NEETS Module 9 − Introduction to Wave− Generation and Wave−Shaping
and Direct Current
||Alternating Current and Transformers
||Circuit Protection, Control, and Measurement
||Electrical Conductors, Wiring Techniques,
and Schematic Reading
||Generators and Motors
||Electronic Emission, Tubes, and Power Supplies
||Solid-State Devices and Power Supplies
||Wave-Generation and Wave-Shaping Circuits
||Wave Propagation, Transmission Lines, and
||Introduction to Number Systems and Logic Circuits
||- Introduction to Microelectronics
||Principles of Synchros, Servos, and Gyros
||Introduction to Test Equipment
||Radio-Frequency Communications Principles
||The Technician's Handbook, Master Glossary
||Test Methods and Practices
||Introduction to Digital Computers
||Introduction to Fiber Optics
|Note: Navy Electricity and Electronics Training
Series (NEETS) content is U.S. Navy property in the public domain.
Upon completion of this chapter you will be able to:
1. Explain the operation of series-limiter circuits.
2. Explain the operation of parallel-limiter circuits.
3. Describe the operation of a dual-diode limiter circuit.
4. Explain the operation of clamper circuits.
5. Explain the composition of nonsinusoidal waves.
6. Explain how RC and RL circuits are used as integrators.
7. Explain how RC and RL circuits are used as differentiators.
8. Explain the operation of a counting circuit.
9. Explain the operation of a step-by-step counter used as
a frequency divider.
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 chapter.
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 understand limiters.
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
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
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 conditions.
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.
Series-Positive 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 limited.
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 limiters.
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 opposite polarity.
Q1. Which portion of a sine-wave input is retained in the output
of a series-positive limiter?
Q2. Which portion of a sine-wave input is retained in the output
of a series-negative limiter?
Q3. How can a series-positive limiter be modified to limit unwanted
negative portions of the input signal?
A PARALLEL-LIMITER 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
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: