Astable Multivibrators
ASTABLE MULTIVIBRATOR
OBJECTIVES
Upon completion of this lesson, the student will be able to:
1.
2.
Identify schematic symbols of an astable multivibrator.
Describe the operating characteristics of an astable multivibrator.
PREREQUISITES None
EQUIPMENT REQUIRED NONE
OVERVIEW
This lesson introduces students to square waves and square wave generation. The lesson opens with the definition and study of
the square wave. Students then learn how an astable multivibrator generates square waves. The in depth study of the astable
multivibrator includes circuit recognition and operation. The included experiment reinforces the classroom lesson by having the
students verify circuit operation with test equipment.
INTRODUCTION
This lesson begins your study of multivibrators. The more advanced the piece of electronic equipment, the more critical system
timing becomes for proper operation. System timing is accomplished through the use of multivibrators. The more you know
about these simple, yet very important, circuits, the more proficient you become as a technician.
SQUARE WAVE
There are many times in electronic studies when a specifically shaped waveform is required. A periodic waveforrn is a pattern of
changes that returns the original pattern and repeats the same pattern of changes. You have already discussed one such waveform,
the sine wave. Other types of periodic waveforms that you will encounter include the square wave, rectangular wave, trapezoidal
wave, sawtooth wave and trigger pulses. Figure 1 illustrates several types of periodic waveforms. This lesson is limited to the
square wave and its generation.
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Astable Multivibrators
Figure 2 illustrates the square wave, showing four cycles. The amplitude of the waveform is measured vertically. One positive or
negative alternation of the waveform is called a pulse. One complete cycle consists of two alternations. The time for one
complete cycle is measured from leading edge to leading edge or trailing edge to trailing edge. The time that it takes for one
complete cycle of the waveform to occur is called the Pulse Repetition Time or PRT.
The leading edge is defined as the edge of the waveform on the left. edge of the waveform on the right.
Refer to Figure 2. The pulse width is the width of either the positive or negative alternation
of the waveform expressed in time. The leading and trailing edges of the waveform require a certain amount of time for the
voltage to go from zero to maximum, or maximum to zero.
The time that it takes for the voltage to change is called the transient interval. The transient interval that occurs when the voltage
increases is called the rise time. The accurate definition of rise time is the time required for the voltage to increase from 10% to
90% of its maximum value. The transient interval that occurs when the voltage decreases is called the fall time. The definition of
fall time is the time required for the voltage to decrease from 90% to 10% of its maximum value.
Another square wave measurement is the frequency of the waveform, which is known as the Pulse Repetition Frequency (PRF),
or also as the Pulse Repetition Rate (PRR). The PRR or PRF is the number of complete cycles of the waveform occurring in one
second.
The formulas for calculating the Pulse Repetition Time (PRT) and Pulse Repetition Frequency (PRF) are:
PRF =
1
1
and PRT =
PRT
PRF
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Astable Multivibrators
Example 1: PRT and PRF Calculations.
Here are some examples to show you how to calculate PRT and PRF.
1.
If each alternation of the square wave in Figure 3 is 50 uS, the time for two alternations or 1 PRT is 100 uS. To calculate
the PRF:
First, write the formula:
PRF =
1
PRT
Now, substitute known values:
PRF =
1
100uS
Divide:
PRF = 10 Khz
Therefore, the square wave has a frequency of 10 kHz.
2.
If each alternation of the square wave in Figure 4 is 60 mS, the time for two alternations or 1 PRT is 120 mS. To
calculate the PRF:
First, write the formula:
PRF =
1
PRT
Now, substitute known values:
PRF =
1
120mS
Divide:
PRF = 8.33hz
Therefore, the square wave has a frequency of 8.33 Hz.
3.
If the frequency of the square wave in Figure 5 is 2,500 Hz, what is the time for one cycle, or 1 PRT?
First, write the formula:
PRT =
1
PRT
Now, substitute known values:
PRT =
1
2500 Hz
Divide:
PRT = 400uS
Therefore, the time for one complete cycle of the square wave is 400 uS.
4.
If the square wave in Figure 6 has a PRF of 100 kHz, what is the time for one cycle, or one PRT?
First, write the formula:
PRT =
1
PRT
Now, substitute known values:
PRT =
1
100 KHz
Divide: PRT = 10uS
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Astable Multivibrators
Therefore, the time for one complete cycle of the square wave is 10 uS.
Exercise 1: PRT and PRF Calculations.
Write the correct response in the blank space provided.
1. If a square wave has a PRT of 75 US, what is the PRF?
_____________________
2. If a square wave has a PRF of 200 kHz, what is the PRT? _____________________
3. If a square wave has a PRT of 275 US, what is the PRF?
_____________________
4. If a square wave has a PRF of 80 Hz, what is the PRT?
_____________________
SQUARE WAVE GENERATION
The most common square wave generator used in electronics is the multivibrator.
DEFINITION
Multivibrator - A multivibrator is two amplifier circuits cross coupled with regenerative feedback.
There are three types of multivibrator circuits. They are the astable, monostable, and bistable multivibrators. Multivibrators are
classified according to the number of stable states they have. A stable state is defined as constant, or unchanging, circuit
operation. A stable state in a multivibrator is when one transistor is saturated and the other is cut off. The discussions in this
lesson are limited to the astable multivibrator.
ASTABLE MULTIVIBRATOR
The astable multivibrator is depicted in Figure 7. The astable does not have any stable states. What that means is that one
transistor amplifier is saturated while the other is cut off. After a predetermined period of time, the amplifier that was saturated
goes into cut-off, and the cut-off amplifier goes into saturation. As long as power is applied to the circuit, the two amplifiers
change states, or flip back and forth.
Another name for the astable multivibrator is the free-running multivibrator. As soon as
power is applied, the circuit begins operation. The two outputs, Q and Q , alternate between saturation (low output) and cut-off
(high output). While one amplifier is saturated, the other one is cut off. The two outputs will always be opposite in a normally
running circuit. If at any time they are both the same, either cut off or saturated, the circuit is defective.
CIRCUIT OPERATION
Looking at Figure 7, you can see that both amplifier circuits are identical. One circuit consists of Ql , Cl, Rl, and R2. The other
circuit consists of Q2, C2, R3, and R4.
When the circuit is first energized, one transistor will conduct before the other. That is because one circuit will offer a little less
resistance than the other. Let's assume that Ql goes into conduction first. When power is initially applied to the circuit, Cl
charges, positive on the left, negative on the right. Ql's conduction will cause its collector to be near zero volts. That will cause Cl
to begin discharging. Cl 's discharge path is from ground, through Ql, Cl to R2, to Vcc. At the same time, C2 begins to charge.
C2's charge path is through Ql's forward biased emitter-base to C2, through R4, and back to Vcc. C2 charges positive on the right,
negative on the left. C 1 's discharging (and charging towards + Vcc) causes the bias on the base of Q2 to increase in the positive
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Astable Multivibrators
direction. As soon as Q2's base becomes positive enough (around +.6 VDC), Q2 goes from cut-off to saturation. As Q2 is
saturated, the low voltage on its collector is fed back to Ql 's base, driving it into cut-off. The two transistors have changed states.
Figure 8 depicts the charge and discharge paths.
Figure 8. Astable Multivibrator, Ql Saturated, Q2 Cut Off Charge Path for C2, Discharge for Cl
As soon as Q2 goes into saturation, C2 begins to discharge. C2's discharge path is from ground, through the saturated Q2, to C2,
and through R3 to Vcc. At the same time, as C2 is discharging, Q2's saturated conduction is charging Cl , negative on the right
and positive on the left. Cl 's charge path is from ground, through Q2's emitter-base to Cl , through Rl, and back to Vcc. C2's
discharge increases the positive bias on the base of Ql . As soon as the positive bias increases enough (around +.6 V), Ql goes
into conduction, cutting off Q2. Once again, the transistors have changed states. Figure 9 illustrates the discharge paths.
+Vcc
Figure 9. Astable Multivibrator, 01 Cut Off, Q2 Saturated Charge Path for Cl, Discharge Path for C2
As long as power is applied to the circuit, the transistors continue to change states. The amount of time that each transistor
remains cut off is determined by the selection of circuit components. The RC time constant of Cl and R2 determines the length of
time that Q2 is cut off . Ql 's cut-off time is determined by the RC time constant of C2 and R3. The larger the RC time constant
of the resistor and capacitors, the larger the PRT of the square wave output. If the PRT is larger, the PRF is smaller.
The astable multivibrator circuit can provide two outputs. Outputs are taken from the collector of Ql and Q2. Depending upon
the requirements of the equipment, one or both outputs may be used.
While all of the illustrations in the lesson have depicted multivibrators using NPN transistors, PNP's may also be used. The only
requirement is that NPN transistors use positive voltages and PNP use negative voltages.
Although the astable multivibrator does not require a trigger, it can be configured to use one. The addition of a trigger improves
the stability of the circuit. Also, the trigger allows the circuit to be synchronized. When a circuit is synchronized, its output is
matched to the rest of the equipment with respect to time. Figure 10 illustrates an astable multivibrator with external triggering.
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Astable Multivibrators
For synchronization, the input triggers must have a higher frequency than the natural PRF of the astable multivibrator. That way,
the input triggers will control when the transistors change states, rather than the RC time constant of the circuit.
SUMMARY
In this lesson, you learned about square waves and square wave generation. Here are some important points to remember.
• A square wave is a waveform with two alternations that are equal in time and square in appearance.
• The time to complete one cycle of a waveform is called the Pulse Repetition Time (PRT).
• The number of complete cycles of a waveform that occurs in one second is called the Pulse Repetition Rate (PRR) or
the Pulse Repetition Frequency (PRF).
• The pulse width is the time duration of either the positive or negative alternation. The leading edge is the left side of the
pulse.
• The trailing edge is the right side of the pulse.
• The astable multivibrator:
o Can also be called a free running multivibrator.
o Has no stable states.
o Has two outputs that must be opposite.
o Has two transistors that alternately switch from cut-off to saturation. The time it takes the transistors to switch
states is determined by the charge and discharge time of the capacitors.
o Does not require an external trigger. - Can be triggered for stability.
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