NEETS Module 9 — Introduction to Wave- Generation and Wave-Shaping
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 1-52
2-1 to 2-10
, 2-11 to 2-20
2-21 to 2-30
, 2-31 to 2-38
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 3-56
4-1 to 4-10
, 4-11 to 4-20
4-21 to 4-30
, 4-31- to 4-40
4-41 to 4-50
, 4-51 to 4-61
Upon completion of this chapter you will be able to:
1. Explain the operation of
2. Explain the operation of parallel-limiter circuits.
3. Describe the operation of a dual-diode limiter circuit.
the operation of clamper circuits.
5. Explain the composition of nonsinusoidal waves.
6. Explain how RC and RL circuits are used as integrators.
how RC and RL circuits are used as differentiators.
8. Explain the operation of a
9. Explain the operation of a step-by-step counter used as a frequency
As a technician, you will be confronted with many different types of LIMITING circuits. A LIMITER is defined as
a device which limits some part of a waveform from exceeding a specified value. Limiting circuits are used
primarily for wave shaping and circuit-protection applications.
A limiter is little more than the
half-wave rectifier you studied in NEETS, Module 6, Introduction to Electronic Emission, Tubes, and Power
Supplies. By using a diode, a resistor, and sometimes a dc bias voltage, you can build a limiter that will
eliminate the positive or negative alternations of an input waveform. Such a circuit can also limit a portion of
the alternations to a specific voltage level. In this chapter you will be introduced to five types of limiters:
SERIES-POSITIVE, SERIES-NEGATIVE, PARALLEL-POSITIVE, PARALLEL-NEGATIVE, and DUAL-DIODE LIMITERS. Both series- and
parallel-positive and negative limiters use biasing to obtain certain wave shapes. They will be discussed in this
The diode in these circuits is the voltage-limiting component. Its polarity and location, with
respect to ground, are the factors that determine circuit action. In series limiters, the diode is in series with
the output. In parallel limiters, the diode is in parallel with the output.
You should remember, from NEETS, Module 7, Introduction to Solid-State Devices and Power Supplies, that a
diode will conduct when the anode voltage is positive with respect to the cathode voltage. The diode will not
conduct when the anode is negative in respect to the cathode. Keeping these two
simple facts in mind as you study limiters will help you understand their operation. Your knowledge of
voltage divider action from NEETS, Module 1, Introduction to Matter, Energy, and Direct Current will also help you
In a SERIES LIMITER, a diode is connected in series with the output, as shown in view
(A) of figure 4-1. The input signal is applied across the diode and resistor and the output is taken across the
resistor. The series-limiter circuit can limit either the positive or negative alternation, depending on the
polarity of the diode connection with respect to ground. The circuit shown in figure 4-1, view (B), is a
SERIES-POSITIVE LIMITER. Reversing D1 would change the circuit to a SERIES-NEGATIVE LIMITER.
Figure 4-1A.—Series-positive limiter.
Figure 4-1B.—Series-positive limiter.
Let's look at the series-positive limiter and its outputs in
figure 4-1. Diode D1 is in series with the output and the output is taken across resistor R1. The input must be
negative with respect to the anode of the diode to make the diode conduct. When the positive alternation of the
input signal (T0 to T1) is applied to the circuit, the cathode is positive with respect to the anode. The diode is
reverse biased and will not conduct. Since no current can flow, no output is developed across the resistor during
the positive alternation of the input signal.
During the negative half cycle of the input signal (T1 to
T2), the cathode is negative with respect to the anode. This causes D1 to be forward biased. Current flows through
R1 and an output is developed.
The output during each negative alternation of the input is approximately the same as the input (-10 volts)
because most of the voltage is developed across the resistor.
Ideally, the output wave shape should be
exactly the same as the input wave shape with only the limited portion removed. When the diode is reverse biased,
the circuit has a small amount of reverse current flow, as shown just above the 0-volt reference line in figure
4-2. During the limiting portion of the input signal, the diode resistance should be high compared to the
resistor. During the time the diode is conducting, the resistance of the diode should be small as compared to that
of the resistor. In other words, the diode should have a very high front-to-back ratio (forward resistance
compared to reverse resistance). This relationship can be better understood if you study the effects that a
front-to-back resistance ratio has on circuit output.
Figure 4-2.—Actual output of a series-positive limiter.
The following formula can be used to determine the output amplitude of the signal:
Let's use the formula to compare the front-to-back ratio of the diode in the forward- and reverse- biased
R1 = 1,000 ohms
Rac = 1 ohm (forward - biased condition)
Rac = 100,000 ohms (reversed biased condition)
Ein = 10 volts
You can readily see that the formula comparison of the forward- and reverse-bias resistance conditions shows
that a small amount of reverse current will flow during the limited portion of the input waveform. This small
amount of reverse current will develop as the small positive voltage (0.09 volt) shown in figure 4-2 (T0 to T1 and
T2 to T3). The actual amount of voltage developed will depend on the type of diode used. For the remainder of this
chapter, we will use only idealized waveforms and disregard this small voltage.
LIMITER WITH BIAS.—In the series-positive limiter (figure 4-1, view (A)), the reference point at the
bottom of resistor R1 is ground, or 0 volts. By placing a dc potential at point (1) in figure 4-3 (views (A) and
(B)), you can change the reference point. The reference point changes by the amount of dc potential that is
supplied by the battery. The battery can either aid or oppose the flow of current in the series-limiter circuit.
POSITIVE BIAS (aiding) is shown in view (A) and NEGATIVE BIAS (opposing) is shown in view (B).
Figure 4-3A.—Positive and negative bias. POSITIVE BIAS.
Figure 4-3B.—Positive and negative bias. NEGATIVE BIAS.
When the dc aids forward bias, as in view (A), the diode conducts even with no signal applied. An input signal
sufficiently positive to overcome the dc bias potential is required to reverse bias and cut off the diode.
Let's look at a series-positive limiter with positive bias as shown in figure 4-4, views (A) and (B). The diode
will conduct until the input signal exceeds +5 at T1 on the positive alternation of the input signal. When the
positive alternation exceeds +5 volts, the diode becomes reverse biased and limits the positive alternation of the
output signal to +5 volts. This is because there is no current flow through resistor R1 and battery voltage is
felt at point (B). The diode will remain reverse biased until the positive alternation of the input signal
decreases to just under +5 volts at T2. At this time, the diode again becomes forward biased and conducts. The
diode will remain forward biased from T2 to T3. During this period the negative alternation of the input is passed
through the diode without being limited. From T3 to T4 the diode is again reverse biased and the output is again
Figure 4-4A.—Series-positive limiter with positive bias.
Figure 4-4B.—Series-positive limiter with positive bias.
Now let's look at what takes place when reverse bias is aided, as shown in figure 4-5, view (A). The diode is
negatively biased with -5 volts from the battery. In view (B), compare the output to the input signal applied.
From T0 to T1 the diode is reverse biased and limiting takes place. The output is at -5 volts (battery voltage)
during this period. As the negative alternation increases toward -10 volts (T1), the cathode of the diode becomes
more negative than the anode and is forward biased. From T1 to T2 the input signal is passed to the output. The
diode remains forward biased until the negative alternation has decreased to -5 volts at T2. At T2 the cathode of
the diode becomes more positive than the anode, and the diode is again reverse biased and remains so until T3.
Figure 4-5A.—Series-positive limiter with negative bias.
Figure 4-5B.—Series-positive limiter with negative bias.
In view (A) of figure 4-6, the SERIES-NEGATIVE LIMITER limits
the negative portion of the waveform, as shown in view (B). Let's consider the input signal and determine how the
output is produced. During T0 to T1 (view (B)), the anode is more positive than the cathode and the diode
conducts. Current flows up through the resistor and the diode, and a positive voltage is developed at the output.
The voltage across the resistor is essentially the same as the voltage applied to the circuit.
Figure 4-6A.—Series-negative limiter.
Figure 4-6B.—Series-negative limiter.
During T1 to T2 the anode is negative with respect to the cathode and the diode does not conduct. This
portion of the output is limited because no current flows through the resistor.
As you can see, the only
difference between series-positive and series-negative limiters is that the diode is reversed in the negative
SERIES-NEGATIVE LIMITER WITH BIAS.—View (A) of figure 4-7 shows a series-negative limiter with
negative bias. The diode is forward biased and conducts with no input signal. In view (B) it will continue to
conduct as the input signal swings first positive and then negative (but only to -5 volts) from T0 through T1. At
T1 the input becomes negative with respect to the -5 volt battery bias. The diode becomes reverse biased and is
cutoff until T2 when the anode again becomes positive with respect to the battery voltage (-5 volts) on the
cathode. No voltage is developed in the output by R1 (no current flow) and the output is held at -5 volts from T1
to T2. With negative bias applied to a series-negative limiter, only a portion of the negative signal is limited.
Figure 4-7A.—Series-negative limiter with negative bias.
Figure 4-7B.—Series-negative limiter with negative bias.
Now let's look at a series-negative limiter with positive bias, as shown in figure 4-8, view (A). Here we will
remove all of the negative alternation and part of the positive alternation of the input signal. We have given a
full explanation of the series-positive limiter, series-positive limiter with bias, series- negative limiter, and
series-negative limiter with negative bias; therefore, you should have little difficulty understanding what is
happening in the circuit in the figure.
Figure 4-8A.—Series-negative limiter with positive bins.
Figure 4-8B.—Series-negative limiter with positive bins.
The series-negative limiter with positive bias is different in only one aspect from the series-positive limiter
with bias (figure 4-5) discussed earlier. The difference is that the diode is reversed and the output is of the
Q1. Which portion of a sine-wave input is retained in the output of a
Q2. Which portion of a sine-wave input is retained in the output of a
Q3. How can a series-positive limiter be modified to limit unwanted
negative portions of the input signal?
circuit uses the same diode theory and voltage divider action as series limiters. A resistor and diode are
connected in series with the input signal and the output signal is developed across the diode. The output is in
parallel with the diode, hence the circuit name, parallel limiter. The parallel limiter can limit either the
positive or negative alternation of the input signal.
Recall that in the series limiter the output was
developed while the diode was conducting. In the parallel limiter the output will develop when the diode is cut
off. You should not try to memorize the outputs of these circuits; rather, you should study their actions and be
able to figure them out.
The schematic diagram shown in figure 4-9, view
(A), is a PARALLEL-POSITIVE LIMITER. The diode is in parallel with the output and only the positive half cycle of
the input is limited. When the positive alternation of the input signal is applied to the circuit (T0 to T1), the
diode is forward biased and conducts. This action may be seen in view (B). As current flows up through the diode
and the resistor, a voltage is dropped across each. Since R1 is much larger than the forward resistance of D1,
most of the input signal is developed across R1. This leaves only a very small voltage across the diode (output).
The positive alternation of the input signal has been limited.
Figure 4-9A.—Parallel-positive limiter.
Figure 4-9B.—Parallel-positive limiter.
From T1 to T2 the diode is reverse biased and acts as an extremely high resistance. The negative alternation of
the input signal appears across the diode at approximately the same amplitude as the input. The negative
alternation of the input is not limited.
As with the series limiter, the parallel limiter should provide
maximum output voltage for the unlimited part of the signal. The reverse-bias resistance of the diode must be very
large compared to the series resistor. To determine the output amplitude, use the following formula:
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 Power Supplies, Introduction
to Amplifiers, Introduction to Wave-Generation
and Wave-Shaping Circuits, Introduction to
Wave Propagation, Transmission Lines, and Antennas,
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