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Chapter 4
Logic Families
Chapter Objectives
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Understand the basics of digital Integrated
Circuits (IC)
Understand the different types of transistor
logic
Understand the concept of Metal-oxide
Semiconductor (MOS) digital ICs
Understand the different types of BiCMOS
logic circuits
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Digital Integrated Circuits
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Digital ICs operate with binary signals and are
constructed with ICs.
Characteristics of digital ICs are:
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Speed of operation
Fan-in
Noise immunity
Digital ICs can be classified into two categories:
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MOS family: It includes the following Metal-oxide
Semiconductor Field-effect Transistor (MOSFETs):
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p-channel MOSFET (PMOS)
n-channel MOSFET (NMOS)
Complementary MOSFET (CMOS)
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Digital Integrated Circuits (Contd..)
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Bipolar logic families: These ICs can be classified as
saturated and non-saturated logic families.
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Saturated logic families consists of:
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Resistor-transistor Logic (RTL)
Resistor-capacitor Transistor Logic (RCTL)
Diode-transistor Logic (DTL)
High Threshold Logic (HTL)
Transistor-transistor Logic (TTL)
Integrated-injection Logic (I2L)
Non-saturated logic families consists of:
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Schottky TTL
Emitter-coupled Logic (ECL)
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Current-sourcing and Currentsinking
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The logic families are categorized on the basis of
current flow from the output of one logic circuit to
the input of another.
If the output of a TTL gate is HIGH, a reverse emitter
current of 40 mA flows from the driver gate transistor
to the load gate transistor. Here, the driver gate
transistor is known as current source.
If the output of the TTL gate is LOW, an emitter
current of 1.6 mA flows from the load gate transistor
to the driver gate transistor. The driver gate transistor
is known as current sink.
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Resistor-transistor Logic
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RTL NOR gate consists of resistors and transistors.
If the inputs of RTL are LOW, then the transistors are
turned OFF. Therefore, the output of the circuit is at
logic 1.
If any of the inputs are at logic 1, then the transistors
are turned ON. Thus, the output of the circuit is at
logic 0.
Characteristics of RTL family are:
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Operation speed is low and average power of dissipation is
high.
Highly sensitive to temperature but poor immunity to noise.
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Resistor-capacitor-transistor Logic
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RCTL circuit consists of a capacitor with an
input resistor to increase the speed and
improve the immunity to noise.
During the transient phase, the resistor is
bypassed by the capacitor. Thus, the base
current increases and the input capacitance is
discharged quickly.
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Diode-transistor Logic
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DTL family removes the problem of output voltage
by increasing the load on the circuit.
If the inputs to the diodes are HIGH, they are reversebiased. If both the diodes and the transistors are
switched ON, then the output is LOW.
If any of the inputs to the diodes is LOW, the current
flows through the other diode and the voltage to that
diode drops down. Then, the base voltage becomes
LOW and the transistor remains at logic 0 and the
output is HIGH.
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High Threshold Logic
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HTL NAND gate is constructed by replacing
the second diode from the DTL NAND gate
with the Zener diode and increasing the power
supply.
Resistor values of this gate are increased to
obtain equal amount of current from DTL and
HTL gates.
These gates are useful in environments where
the noise level is high.
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Transistor-transistor Logic
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The 54/74 series of TTL family is classified
into 5 divisions:
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Standard (SN 54/74)
High-speed (SN54H/74H)
Low-power (SN54L/74L)
Schottky-diode-clamped (SN54S/74S)
Low power schottky (SN54LS/74LS)
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Transistor-transistor Logic (Contd..)
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Common characteristics are:
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Supply voltage for all the divisions is 5.0 V.
Logical 0 output voltage is 0V to 0.4V.
Logical 1 output voltage is 2.4V to 5V.
Logical 0 input voltage is 0V to 0.8V.
Logical 1 input voltage is 2V to 5V.
Noise immunity is 0.4 V.
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Integrated-injection Logic
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I2L is also called Merged Transistor Logic
(MTL).
It uses two types of bipolar junction to form a
large number of IC gate on the chip. Types of
bipolar junction are:
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n-p-n
p-n-p
It dissipates less power even if it is operated at
high speed.
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Emitter-coupled Logic
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ECL is a Current-mode Logic (CML).
It reduces the saturated transistor delay by
operating in active mode.
The differential amplifier, which is the basic
circuit of ECL, draws the constant current
during the transition of circuit from one state
to another.
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MOS Digital Integrated Circuits
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MOS technology is derived from the basic
MOS structure of metal electrode on the oxide
insulator over a semiconductor substrate.
They are commonly used in digital devices
such as logic gates and registers.
They can accommodate more number of
circuit elements than bipolar ICs.
Operating speed of MOS IC is less than
bipolar ICs.
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MOS Digital Integrated Circuits
(Contd..)
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Characteristics of MOS logic are:
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Propagation delay is 50ns.
Fan-out capacity of is unlimited.
Draw less power because of large resistance.
Simplest as they use only one basic element,
NMOS transistor.
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Complementary MOS Logic
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They are also called COSMOS or CMOS and are
made using PMOS and NMOS transistors.
Characteristics of CMOS are:
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Power consumption is very low.
Propagation delay ranges from 25ns to 150ns that depends
upon the power supply.
CMOS can operate at voltage range of +3V to +15V.
Fan-out of CMOS is 10.
They have small power dissipation and improved noise
immunity.
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BiCMOS Logic Circuits
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BiCMOS is used for developing low voltage analog
circuits, Very Large Scale Integration (VLSI) circuits
and Application Specific Integrated Circuits (ASIC).
Basic BiCMOS inverter circuit is formed from the
complementary pairs of PMOSFET and NPMOSFET
with NPN transistors.
The switching speed of basic inverter can be
improved by discharging the excess carriers from the
transistors with additional NMOS devices.
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Compatibility or Interfacing
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The output of the circuit should match with the inputs
of other circuits. This is referred to as compatibility.
An interface circuit is the one, which is connected
between the drivers and the loads.
The designer of the circuit should take care while
matching the characteristics of current and voltage of
two circuits of two different families.
An interface circuit between the two families is
required to match the output characteristics of the
driver and the load.
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