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Logic Families
Logic Families
• A logic family refer to a set of techniques used to
implement logic within very large scale integrated
circuits.
• These families may vary by speed, power consumption,
cost, voltage and current levels.
• The digital integrated circuits are designed using bipolar
devices or Metal Oxide Semiconductor (MOS) or a
combination of both.
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Logic Families
TYPES OF LOGIC FAMILY
• The logic family which falls under the first kind is Bipolar
logic family and the other is Unipolar logic family.
vBIPOLAR LOGIC FAMILY:
• There are two kinds of operations in bipolar integrated
circuits: Saturated Bipolar Logic family and Non-saturated Bipolar
Logic family.
vUNIPOLAR LOGIC FAMILY:
• Unipolar logic family consists of Metal Oxide Semiconductor (MOS)
logic families.
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Saturated Bipolar Logic Families
• Bipolar Logic Families uses bipolar devices like diodes, transistors in
addition to passive elements like resistors and capacitors.
• In this family the transistors used in ICs are driven into saturation.
• Transistor-Transistor Logic (TTL)
• Resistor-Transistor Logic (RTL)
• Direct Coupled Transistor Logic (DCTL)
• Diode Transistor Logic (DTL)
• High Threshold Logic(HTL)
• Integrated Injection Logic (IIL or 𝐼 ! L)
Unsaturated bipolar logic family
• In this family the transistors used in IC is not driven into saturation.
• Schottky TTL
• Emitter Coupled Logic(ECL)
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Unipolar Logic Families
• It mainly uses Unipolar devices like MOSFETs in addition
to passive elements like resistors and capacitors.
• These logic families have the advantages of high speed and
lower power consumption than Bipolar families.
• PMOS or P-Channel MOS Logic Family
• NMOS or N-Channel MOS Logic Family
• CMOS Logic Family
FEATURES OF LOGIC FAMILIES
• TTL - Transistor-Transistor Logic: Standard logic family; used for the
longest time.
• ECL - Emitter Coupled Logic: Suitable for systems requiring high-speed
operations.
• MOS - Metal Oxide Semiconductor Logic: Suitable for systems with
high component density.
• CMOS - Complementary Metal Oxide Semiconductor Logic: Suitable
for systems with low power consumption (VLSI circuits). Gradually
becomes the dominant logic family.
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Transistor-transistor logic:
• Transistor-transistor logic (TTL or T2L) integrated circuits were
introduced in the late 1960s.
• Today TTL continues to be a popular choice of integrated circuits.
• TTL is best represented by the SN 54/74 series of digital integrated
circuits.
• The SN 54 devices will operate over a range of temperatures from -55
to +125 °C, whereas the SN 74 devices operate over a range from 0 to
70 °C.
Transistor-transistor logic
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Resistor Transistor Logic
• In RTL (resistor transistor logic), all the logic are implemented using
resistors and transistors.
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Diode Transistor Logic
• In DTL (Diode transistor logic), all the logic is implemented using
diodes and transistors
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CMOS
• Complementary Metal Oxide Semiconductor.
• CMOS comprises of Pull-up and Pull-down Networks.
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CMOS
• Pull-up Network:
Øwhen the input is high, NMOS gets ON and pulls down the output to
ground.
• Pull-down Network:
Øwhen the input voltage is low, PMOS gets ON and pulls the
output up from the ground.
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CMOS Inverter
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CMOS NAND
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CMOS NOR
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Noise Margin
• The noise margin (immunity) of a digital circuit is the ability of that
circuit to maintain a logic state in the presence of noise.
• Where:
• NL - noise immunity of the digital circuit input when the input signal is LOW,
NH - noise immunity of the digital circuit input when the input signal is HIGH,
VILmax - maximum input voltage that can be read by the circuit as LOW,
VOLmax - maximum output voltage that can represent LOW,
VOHmin - minimum output voltage than can represent HIGH,
VIHmin - minimum input voltage that can be read by the circuit as HIGH.
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Fan in and Fan out
• Fan-in refers to the number of inputs in a digital logic
gate family.
• For the example given in the figure below, the EX-OR
gate has three inputs. So fan-in for the given EX-OR
gate is 3.
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Fan in and Fan out
• Fan-out refers to the number of inputs that is driven
by the output of another logic gates.
• For example, the following circuit has an EX-OR gate,
which drives 4 NOT gates. So fan-out of EX-OR gate is
4.
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