to 1-56,
2-1 to 2-10,
2-11 to 2-20,
2-21 to 2-30,
2-31 to 2-39,
3-1 to 3-10,
3-11 to 3-20,
3-21 to 3-30,
3-31 to 3-40,
3-41 to 3-50,
3-51 to 3-60, AI-1 to AI-3, Index
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).
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
As we mentioned earlier, the bias voltage will vary with the input unless Ck, the cathode bypass capacitor, is
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.
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
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
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
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.
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.
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, 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
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.
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.
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
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
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.
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.
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
TRANSIENT TIME, INTERELECTRODE CAPACITANCE, TRANSCONDUCTANCE,
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.
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:
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).
in plate voltages (Ep) is then 100 volts. The amplification factor (µ) of just the tube is then
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.
NEETS Table of Contents
- Introduction to Matter, Energy,
and Direct Current
- Introduction to Alternating Current and Transformers
- Introduction to Circuit Protection,
Control, and Measurement
- Introduction to Electrical Conductors, Wiring
Techniques, and Schematic Reading
- Introduction to Generators and Motors
- Introduction to Electronic Emission, Tubes,
and Power Supplies
- Introduction to Solid-State Devices and
- Introduction to Amplifiers
- Introduction to Wave-Generation and Wave-Shaping
- Introduction to Wave Propagation, Transmission
Lines, and Antennas
- Microwave Principles
- Modulation Principles
- Introduction to Number Systems and Logic Circuits
- Introduction to Microelectronics
- Principles of Synchros, Servos, and Gyros
- Introduction to Test Equipment
- Radio-Frequency Communications Principles
- Radar Principles
- The Technician's Handbook, Master Glossary
- Test Methods and Practices
- Introduction to Digital Computers
- Magnetic Recording
- Introduction to Fiber Optics