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connected to ground to ensure that it cannot reset the bistable circuit. The
discharge terminal (pin 7) is not used.
Trigger
1/3(V
Reset
1/3(V
V
+V
cc
cc
out
cc
)
0
t
)
0
t
t
pulse
t
Trigger
R1
Reset
R2
7
4
6
2
8
Supply
Discharge
Reset
Threshold
Trigger
GND
1
+V
cc
Output
Control
V
out
3
5
C2
0
Bistable
triggered
Bistable
reset
Fig. 9.43. Bistable 555 timer (Flip-Flop)
555 Timer Output
We could not finish this 555 Timer description without discussing
something about the switching and drive capabilities of the 555 timer or
indeed the dual 556 Timer IC. The output (pin 3) of the 555 timer, has the
ability to either ‘Sink’ or ‘Source’ a load current up to 200 mA, which is
sufficient to directly drive output transducers such as relays, LEDs, or
speakers, etc. The ability to both ‘Sink’ (absorb) and ‘Source’ (supply) means
that the output device can be connected between the output terminal of the
555 timer and the supply to sink the load current or between the output
terminal and ground to source the load current. For example,
V
+
cc
84
7
6
2
LED
Supply
Discharge
Threshold
Trigger
GND
‘ON’ when pin 3
is LOW (Sinking)
V
+
cc
R1
84
Reset
Output
Control
51
3
VD
Current
Sink
7
Discharge
6
Threshold
2
Trigger
Supply
GND
Reset
Output
Control
51
LED ‘ON’ when pin 3
is HIGH (Sourcing)
Fig. 9.44. Sinking and sourcing the 555 timer
131
Source
Current
3
VD
R1
In the first circuit above, the LED is connected between the positive
supply rail (+V
) and the output pin 3. This means that the current is ‘Sink’
CC
(absorb) or flows into the 555 timer output terminal and the LED is ‘ON’
when the output is ‘LOW’. The second circuit above shows that the LED is
connected between the output (pin 3) and ground (0 V). This means that the
current is ‘Source’ (supply) or flows out of the 555 timer’s output terminal
and the LED is ‘ON’ when the output is ‘HIGH’. Resistor R
is used to limit
1
the LED current to below 20 mA.
The ability of the 555 timer to both sink and source its output load
current means that both LEDs can be connected to the output terminal at the
same time but only one is switched ‘ON’ depending on whether the output
state is ‘HIGH’ or ‘LOW’.
We said earlier that the maximum output current to either sink or source
the load current via pin 3 is about 200 mA and this value is more than enough
to drive or switch other logic ICs, LEDs or small lamps, etc. If we wanted to
switch or control higher power devices such as motors, electromagnets or
loudspeakers, we would need to use a transistor to amplify the 555 timers
output in order to provide a current that is high enough to drive the load.
Thus, we have discussed the use of the 555 Timer for generating
monostable and bistable output pulses. Below we will discuss the application
of the 555 as an astable multivibrator. When used in the astable mode both
the frequency and the duty cycle of the output waveform can be accurately
controlled to produce a very versatile waveform generator.
555 Oscillator
The 555 Oscillator is another type of the relaxation oscillator for
generating stabilized square wave output waveforms of either a fixed
frequency of up to 500 kHz or of varying duty cycles from 50 to 100 %. In
the previous 555 Timer section we saw that the Monostable circuit produces
a single output one-shot pulse when triggered on its pin 2 trigger input. In
order to get the 555 Oscillator to operate as an astable multivibrator, it is
necessary to continuously re-trigger the 555 IC after each and every timing
cycle. This is basically achieved by connecting the trigger input (pin 2) and
the threshold input (pin 6) together, thereby allowing the device to act as an
astable oscillator. Then the 555 Oscillator has no stable states as it
continuously switches from one state to the other. Also the single timing
resistor of the previous monostable multivibrator circuit has been split into
two separate resistors, R
and R2, with their junction connected to the
1
discharge input (pin 7) as shown below.
132
R
R
+V
R1
R2
C1
7
Discharge
6
Threshold
2
Trigger
cc
84
Supply
GND
1
Reset
Output
Control
2/3(V
1/3(V
V
3
5
out
C2
V
V
+V
C1
cc
cc
0
out
cc
+V
cc
)
)
0
T
t
t
1
2t1
t
2
t
0
t
Fig. 9.45. Astable 555 oscillator
In the 555 Oscillator above, pin 2 and pin 6 are connected together
allowing the circuit to re-trigger itself on each and every cycle. This allows
the circuit to operate as a free running oscillator. During each cycle the
capacitor, C charges up through both timing resistors, R
discharges itself only through resistor, R
as the other side of R2 is connected
2
to the discharge terminal, pin 7. Then the capacitor charges up to 2/3V
upper comparator limit) which is determined by the
combination and discharges itself down to 1/3V
limit) determined by the
0.693
C
combination. This results in an output
2
(the lower comparator
CC
waveform whose voltage level is approximately equal to V
and R2 but
1
CC
0.693
12
–1.5V and
CC
(the
RC
whose output ‘ON’ and ‘OFF’ time periods are determined by the capacitor
and resistors combinations. The individual times required to complete one
charge and discharge cycle of the output are therefore given as:
Astable 555 Oscillator Charge and Discharge Times
0.693tRRC
112
and
0.693tRC
22
,
where R is in ’s and C is in Farads.
When connected as an astable multivibrator, the output from the
555 Oscillator will continue indefinitely charging and discharging between
2/3V
and 1/3VCC until the power supply is removed. As with the
CC
monostable multivibrator these charge and discharge times and therefore the
frequency are independent of the supply voltage. The duration of one full
cycle is therefore equal to the sum of the two individual times that the
capacitor charges and discharges and is given as:
555 Oscillator Cycle Time
12 1 2
0.693 2Tt t R RC
133
.
The output frequency of oscillations can be found by inverting the
equation above for the total cycle time giving a final equation for the output
frequency of an Astable 555 Oscillator as:
555 Oscillator Frequency Equation
11.44
f
TRRC
2
12
.
By altering the time constant of just one of the RC combinations, the
Duty Cycle of the output waveform can be accurately set and is given as the
ratio of resistor R2 to resistor R1. The Duty Cycle for the 555 Oscillator,
which is the ratio of the ‘ON’ time divided by the ‘TOTAL’ time, is given
555 Oscillator Duty Cycle
by:
Duty Cycle
TtRR
ON
TT ttRR
ON OFF
112
12 1 2
.
2
The duty cycle has no units as it is a ratio, but it can be expressed as a
percentage (%). If both timing resistors, R
and R2, are equal the output duty
1
cycle is given as 2:1 or 33%.
EXAMPLE:
Astable 555 Oscillator is constructed, using the following
An
components, R
= 1 k, R2 = 2 k and capacitor C = 10 uF. Calculate the
1
output frequency from the 555 oscillator and the duty cycle of the output
waveform.
Solution:
t
– Charge ‘ON’ time is calculated as:
1
6
0.693 0.693 1000 2000 10 10 0.021s 21 mstRRC
112
t
– Discharge ‘OFF’ time is calculated as:
2
0.693 0.693 2000 10 10 0.014 s 14 mstRC
22
Total periodic time is calculated as:
12
21 ms 14 ms 35 msTt t
The output frequency, ƒ is therefore given as:
11
f
35 ms
T
Giving a duty cycle value of:
Duty Cycle
RR
12
2 1000 2 2000
RR
12
1000 2000
134
6
28.6 Hz
0.6or60%
As the timing capacitor, C charges through resistors R1 and R2 but
R
R
R
R
D
discharges only through resistor R
between 50 and 100 % by changing the value of resistor R
the value of R
the duty cycle increases towards 100% and by increasing R2
2
the duty cycle reduces towards 50 %. If resistor, R
resistor R
by the product
the output frequency of the 555 astable circuit is determined only
1
C
. The problem with this basic astable 555 oscillator
2
the output duty cycle can be varied
2
. By decreasing
2
is very large relative to
2
configuration is that the duty cycle is never below 50 % as the presence of
resistor R
shorter than the ‘OFF’ time as
prevents this. In other words we cannot make the ‘ON’ time
2
RC
12
is always greater than
C
2
. One
way to overcome this problem is to connect a bypassing diode in parallel
with resistor R
R1
VD1
R2
C1
as shown below.
2
V
+
cc
84
7
Discharge
6
Threshold
2
Trigger
Supply
GND
1
Reset
Output
Control
C1
V
2/3(V
1/3(V
cc
cc
)
)
0
V
3
5
out
V
+V
out
cc
T
t
C2
t
t
1
2
0
t
Fig. 9.46. Improved 555 oscillator duty cycle
By connecting this diode, VD1 between the trigger input and the
discharge input, the timing capacitor now charges up directly through the
resistor R
capacitor discharges through the resistor, R
of
112
is given as
Then, in order to generate a duty cycle of less than 50 %, the resistor R
be less than the resistor R
only, as the resistor R2 is effectively shorted out by the diode. The
1
. Now the previous charging time
2
0.693tRRC
0.693
is modified to consider this new charging circuit and
C
. The duty cycle is, therefore, given as
1
.
2
RRR
11 2
must
1
.
555 Oscillator Applications
We have previously mentioned that the maximum output to either the
sink or the source load current via pin 3 is about 200 mA and this value is
more than enough to drive or switch other logic ICs, a few LEDs or a small
lamp etc. and that we would need to use a bipolar transistor or MOSFET to
amplify the 555’s output to drive larger current loads such as motor or relays.
But the
555 Oscillator can be used in a wide range of waveform generator
135
circuits and applications that require very little output current. They can be
used in electronic test equipment for producing a whole range of different
output test frequencies from very accurate sine, square and pulse waveforms
or as LED or lamp flashers and dimmers to simple noise making circuits such
as metronomes, tone and sound effects generators.
Self-Assessment Questions
Applications of Operational Amplifiers
1. Draw the connection diagrams for an op-amp. How must we connect
the power supply networks to the op-amp?
What will be the output voltage polarity if the voltage across
2.
inverting input terminal is more than that of noninverting terminal?
Design an op-amp noninverting amplifier. Draw the input and output
3.
voltage waveforms. What is the voltage at the noninverting input of the opamp device? Derive an expression for voltage gain in terms of the feedback
resistances.
Design an op-amp inverting amplifier. Draw the input and output
4.
voltage waveforms. Derive an expression for voltage gain.
Draw the circuit of the voltage follower. Why is this circuit used as a
5.
buffer?
Draw the amplitude curve if the voltage gain is equal to: a) 1; b) 7.
6.
What does the term ‘saturation voltage’ mean when applied to an op-amp?
Draw the frequency response of an op-amp amplifier. Mark the
7.
bandwidth. Explain the difference between the cut-off frequency and the
unity-gain frequency.
Draw the load characteristic of the op-amp amplifier. How would
8.
you measure the output resistance of an amplifier?
Design the two-input inverting adder. Derive an expression for the
9.
output voltage in terms of the feedback resistances and input voltage.
What does the term ‘weighting’ mean when applied to an adder?
10.
Design a three-input inverting adder. Choose components to get the
11.
output voltage
out
if R1 = 33 k. Suggest a suitable
321
103 VVVV
value of the resistor that is series connected to the op-amp noninverting input.
Sinusoidal Oscillators
1. What conditions for oscillation do you know?
Design the Wien bridge network. Draw the amplitude-frequency and
2.
phase-frequency responses.
Write the expression for the centre frequency of the Wien bridge
3.
network. Draw the amplitude curve at the centre frequency.
136
4. Design an op-amp Wien bridge oscillator. Draw the input and output
voltage waveforms. Explain the principle of operation.
How are the conditions for oscillation fulfilled in the Wien bridge
5.
oscillator?
What is the original signal source of the oscillation?
6.
How can the output frequency of oscillator be varied?
7.
8.
Resistor R equals 10 k in the Wien bridge oscillator. Find
capacitor C to obtain oscillating frequency of 2 kHz.
Rectangular Waveform Generators
1. Design the op-amp astable multivibrator. Draw the inputs and output
voltage waveforms marking the value of the voltages of the waveform
extremities. Explain the principle of operation.
Draw the hysteresis curve for the op-amp astable multivibrator.
2.
What does the hysteresis loop width depend on?
Derive an expression for pulse durations and frequency of the output
3.
waveform of the astable M/V circuit.
What does the term ‘duty cycle’ mean? Illustrate it by using
4.
waveform.
How can the output frequency of multivibrator be varied?
5.
Design the asymmetrical op-amp M/V. Draw the input voltage and
6.
output voltage waveforms marking the value of the voltages of the waveform
extremities. Explain the principle of operation.
Derive an expression for the pulse durations and waveform
7.
frequency of the asymmetrical op-amp M/V.
Calculate all components of the op-amp astable M/V circuit if
8.
f
= 5 kHz, Vcc = 12 V.
0
Design the 555 timer astable multivibrator. Draw the capacitor
9.
voltage and output voltage waveforms marking the value of the voltages of
the waveform extremities. Explain the principle of operation.
Design the monostable multivibrator circuit using the 555 timer.
10.
Draw the input and output voltage waveforms marking the value of the
voltages. Explain the principle of operation.
137
SUMMARY
The aim of this textbook is to develop a clear understanding of the
principles of circuit design. Basic electrical concepts are briefly considered
before taking the subject further. This textbook is for a university course in
electrical circuits and analogue electronics.
In first three chapters, signals and basic principles of d.c. and a.c. circuit
analysis are considered. The following five chapters describe the properties
of diodes, transistors and thyristors and cover their different applications. The
final chapter describes the basic principles of analogue integrated circuits,
including operational amplifiers and the 555 timer. Nine chapters of this
textbook help the student get basic understanding of the principles of
analogue electronics.
The material is structured so that it will be supportive to the student.
Each chapter has clear objectives and the text contains worked examples.
Necessary mathematics is introduced when needed. One can use the
computer and many versions of circuit simulation software that help
understand how a circuit works.
In this textbook we have tried to make it easy for you to find the
answers you are looking for. We really hope that you will find this textbook
useful and we want to help you open the door to the world of electronics.
138
BIBLIOGRAPHY
1. Analog and digital electronics : a first course / Beards H. Peter. –
nd
revised ed. – London : Prentice Hall Europe, 1996. – 646 p.
2
A practical introduction to electronic circuits / Jones H. Martin. –
2.
rd
ed. – Cambridge : Cambridge University Press, 1995. – 548 p.
3
An introduction to operational amplifiers with linear IC applications /
3.
Luces M. Faulkenberry. – 2
The essence of Analog Electronics / C. Lunn. – London : Prentice
4.
Hall, 1997. – 325 p. : il.
The art of electronics / Paul Horowitz, Winfield Hill. – 2
5.
Cambridge : Cambridge University Press, 1989. – 1105 p.
Electronics: a complete course / Nigel P. Cook. – 2
6.
Prentice Hall, 2004. – 1037 p.
Electronics / D.I. Crecraft, D.A. Gorham. – 2
7.
Nelson Thornes in association with the Open University, 2003. – 428 p.
Electronics: a system approach / Neil Storey. – 3
8.
Prentice Hall, 2006. – 645 p.
Thyristor [Online] / Wikipedia, The free encyclopedia, 2019. –
9.
URL:
https://en.wikipedia.org/wiki/Thyristor#Types.
555 Timer Tutorial [Online] / Electronics Tutorials, AspenCore Inc.,
10.
2019. – URL: https://www.electronics-tutorials.ws/waveforms/555_timer.html.
nd
ed. – New York : Wiley, 1982. – 530 p. : il.
nd
ed. – London :
nd
ed. – London :
rd
ed. – London :
nd
ed. –
139
APPENDIX A
H
Symbols, SI units and Abbreviations
Symbol Representing Unit (SI)
A
A, AV
Area square meter
Voltage gain (open loop) of an amplifier dimensionless
a.c. Adjective applied to alternating voltages and
currents
B
b, B
BW
C
c, C
CMRR
d
D
d.c.
Magnetic flux density tesla
Base of a bipolar transistor
Bandwidth hertz
Capacitance farad
Collector of a bipolar transistor
Common-mode rejection ratio dimensionless
Distance meter
Drain of a MOSFET or JFET
Adjective applied to non-alternating voltages
and currents (direct current)
e, E
e, E
f
G
G
G
H
I
Electromotive force (e.m.f.), e.m.f. source volt
Emitter of a bipolar transistor
Frequency hertz
Closed-loop gain of feedback amplifier dimensionless
Conductance; reciprocal of resistance siemens
Gate of a MOSFET or JFET
Strength of magnetic field ampere/second
Current; d.c. value; or amplitude or r.m.s.
value of a.c. current
IB
IF
I
IS
i
L
n
Np, Ns
P
p
Input bias current of an amplifier ampere
Forward current for a diode ampere
Holding current of a thyristor ampere
Diode saturation current ampere
Magnetizing current ampere
Inductance henry
N-region of a semiconductor
Primary, secondary turns in a transformer dimensionless
Power watt
P-region of a semiconductor
or oersted
ampere
140
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