Download 1 Digital Logic: Elementary Building Blocks

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Digital Logic: Elementary Building Blocks
• The transistor was invented in 1949 at Bell Laboratories.
• It has revolutionized electrical engineering (and our society?).
• We will focus on using the transistor as an electrically controlled switch.
• We will consider the bipolar junction transistor (BJT) and the field
effect transistor (FET).
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1.1
What are transistors made of ?
• Transistors are made of chemical elements in Group IV in the periodic
table of elements.
• Most commonly Silicon is used; Germanium is less common.
• These elements form crystals in such a way that the four valence electrons of each atom are shared between neighbors.
• Then, at any time each atom has eight valence electrons on its outer
shell.
• This is a very stable state and there are no freely available electrons
for conducting a current.
• However, this perfect balance can be disturbed by replacing some of
the silicon atoms with atoms from Group V (phosphorus or arsenic) or
group III (boron).
• For example, if phosphorus is introduced an extra freely available electron per phosphorus atom is introduced.
• The resulting material is called n-type material.
• Similarly, by introducing Group III atoms, p-type material results.
• By placing p-type and n-type materials next to each other p-n junctions
are formed.
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1.2
P-N Junction: Diode
• If the p-type material is at a lower voltage potential than the n-type
material no current can flow through the diode.
• This condition is called reverse biased.
• On the other hand, if the p-region is at a voltage potential about 0.7 V
higher than the n-region, the diode becomes conducting.
• A significant current can flow through the diode.
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1.3
Bipolar Junction Transistor
• A BJT is a device with three terminals: collector, base, and emitter.
• In an npn-BJT, collector and emitter are made of n-type material and
the base is made of p-type material.
• If the base-emitter diode becomes conducting, then a significant current
can flow from collector to emitter.
• In other words, by adjusting the base-emitter voltage, the collectoremitter current can be controlled.
• This behaviour can be used to construct an amplifier.
• Also, it can be exploited to build an electrically controlled switch.
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1.4
Field Effect Transistor
• In switching applications, field effect transistors are preferred because
they consume less power.
• FET’s also have a NPN (or PNP) sequence of materials.
• However, the conductivity is controlled through the so-called gate that
is electrically insulated from the device itself.
• By adjusting the voltage between the gate and the substrate, the width
of a conducting channel between source and drain can be controlled.
• Functionally, the following correspondences between BJT and FET exist:
– base - gate
– source - collector
– drain - emitter
• The most common FET technology is called MOSFET for metal-oxidesemiconductor FET.
• In digital electronics, P-type and N-type MOSFETs are combined to
form CMOS (complementary MOS ) logic gates.
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CMOS Logic Gates
• CMOS (complementary metal-oxide-semiconductor ) technology is used
predominantly to create digital circuitry.
• The fundamental building blocks of CMOS circuits are P-type and Ntype MOSFET transistors.
• A P-type MOSFET can be modeled as a switch that is closed when
the input voltage is low (0 V) and open when the input voltage is high
(5 V).
• A N-type MOSFET can be modeled as a switch that is closed when
the input voltage is high (5 V) and open when the input voltage is low
(0 V).
• The basic idea for CMOS technology is to combine P-type and N-type
MOSFETs such that there is never a conducting path from the supply
voltage (5 V) to ground.
• As a consequence, CMOS circuits consume very little energy.
Figure 1: P-type and N-type MOSFETS.
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2.1
CMOS Inverter
• The circuit below is the simplest CMOS logic gate.
• When a low voltage (0 V) is applied at the input, the top transitor (Ptype) is conducting (switch closed) while the bottom transitor behaves
like an open circuit.
• Therefore, the supply voltage (5 V) appears at the output.
• Conversely, when a high voltage (5 V) is applied at the input, the
bottom transitor (N-type) is conducting (switch closed) while the top
transitor behaves like an open circuit.
• Hence, the ouput voltage is low (0 V).
• The function of this gate can be summarized by the following table:
Input
High
Low
Output
Low
High
• The output is the opposite of the input - this gate inverts the input.
• Notice that always one of the transistor will be an open circuit and no
current flows from the supply voltage to ground.
Figure 2: Inverter Circuit and Standard Symbol
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2.2
NAND Gate
• The circuit below has two inputs and one output.
• Whenever at least one of the inputs is low, the corresponding P-type
transistor will be conducting while the N-type transistor will be closed.
• Consequently, the ouput voltage will be high.
• Conversely, if both inputs are high, then both P-type transistors at the
top will be open circuits and both N-type transistors will be conducting.
• Hence, the output voltage is low.
• The function of this gate can be summarized by the following table:
V1
Low
Low
High
High
V2
Low
High
Low
High
Output
High
High
High
Low
• If logical 1’s are associated with high voltages then the function of this
gate is called NAND for negated AND.
• Again, there is never a conducting path from the supply voltage to
ground.
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Figure 3: NAND Circuit and Standard Symbol
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2.3
NOR Gate
• The circuit below has two inputs and one output.
• Whenever at least one of the inputs is high, the corresponding N-type
transistor will be closed while the P-type transistor will be open.
• Consequently, the ouput voltage will be low.
• Conversely, if both inputs are low, then both P-type transistors at the
top will be closed circuits and the N-type transistors will be open.
• Hence, the output voltage is high.
• The function of this gate can be summarized by the following table:
V1
Low
Low
High
High
V2
Low
High
Low
High
Output
High
Low
Low
Low
• If logical 1’s are associated with high voltages then the function of this
gate is called NOR for negated OR.
• Again, there is never a conducting path from the supply voltage to
ground.
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Figure 4: NOR Circuit and Standard Symbol
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2.4
Other Logic Functions
• Other logic functions can be realized from these three gates.
• For example, the AND function can be realized by
– combining a NAND gate followed by an inverter,
– using inverters at the input of a NOR gate.
• The AND function can be summarized by the following table:
V1
Low
Low
High
High
V2
Low
High
Low
High
Output
LOW
Low
Low
High
• Similarly, the OR function can be realized by
– combining a NOR gate followed by an inverter,
– using inverters at the input of a NAND gate.
Figure 5: AND gate constructed from standard gates
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2.5
Summary
• CMOS logic gates use P-type and N-type MOSFET.
• The logic gates in CMOS technology are very energy efficient since no
currents flow in the switched states.
• Design of logic circuits can be done with the standard gates introduced
here.
• That means, circuits do not need to be designed at the transistor level.
• It is possible to design circuits with very large numbers of logic gates
and realize them on a single chip.
• With modern tools, it is not very difficult to design custom chips based
on libraries of standard cells.
• Such custom circuits are called ASICs (application specific integrated
circuits).
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