Chapter 2
Digital Electronic Signals and
Switches
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2-1 Digital Signals
• Timing diagram
– Voltage is measured on the y-axis; time on the
x-axis
• 0 and 1 correspond to 0 V and 5 V
• Can be viewed using an oscilloscope.
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2-2 Clock Waveform Timing
• Periodic waveform are
– Repetitive
– Have a specific time interval
– Successive pulses are identical
• Period – the time from one edge to the
corresponding edge on the next cycle
– tp = 1/f
• Frequency – reciprocal of waveform
period
– f = 1/tp
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Engineering Notation
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Solving for Period and Frequency
• Example 2-1 If a clock waveform has a
period of 2 µs, its frequency is
f = 1/tp = 1/2 µs = 500 kHz
• Example 2-2 A 4.17 MHz waveform has a
period of
tp = 1/f = 1/4.17 MHz = 240 ns
2-3 Serial Representation
• Uses a single electrical conductor
-> inexpensive
• Slow since only one bit for each clock period
– Used for telephone lines, computer-to-computer
networks
• Computer COM ports are most often used for
serial communications
• Ethernet plug-in cards are for serial
networking
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Figure 2.5 Serial communication
between computers.
Figure 2-6 Serial Representation of a Binary
Number
• A serial representation of the binary number
01101100 is illustrated below.
– Note that there is one bit per clock period and the
bit-to-bit change occurs on the falling edge.
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2- 4 Parallel Representation
• Separate electrical conductor for each bit
-> Expensive
• Very fast
• Inside a computer
– Data bus
• External Devices
– Centronics printer interface (LPT1)
– SCSI (Small Computer Systems Interface)
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Figure 2.7 Original parallel
communication between a computer
and a printer.
Figure 2.8 Parallel representation of
the binary number 01101100.
2-5 Switches in Electronic Circuits
• Make or break connections between
conductors
– Manual switches
– Electromechanical relays
• Semiconductor devices
– Diodes
– Transistors
• Manual Switches - ideal resistances:
– ON - 0 W
– OFF -  W
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Switches and Logic Levels
• A double-throw switch can be used to
produce a high (1) or a low (0) at an output.
– This circuit produces a high (1) at the output.
– This circuit produces a low (0) at the output.
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Figure 2.14 Physical representation of
an electromechanical relay
(a) normally closed (NC) relay;
(b) normally open (NO) relay;
(c) photograph of actual relays.
Figure 2.15 Symbolic representation
of an electromechanical relay
2.7 A Diode as a Switch
• A diode is a semiconductor device that allows
current to flow in one direction but not the other.
Figure 2.22 Diode in a series circuit: (a) forward biased and (b) reverse biased.
Forward biased: a diode whose Anode voltage is more positive than its Cathode.
Reversed biased: its anode voltage is equal to or more negative than its cathode.
Thus not allow current flow.
Figure 2.23
Water system check valve.
• A diode is analogous to a check valve in a
water system.
Figure 2.24 Diode voltage versus
current characteristic curve.
• A diode is not a perfect
short in the forwardbiased condition.
– No current flows until
Vforw reaches a 0.7v cutin voltage.
• As Vrev becomes more
negative, current flow
is still practically zero.
Figure 2.25 Forward-biased diode in
an electric circuit
(a)original circuit
(b)equivalent circuit showing the diode
voltage drop and Vout = 5 – 0.7 = 4.3 V.
V1 = 5-0.7=4.3v
V2 = 0v
V3 = 0.7v
V4 = 5v
V5 = 5-0.7=4.3v
V6 = 0.7v
2.8 A Transistor as a Switch
• Bipolar transistor is a very commonly used
switch in digital electronic circuits.
– A three-terminal semiconductor component
– An input signal at one terminal generates a short
or open between the other two terminals
• Transistor is made by bombarding pure
silicon into N-type and P-type materials
– N-type: having one more electron than silicon
– P-type: having one less electron than silicon
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2.8 A Transistor as a Switch
• Three distinct regions: base, emitter, and
collector
– Can be a combination of N-P-N-type or P-N-Ptype material bonded together
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NPN transistor
• Positive base-to-emitter voltage shorts the
transistor output (collector-to-emitter)
– Transistor is said to be ON
• Negative voltage (or 0 V) base-to-emitter
opens the transistor output
– Transistor is said to be OFF
Vbe > 0v
C
Vbe <= 0v
E
C
E
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Figure 2.28 NPN transistor switch: (a)
transistor ON and (b) transistor OFF.
Example 2.11
Example 2.11 Continued
Example 2.12
Example 2.12 Continued
PNP transistor
• Positive voltage (or 0 V) base-to-emitter
opens the transistor output (collector-toemitter)
– Transistor is said to be OFF
• Negative base-to-emitter voltage shorts the
transistor output
– Transistor is said to be ON
Vbe > 0v
C
Vbe <= 0v
E
C
E
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2.9 The TTL Integrated Circuit
• Transistor-Transistor Logic (TTL) is one of
the most widely used integrated circuit
technology
– A combination of transistors, diodes, and resistors
integrated in a single package.
• One basic function of a TTL IC is as a
complementing switch or inverter
– The output is the complement of the input.
(Converts 1s to 0s and vice-versa)
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Figure 2.35 Common-emitter
transistor circuit operating as an
inverter.
• Vin equals 1 (+5v), the
transistor is turned on
(saturation) and Vout
equals 0 (0v),
• Vin equals 0 (0v), the
transistor is turned off
(cutoff) and Vout
equals 1 (≈ 5v)
– Assume Rload >> Rc
Example 2-13
Let’s assume that Rc=1kΩ, RL=10kΩ, Vin=0v.
Vout =
5𝑣×10𝑘Ω
1𝑘Ω+10𝑘Ω
= 4.55𝑣
But if RL decreases to 1kΩ by adding more
loads in parallel with it, Vout will drop to 2.5v
Vout =
5𝑣×1𝑘Ω
1𝑘Ω+1𝑘Ω
= 2.5𝑣
Figure 2.36 Common-emitter
calculations.
When the transistor is
cut off, Rc is expected to
be small to ensure Vout
close to 5v.
When the transistor is
saturated, Rc is
expected to be large to
avoid excessive
collector current.
Need a variable Rc resistance
Figure 2.37 Schematic of a TTL
inverter circuit.
• Q4 replaces Rc, acts in opposite
to Q3.
– Such combination is called the
totem-pole arrangement.
• Q1 (input transistor) drives Q2.
• D1 protects Q1 from negative
voltages at input.
• D2 ensures that Q4 cuts off
totally when Q3 is saturated.
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A 7404 TTL IC chip is called 14 pin DIP(
Dual-in-line package)
Figure 2.40 Photograph of three
commonly used ICs: the 74HC00,
74ACT244, and 74150.
2-11 The CMOS IC
• Another common IC technology is the CMOS
(Complementary metal oxide semiconductor)
– Use a complementary pair of metal oxide
semiconductor field-effect transistors (MOSFETs)
– NOT bipolar transistors, which relies on the
contact of two types of semiconductor for its
operation, in TTL chips
– VS. TTL: Lower power consumption (commonly
used in battery-powered devices); slower
switching speed.
Figure 2.42 A 4049 CMOS hex inverter
pin configuration.
2-12 Surface-Mounted Devices
(SMD)
• Modern electronics depends on the ability to
design smaller, more dense components and
systems.
–
–
–
–
–
SMDs reduce the size of DIP-style logic by 70%
SMDs reduce the weight by 90%
7400 IC in the DIP style:19.23mm X 6.48mm
7400 SMD: 8.75mm X 6.20mm
SMDs significantly lower the manufacture cost
Popular SMD package styles
(a) small outline
(b) plastic leaded chip
carrier (PLCC)
c) ball grid array (BGA)
configuration
(d) photograph of actual SMDs
(e) photograph of SMDs mounted on
a printed-circuit board
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Summary
• The digital level for 1 is commonly
represented by a voltage of 5 V in digital
systems. A voltage of 0 V is used for the 0
level.
• An oscilloscope can be used to observe the
rapidly changing voltage-versus-time
waveform in digital systems.
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Summary
• The frequency of a clock waveform is equal
to the reciprocal of the waveform’s speed
• The transmission of binary data in the serial
format requires only a single conductor with a
ground reference. The parallel format
requires several conductors but is much
faster than serial.
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Summary
• The transistor is the basic building block of
the modern digital integrated circuit. It can
be switched on or off by applying the
appropriate voltage at its base connection.
• TTL vs. CMOS circuits (Complete
coverage in Chapter 9)
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