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IC digital logic families
RTL Resistor-transistor logic
DTL Diode-transistor logic TTL Transistor -transistor logic ECL Emitter-coupled logic MOS Metal-oxide semiconductor
CMOS (Complementary MetalOxide Semiconductor)
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Characteristics of an
Ideal Logic Family The ideal logic family should have or be:
Low power
High speed Easy to use
Many different logic functions
Clear voltage levels for 0 (LOW) and 1 (HIGH)
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Feature fan-out
The no. of standard loads can be connected
to the output of the gate without degradingits normal operation
Sometimes the term loading is used
Power dissipation
The power needed by the gate Expressed in mW
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Computing fan-out
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Power dissipation
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Propagation delay50% VH
50% VH
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Noise margin
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Noise margin (Contd)
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The Ideal Switching Action
of the BJT Consider a bipolar transistor in logic circuits
It is operated in either two states
produces the two logic levelsFully conducting state - saturated/turned on
or
Fully non-conducting state -cut-off state
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Typical npn Transistor
ParametersRegion VBE (V) VCE (V) Current
Relation
Cutoff < 0.6 Opencircuit IB=IC=0
Active 0.6-0.7 > 0.8 IC =hFEIB
Saturation 0.7-0.8 0.2 IB IC/hFE
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Transistor-Transistor Logic
(TTL) TTL was developed in 1965 The original basic TTL gate was a slight improvement over the
DTL gate. There are several TTL subfamilies or series of the TTL
technology. For example
Standard TTL 74 High Speed TTL 74H (twice as fast, twice as much
power)
Low Power TTL -74L (1/10 the speed, 1/10 the powerof standard" TTL)
Schottky TTL 74S(for high-frequency uses ) Low-power Schottky TTL 74LS Advanced Schottky TTL 74ALS
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Part Numbers Part numbers for 7400 TTL series logic devices
often use the following naming convention
SN 74 ALS 245 A
manufacturer temperature range logic subfamily deviceLS - Low Power Schottky
AS - Advanced Schottky
ALS - Advanced Low
Power Schottky
SN = Texas Instruments
DM = Fairchild
74 - commercial
54 military (extended)
hundreds of different
devices in each family
package type, quality
grade, etc. (varies widely
by manufacturer)
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Open-collector TTL NAND
Gate
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Open Collector Outputs
(Cont.) An open-collector output can present a logic LO
output Since there is no internal path from the output Y
to the supply voltage VCC , the circuit cannotpresent a logic HIGH on its own To function properly an external pull-up resistor,
Rp is being used as shown
Use this symbolto Indicates opencollector output
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Advantages of Open
Collector Outputs Why should we use open-collector
gates which require the addition of a
pull-up resistor in order to functionproperly when we could use a gatewith a totem-pole output instead?
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1. Wired-ANDingOpen-collector outputs can be tieddirectly together which results in the
logical ANDing of the outputs. Thusthe equivalent of an AND gate can beformed by simply connecting theoutputs
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Advantages (cont)2. Increased current levels - Standard TTL
gates with totem-pole outputs can onlyprovide a HIGH current output of 0.4 mA and
a LOW current of 1.6 mA. Many open-collector gates have increased currentratings
3. Different voltage levels - variety of outputHIGH voltages can be achieved. This is useful in
interfacing different logic families that havedifferent voltage and current levelrequirements
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Disadvantages : The big disadvantage of open-
collector gates is their slow
switching speed. This is because thevalue of pull-up resistor is in k;,which results in a relatively long timeconstants