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Digital Electronic Circuits
Lecture 01
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Electronic Logic Values
01
Digital circuits contain many elements.
The inputs and outputs of the elements are
connected together.
The connections are nodes in the circuits.
The nodes are always in a state that is
described as being either 1 or 0.
Electronic Digital Circuits use two defined
voltages to represent the logic values.
One possible choice is to define the
terminals of a battery as being the logic
values.A node in the circuit will have a logic 1 if
it is somehow connected to the +veterminal
of the battery.
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Basic Switching
The node on the right is always a 1 or a 0 except while the switch is being
changed. We shall consider this change to be instantaneous.
The switch changes from one position to the other because of a mechanical input.
The switch is actuated by the mechanical action of a human.
Digital circuits are constructed from many electronically actuated switches.
1
0
1
0
1
1
0
0
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Electronically Actuated Switches
The terminal that is drawn touching the bar is the actuatingterminal.
The switch is EITHER
open = not connecting = not conducting
OR
closed = connecting = conducting
We have TWO possible types of switches
TYPE 1: 1 actuates the switch to open and 0 actuates the switch to closed
TYPE 2: 0 actuates the switch to open and 1 actuates the switch to closed
Right
Left
TYPE 2 TYPE 1
We draw the symbol in the 0 actuated state.
Actuating terminal
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Electronic Relays
Many years ago digital circuits used Relays as electronically actuated switches.
A relay contains a single pair of contacts or sometimes a complicated arrangement
of contacts.
There is a coil of wire which becomes an electromagnet when a current is passed
through it. The magnet attracts the contacts pulling them into a position that is
different from the unmagnetised position.
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MOSFET (a)
np
SiO2 SiO2SiO2
SiO2
TOP VIEW
CROSS-SECTIONAL VIEW
Today we use transistors as electronically actuated switches.
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MOSFET (b)
PolySilicon Gate
n
+
p
+Ion Implantation
Contacts
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MOSFET (c)
Insulator
Metal
Each transistor has 3 terminals called the source, gate and drain
The gate is the actuating terminal.
MetallicInterconnect
nmos pmos
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MOSFET (d)
NMOSthe channel conducts when the gate is connected to a logic 1
PMOSthe channel conducts when the gate is connected to a logic 0
nmos
pmos
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CMOS Inverter
1
0
1 0
1
0
0 1
A pair of transistors, nmos and pmos can be connected to form an inverter.
input output
1
0
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CMOS NAND gate
input 1
output
1
0
input 2
input 1
output
1
0
input 2
1
1
0
input 1
output
1
0
input 2
1
0
1
input 1
output
1
0
input 2
0
1
1
Can you build a NOR gate?
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SSI Logic
30 years ago we would buy ICs with 2030 transistors per chip. We would
wire together the individual gates by connecting the leads of the packaged IC.
The silicon die is placed inside a package to protect
the circuit and also to allow us to connect chips
together using the package leads.
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IC packages
ICs today contain millions
of transistors that are
interconnected to form
very complicated digital
circuits.
Modern packages have
many leads to
externally connect to theI/O of the internal circuit.
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Board-Level Systems
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Motherboards
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PALs
We will continue our discussion next time.
