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Logic Families
INTRODUCTION
ICs are integrated using following integration techniques•SSI < 12 gates/chip•MSI < 100 gates/chip•LSI …1K gates/chip•VLSI …10K gates/chip•ULSI …100K gates/chip
Logic Family
It is a group of compatible ICs with the same logic levels and the supply voltages for performing various logic functions
They are the building block of logic circuits.
Types of logic families
BIPOLAR ICs
The main element of a bipolar ICs are Resistors, Diodes, Capacitors and Transistors.
They can be operated in two ways:SaturatedNon-Saturated
Saturated Logic: The transistors in the IC are driven to saturation
Non-Saturated Logic: The transistors in the IC are not driven to saturation.
BIPOLAR ICsSaturated Bipolar families are:
Resistor-Transistor logic (RTL),Direct-Coupled transistor logic (DCTL),Integrated-injection logic (IIL),Diode-transistor logic (DTL),High-Threshold logic (HTL) andTransistor-transistor logic (TTL)
Non-Saturated Bipolar families are:Schottkey TTLEmitter-Coupled logic (ECL)
UNIPOLAR LOGIC FAMILIES
MOS devices are unipolar devices and only MOSFETs are employed in MOS logic circuits.
These families are:PMOS (p-channel MOSFETs)NMOS (n-channel MOSFETs)CMOS (Both p- and n- channel MOSFETs are
fabricated on same silicon chip)
Basic Characteristics of ICsPropagation delayPower dissipationFan in and fan outNoise immunityPower supply requirementFigure of merits i.e. speed power productOperating temperatureCurrent and voltage parameters
Power supply requirement
CMOS and TTL are available in different supply voltage categories
In each IC, Vcc pin is connected to positive supply and GND pin is connected to ground of supply.
Current and voltage parameters
Four different kind of Logic level specifications are defined: VIL, VIH, VOL, VOH
VIL, VIH : These are the input logic levels (Low & High)
VOL, VOH : These are the output logic levels (Low & High)
Noise Immunity
Noise is unwanted voltage that is induced in electrical circuits and can cause threat to proper operation of circuit.
Noise immunity is the ability to tolerate a certain amount of unwanted voltage fluctuations on its inputs without changing outputs
Noise MarginA quantitative measure of noise immunity is
called Noise margin. It is expressed in volts.Two Noise margins are specified for logic circuits,
High level Noise margin (VNH) and Low level Noise margin (VNL), expressed as:
Power Dissipation
This is the amount of power dissipated in an IC.
It is Determined by the current Icc, that it draws from the Vcc supply, and is given by , Pd = Vcc X Icc.
Propagation Delay
Fan in and Fan outFan-in:
Number of inputs a gate has. For example, a two input gate will have fan-in equal to 2
Fan-out:
Maximum number of inputs of the same IC family that a gate can drive without falling outside the specified output voltage limits.
Figure of merits/speed power product
This is obtained by multiplying gate propagation delay by gate power dissipation
Operating temperatureFor consumer application , the temperature is
0 to +70°CFor industrial application , the temperature is
-55°C to +125 °C
Transistor as a switch
Transistor in cut off
Transistor as a switch
Transistor in saturation
TTL(Transistor Transistor logic)
Diode equivalent of TTL with single emitter transistor
Diode equivalent of a TTL multiple emitter Transistor
TTL Inverter
Basic integrated circuit TTL NAND gate
Basic TTLNANDNAND gate.
CMOS-Complementary metal oxide semiconductor
most widely used family for large-scale devices
combines high speed with low power consumption
usually operates from a single supply of 5 – 15 V
excellent noise immunity of about 30% of supply voltage
CMOS inverterA Q1 Q2 O/P
0V(Logic 0)
OFF ON VDD
(Logic 1)
VDD
(Logic 1) ON OFF 0V
(Logic 0)
CMOS NAND gateInput Transistor O/
P
A B Q1 Q2 Q3 Q4 Y
0 0 ON ON OFF OFF 1
0 1 ON OFF OFF ON 1
1 0 OFF ON ON OFF 1
1 1 OFF OFF ON ON 0
CMOS AND gate
CMOS NOR gate
CMOS OR gate