Unit-5
Classification of Logic Families
Characteristics of Digital IC’s
Propagation Delay:
The propagation delay of a gate is basically the time interval between the application of an input pulse and the occurrence of the resulting
output pulse.
Very important characteristics of logic circuit because it limits the speed at which they can operate.
shorter the propagation delay, the higher the speed of the circuit and vice-versa
determined using two basic values
i. tP L H : It is the delay time measured when output is changing from logic 0 to logic 1 state (LOW TO HIGH)
ii. tP H L : It is the delay time measured when output is changing from logic 1 to logic 0 state (HIGH TO LOW)
When the two above are not equal, the larger value is considered as a propagation delay time for that logic gate, i.e.
tp = max(tP L H , tP H L )
OR tp = 12 (tP LH + tP H L )
Power Dissipation:
The amount of power that an IC dissipates is determined by the average supply current IC C , that it draws from the VC C supply. It
is product of IC C and VC C . For ICs, the value of IC C for a LOW gate output (IC C L ) is different from a HIGH output (IC C H ).
Unit-5 1
, Therefore, average IC C is determined based on the 50% duty cycle operation of the gate(LOW half of the time and HIGH half of
the time).
ICCH +ICCL
Ic c(av g ) = 2
This can be used to calculate average power dissipation as
PD (av g ) = IC C (av g ) x VC C
Current Voltage Parameter:
VIH (min) - High-Level Input Voltage:
It is the minimum voltage level required for a logical 1 at an input. Any voltage below this level will not be accepted as a HIGH by
the logic circuit.
VIL(max) - Low-Level Input Voltage:
It is the maximum voltage level required for a logical 0 at an input. Any voltage above this level will not be accepted as a LOW by
the logic circuit.
VOH (min) - High-Level Output Voltage:
It is the minimum voltage level at a logic circuit output in the logical 1 state under defined load conditions.
VOL(max) - Low-Level Output Voltage:
It is the maximum voltage level at a logic circuit output in the logical 0 state under defined load conditions.
IIH - High-Level Input Current:
It is the current that flows into an input when a specified high-level voltage is applied to that input.
IIL - Low-Level Input Current:
It is the current that flows into an input when a specified low-level voltage is applied to that input.
IOH -High-Level Output Current:
It is the current that flows from an output in the logical 1 state under specified load conditions.
IOL -Low-Level Output Current:
It is the current that flows from an output in the logical 0 state under specified load conditions.
Noise Margin and Logic Voltages Levels:
Each logic family has a different standard
Family Logic 0 Logic 1
TTL 0V +5V
ECL -1.7. V - 0.9 V
CMOS 0V 3 -15 V
Stray electric and magnetic field can induce voltages on the connecting wires between logic circuits. These unwanted signals are
called noise and can sometimes cause the voltage at the input to a logic circuit to drop below VIH (min) or rise above VIL(max) ,
which could produce unpredictable operations.
The noise immunity of at a logic circuit refers to the circuit’s ability to tolerate the noise without causing spurious changes in the
output voltage. To avoid this problem due to noise, voltage level VIH (min) is kept at a few fraction of volts below VOH (min) and
voltage level VIL(max) is kept above VOL(max) , at the design time.
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, VN H (high/1) = VOH (min) − VIH (min)
VN L (low/1) = VIL(max) − VOL(max)
The maximum number of inputs of several gates that be driven by the output of a logic gate is decided by the parameter called fan-out.
IOH(max)
fan-out = I
IH(max)
Current Sinking:
A device output is said to sink current when current flows from power supply, through the load and through the device output to ground.
Current Sourcing:
A device output is said to source current when current flows from power supply, out of the device output and through the load to the
ground.
Speed Power Product( Figure of Merit) (SPP):
A common means for measuring and comparing the overall performance of an IC family is the Speed Power Product (SPP). It is also
called Figure of Merit.
SPP = Propagation delay X Average power dissipation
Operating Temperature Range:
It is the temperature range specified by the logic family within which devices are guaranteed to work reliably.
Unit-5 3
, Power Supply Requirements:
Power supply requirements differ from logic family to family.
Transistor- Transistor Logic (TTL):
depends on transistor alone to perform basic logic operations.
First version known as standard TTL developed in 1965(rarely used today)
2-INPUT TTL NAND GATE :
The above structure consists of multiple-emitter transistor and output structure consists of totem-pole output.
Here Q1 is an NPN transistor having two emitters, one for each input to the gate.
Diode equivalent of Q1 is given below
In the figure, diodes, DA and DB represent the 2-input emitter (E-B)junction of transistor Q1 . Diode DC represents the collector-
base(C-B)junction of transistor Q2
When both inputs A and B are low, both the diodes are forward biased. So the current due to the supply voltage +VCC = 5 V will go
to the ground through R1 and the two diodes DA and DB.
The supply voltage gets dropped in the resistor R1 and it will not be sufficient to turn ON the transistor Q2. With Q2 open, the
transistor Q4 will also cut off. But the transistor Q3 is pulled high. Since Q3 is an emitter follower, the output at the terminal will
also be HIGH, which is at logic 1.
When any one input, either A or B is low, the diode with low input will be forward biased. The same operation will take place as
explained above. In this case, the output will be HIGH.
When both the inputs A and B are high, both the diodes at the emitter-base junction will be reverse biased. The diode DC at the
collector-base junction is forward biased. It will turn on the transistor Q2. With Q2 turned ON, transistor Q4 will also be turned ON.
Unit-5 4
Unit-5
Classification of Logic Families
Characteristics of Digital IC’s
Propagation Delay:
The propagation delay of a gate is basically the time interval between the application of an input pulse and the occurrence of the resulting
output pulse.
Very important characteristics of logic circuit because it limits the speed at which they can operate.
shorter the propagation delay, the higher the speed of the circuit and vice-versa
determined using two basic values
i. tP L H : It is the delay time measured when output is changing from logic 0 to logic 1 state (LOW TO HIGH)
ii. tP H L : It is the delay time measured when output is changing from logic 1 to logic 0 state (HIGH TO LOW)
When the two above are not equal, the larger value is considered as a propagation delay time for that logic gate, i.e.
tp = max(tP L H , tP H L )
OR tp = 12 (tP LH + tP H L )
Power Dissipation:
The amount of power that an IC dissipates is determined by the average supply current IC C , that it draws from the VC C supply. It
is product of IC C and VC C . For ICs, the value of IC C for a LOW gate output (IC C L ) is different from a HIGH output (IC C H ).
Unit-5 1
, Therefore, average IC C is determined based on the 50% duty cycle operation of the gate(LOW half of the time and HIGH half of
the time).
ICCH +ICCL
Ic c(av g ) = 2
This can be used to calculate average power dissipation as
PD (av g ) = IC C (av g ) x VC C
Current Voltage Parameter:
VIH (min) - High-Level Input Voltage:
It is the minimum voltage level required for a logical 1 at an input. Any voltage below this level will not be accepted as a HIGH by
the logic circuit.
VIL(max) - Low-Level Input Voltage:
It is the maximum voltage level required for a logical 0 at an input. Any voltage above this level will not be accepted as a LOW by
the logic circuit.
VOH (min) - High-Level Output Voltage:
It is the minimum voltage level at a logic circuit output in the logical 1 state under defined load conditions.
VOL(max) - Low-Level Output Voltage:
It is the maximum voltage level at a logic circuit output in the logical 0 state under defined load conditions.
IIH - High-Level Input Current:
It is the current that flows into an input when a specified high-level voltage is applied to that input.
IIL - Low-Level Input Current:
It is the current that flows into an input when a specified low-level voltage is applied to that input.
IOH -High-Level Output Current:
It is the current that flows from an output in the logical 1 state under specified load conditions.
IOL -Low-Level Output Current:
It is the current that flows from an output in the logical 0 state under specified load conditions.
Noise Margin and Logic Voltages Levels:
Each logic family has a different standard
Family Logic 0 Logic 1
TTL 0V +5V
ECL -1.7. V - 0.9 V
CMOS 0V 3 -15 V
Stray electric and magnetic field can induce voltages on the connecting wires between logic circuits. These unwanted signals are
called noise and can sometimes cause the voltage at the input to a logic circuit to drop below VIH (min) or rise above VIL(max) ,
which could produce unpredictable operations.
The noise immunity of at a logic circuit refers to the circuit’s ability to tolerate the noise without causing spurious changes in the
output voltage. To avoid this problem due to noise, voltage level VIH (min) is kept at a few fraction of volts below VOH (min) and
voltage level VIL(max) is kept above VOL(max) , at the design time.
Unit-5 2
, VN H (high/1) = VOH (min) − VIH (min)
VN L (low/1) = VIL(max) − VOL(max)
The maximum number of inputs of several gates that be driven by the output of a logic gate is decided by the parameter called fan-out.
IOH(max)
fan-out = I
IH(max)
Current Sinking:
A device output is said to sink current when current flows from power supply, through the load and through the device output to ground.
Current Sourcing:
A device output is said to source current when current flows from power supply, out of the device output and through the load to the
ground.
Speed Power Product( Figure of Merit) (SPP):
A common means for measuring and comparing the overall performance of an IC family is the Speed Power Product (SPP). It is also
called Figure of Merit.
SPP = Propagation delay X Average power dissipation
Operating Temperature Range:
It is the temperature range specified by the logic family within which devices are guaranteed to work reliably.
Unit-5 3
, Power Supply Requirements:
Power supply requirements differ from logic family to family.
Transistor- Transistor Logic (TTL):
depends on transistor alone to perform basic logic operations.
First version known as standard TTL developed in 1965(rarely used today)
2-INPUT TTL NAND GATE :
The above structure consists of multiple-emitter transistor and output structure consists of totem-pole output.
Here Q1 is an NPN transistor having two emitters, one for each input to the gate.
Diode equivalent of Q1 is given below
In the figure, diodes, DA and DB represent the 2-input emitter (E-B)junction of transistor Q1 . Diode DC represents the collector-
base(C-B)junction of transistor Q2
When both inputs A and B are low, both the diodes are forward biased. So the current due to the supply voltage +VCC = 5 V will go
to the ground through R1 and the two diodes DA and DB.
The supply voltage gets dropped in the resistor R1 and it will not be sufficient to turn ON the transistor Q2. With Q2 open, the
transistor Q4 will also cut off. But the transistor Q3 is pulled high. Since Q3 is an emitter follower, the output at the terminal will
also be HIGH, which is at logic 1.
When any one input, either A or B is low, the diode with low input will be forward biased. The same operation will take place as
explained above. In this case, the output will be HIGH.
When both the inputs A and B are high, both the diodes at the emitter-base junction will be reverse biased. The diode DC at the
collector-base junction is forward biased. It will turn on the transistor Q2. With Q2 turned ON, transistor Q4 will also be turned ON.
Unit-5 4
