Transistor and Gate
Transcript of Transistor and Gate
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8/8/2019 Transistor and Gate
1/16
Transistor AND Gate
AND Gate
OR GateNAND
Gate
NOR Gate
Double
Transistor
NOR Gate
Single
Transistor
The use oftransistorsfor
the construction oflogic gatesdepends upon
their utility asfastswitches. When the
base-emitter diode is
turned on enough to be
driven intosaturation, thecollector voltage with
respect to ground may be
less than a volt and can beused as a logic 0 in
the TTL logic family.
Basic Gates
Index
Electronicsconcepts
Digital
Electronics
Reference
MimsDigital
Logic
Circuits
HyperPhysics*****Electricity and magnetismR
Nave
Go Back
http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/trangate.html#c2%23c2http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/trangate.html#c3%23c3http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/trangate.html#c3%23c3http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/trangate.html#c4%23c4http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/trangate.html#c4%23c4http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/trangate.html#c4%23c4http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/trangate.html#c5%23c5http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/trangate.html#c5%23c5http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/trangate.html#c5%23c5http://hyperphysics.phy-astr.gsu.edu/hbase/solids/trans.html#c1http://hyperphysics.phy-astr.gsu.edu/hbase/solids/trans.html#c1http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/gate.html#c1http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/transwitch.html#c1http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/transwitch.html#c1http://hyperphysics.phy-astr.gsu.edu/hbase/solids/trans2.html#c1http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/logfam.html#c2http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/logfam.html#c2http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/gate.html#c1http://hyperphysics.phy-astr.gsu.edu/hbase/hframe.htmlhttp://hyperphysics.phy-astr.gsu.edu/hbase/electronic/etroncon.html#c1http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/etroncon.html#c1http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/digcktcon.html#c1http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/digcktcon.html#c1http://hyperphysics.phy-astr.gsu.edu/hbase/electric/eleref.html#c1http://hyperphysics.phy-astr.gsu.edu/hbase/hph.htmlhttp://hyperphysics.phy-astr.gsu.edu/hbase/emcon.html#c1http://history.go%28-1%29/http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/trangate.html#c5%23c5http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/trangate.html#c4%23c4http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/trangate.html#c3%23c3http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/trangate.html#c2%23c2http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/trangate.html#c2%23c2http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/trangate.html#c3%23c3http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/trangate.html#c3%23c3http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/trangate.html#c4%23c4http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/trangate.html#c4%23c4http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/trangate.html#c4%23c4http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/trangate.html#c5%23c5http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/trangate.html#c5%23c5http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/trangate.html#c5%23c5http://hyperphysics.phy-astr.gsu.edu/hbase/solids/trans.html#c1http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/gate.html#c1http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/transwitch.html#c1http://hyperphysics.phy-astr.gsu.edu/hbase/solids/trans2.html#c1http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/logfam.html#c2http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/gate.html#c1http://hyperphysics.phy-astr.gsu.edu/hbase/hframe.htmlhttp://hyperphysics.phy-astr.gsu.edu/hbase/electronic/etroncon.html#c1http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/etroncon.html#c1http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/digcktcon.html#c1http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/digcktcon.html#c1http://hyperphysics.phy-astr.gsu.edu/hbase/electric/eleref.html#c1http://hyperphysics.phy-astr.gsu.edu/hbase/hph.htmlhttp://hyperphysics.phy-astr.gsu.edu/hbase/emcon.html#c1http://history.go%28-1%29/ -
8/8/2019 Transistor and Gate
2/16
Transistor OR Gate
AND Gate
OR GateNAND
Gate
NOR Gate
Double
Transistor
NOR Gate
Single
Transistor
The use oftransistors for
the construction oflogic gatesdepends upon
their utility asfast switches. When the
base-emitter diode is
turned on enough to bedriven intosaturation, the
collector voltage with
respect to ground may beless than a volt and can be
used as a logic 0 in
theTTL logic family.
Basic Gates
Index
Electronicsconcepts
DigitalElectronics
ReferenceMims
Digital
Logic
Circuits
HyperPhysics*****Electricity and magnetismR
Nave
Go Back
http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/trangate.html#c1%23c1http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/trangate.html#c3%23c3http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/trangate.html#c3%23c3http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/trangate.html#c4%23c4http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/trangate.html#c4%23c4http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/trangate.html#c4%23c4http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/trangate.html#c5%23c5http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/trangate.html#c5%23c5http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/trangate.html#c5%23c5http://hyperphysics.phy-astr.gsu.edu/hbase/solids/trans.html#c1http://hyperphysics.phy-astr.gsu.edu/hbase/solids/trans.html#c1http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/gate.html#c1http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/transwitch.html#c1http://hyperphysics.phy-astr.gsu.edu/hbase/solids/trans2.html#c1http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/logfam.html#c2http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/logfam.html#c2http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/gate.html#c1http://hyperphysics.phy-astr.gsu.edu/hbase/hframe.htmlhttp://hyperphysics.phy-astr.gsu.edu/hbase/electronic/etroncon.html#c1http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/etroncon.html#c1http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/digcktcon.html#c1http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/digcktcon.html#c1http://hyperphysics.phy-astr.gsu.edu/hbase/electric/eleref.html#c1http://hyperphysics.phy-astr.gsu.edu/hbase/hph.htmlhttp://hyperphysics.phy-astr.gsu.edu/hbase/emcon.html#c1http://history.go%28-1%29/http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/trangate.html#c5%23c5http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/trangate.html#c4%23c4http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/trangate.html#c3%23c3http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/trangate.html#c1%23c1http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/trangate.html#c1%23c1http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/trangate.html#c3%23c3http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/trangate.html#c3%23c3http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/trangate.html#c4%23c4http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/trangate.html#c4%23c4http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/trangate.html#c4%23c4http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/trangate.html#c5%23c5http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/trangate.html#c5%23c5http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/trangate.html#c5%23c5http://hyperphysics.phy-astr.gsu.edu/hbase/solids/trans.html#c1http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/gate.html#c1http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/transwitch.html#c1http://hyperphysics.phy-astr.gsu.edu/hbase/solids/trans2.html#c1http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/logfam.html#c2http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/gate.html#c1http://hyperphysics.phy-astr.gsu.edu/hbase/hframe.htmlhttp://hyperphysics.phy-astr.gsu.edu/hbase/electronic/etroncon.html#c1http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/etroncon.html#c1http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/digcktcon.html#c1http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/digcktcon.html#c1http://hyperphysics.phy-astr.gsu.edu/hbase/electric/eleref.html#c1http://hyperphysics.phy-astr.gsu.edu/hbase/hph.htmlhttp://hyperphysics.phy-astr.gsu.edu/hbase/emcon.html#c1http://history.go%28-1%29/ -
8/8/2019 Transistor and Gate
3/16
Transistor NAND Gate
AND Gate
OR GateNAND
Gate
NOR Gate
Double
Transistor
NOR Gate
Single
Transistor
The use oftransistors for
the construction of
logicgates depends upontheir utility as
fast switches. When the
base-emitter diode isturned on enough to be
driven into saturation,
the collector voltage withrespect to ground may be
less than a volt and can
be used as a logic 0 in
theTTL logic family.
Basic Gates
Index
Electronics
concepts
Digital
Electronics
ReferenceMims
Digital
LogicCircuits
HyperPhysics*****Electricity and magnetismR
Nave
Go Back
Transistor NOR Gate
AND Gate
OR Gate
NAND
Gate
NOR Gate
DoubleTransistor
NOR Gate
Single
Transistor
The use oftransistors for
the construction oflogic gatesdepends upon
their utility as
fastswitches. When thebase-emitter diode is
turned on enough to be
driven intosaturation, thecollector voltage with
respect to ground may be
less than a volt and can beused as a logic 0 in
the TTL logic family.
Basic Gates
Index
Electronics
concepts
Digital
Electronics
Reference
Mims
DigitalLogic
Circuits
http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/trangate.html#c1%23c1http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/trangate.html#c2%23c2http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/trangate.html#c4%23c4http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/trangate.html#c4%23c4http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/trangate.html#c4%23c4http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/trangate.html#c5%23c5http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/trangate.html#c5%23c5http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/trangate.html#c5%23c5http://hyperphysics.phy-astr.gsu.edu/hbase/solids/trans.html#c1http://hyperphysics.phy-astr.gsu.edu/hbase/solids/trans.html#c1http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/gate.html#c1http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/transwitch.html#c1http://hyperphysics.phy-astr.gsu.edu/hbase/solids/trans2.html#c1http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/logfam.html#c2http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/logfam.html#c2http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/gate.html#c1http://hyperphysics.phy-astr.gsu.edu/hbase/hframe.htmlhttp://hyperphysics.phy-astr.gsu.edu/hbase/electronic/etroncon.html#c1http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/etroncon.html#c1http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/digcktcon.html#c1http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/digcktcon.html#c1http://hyperphysics.phy-astr.gsu.edu/hbase/electric/eleref.html#c1http://hyperphysics.phy-astr.gsu.edu/hbase/hph.htmlhttp://hyperphysics.phy-astr.gsu.edu/hbase/emcon.html#c1http://history.go%28-1%29/http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/trangate.html#c1%23c1http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/trangate.html#c2%23c2http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/trangate.html#c3%23c3http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/trangate.html#c3%23c3http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/trangate.html#c5%23c5http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/trangate.html#c5%23c5http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/trangate.html#c5%23c5http://hyperphysics.phy-astr.gsu.edu/hbase/solids/trans.html#c1http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/gate.html#c1http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/transwitch.html#c1http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/transwitch.html#c1http://hyperphysics.phy-astr.gsu.edu/hbase/solids/trans2.html#c1http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/logfam.html#c2http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/gate.html#c1http://hyperphysics.phy-astr.gsu.edu/hbase/hframe.htmlhttp://hyperphysics.phy-astr.gsu.edu/hbase/electronic/etroncon.html#c1http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/etroncon.html#c1http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/digcktcon.html#c1http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/digcktcon.html#c1http://hyperphysics.phy-astr.gsu.edu/hbase/electric/eleref.html#c1http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/trangate.html#c5%23c5http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/trangate.html#c3%23c3http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/trangate.html#c2%23c2http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/trangate.html#c1%23c1http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/trangate.html#c5%23c5http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/trangate.html#c4%23c4http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/trangate.html#c2%23c2http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/trangate.html#c1%23c1http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/trangate.html#c1%23c1http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/trangate.html#c2%23c2http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/trangate.html#c4%23c4http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/trangate.html#c4%23c4http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/trangate.html#c4%23c4http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/trangate.html#c5%23c5http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/trangate.html#c5%23c5http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/trangate.html#c5%23c5http://hyperphysics.phy-astr.gsu.edu/hbase/solids/trans.html#c1http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/gate.html#c1http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/transwitch.html#c1http://hyperphysics.phy-astr.gsu.edu/hbase/solids/trans2.html#c1http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/logfam.html#c2http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/gate.html#c1http://hyperphysics.phy-astr.gsu.edu/hbase/hframe.htmlhttp://hyperphysics.phy-astr.gsu.edu/hbase/electronic/etroncon.html#c1http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/etroncon.html#c1http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/digcktcon.html#c1http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/digcktcon.html#c1http://hyperphysics.phy-astr.gsu.edu/hbase/electric/eleref.html#c1http://hyperphysics.phy-astr.gsu.edu/hbase/hph.htmlhttp://hyperphysics.phy-astr.gsu.edu/hbase/emcon.html#c1http://history.go%28-1%29/http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/trangate.html#c1%23c1http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/trangate.html#c2%23c2http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/trangate.html#c3%23c3http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/trangate.html#c3%23c3http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/trangate.html#c5%23c5http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/trangate.html#c5%23c5http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/trangate.html#c5%23c5http://hyperphysics.phy-astr.gsu.edu/hbase/solids/trans.html#c1http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/gate.html#c1http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/transwitch.html#c1http://hyperphysics.phy-astr.gsu.edu/hbase/solids/trans2.html#c1http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/logfam.html#c2http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/gate.html#c1http://hyperphysics.phy-astr.gsu.edu/hbase/hframe.htmlhttp://hyperphysics.phy-astr.gsu.edu/hbase/electronic/etroncon.html#c1http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/etroncon.html#c1http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/digcktcon.html#c1http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/digcktcon.html#c1http://hyperphysics.phy-astr.gsu.edu/hbase/electric/eleref.html#c1 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8/8/2019 Transistor and Gate
4/16
HyperPhysics*****Electricity and magnetismR
Nave
Go Back
Transistor NOR Gate
AND Gate
OR Gate
NAND
Gate
NOR Gate
Double
Transistor
NOR Gate
Single
Transistor
The use oftransistors forthe construction of
logicgates depends upon
their utility asfast switches. When the
base-emitter diode is
turned on enough to be
driven into saturation, thecollector voltage with
respect to ground may be
less than a volt and canbe used as a logic 0 in
theTTL logic family.
Basic Gates
Index
Electronics
concepts
Digital
Electronics
Reference
MimsDigital
Logic
Circuits
HyperPhysics*****Electricity and magnetismR
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written byJonathan Dunder
Logic gates take binary values and perform
functions on them, similar to the functions found
in simple algebra. Binary algebra is the set of
mathematical laws that are valid for binary values.A binary value can only be a 1 or a 0. 1 is a high
value, representing true and high voltage. 0 is a
low value, representing a false value and low
voltage.
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Logic gates are typically packaged in integrated
circuits, although they can be constructed using
analogue components. Integrated circuits allow
multiple logic gates to be packaged in one chipand are usually quite reliable. Logic gates
typically come in two flavors, TTL (transistor-
transistor logic) and CMOS (Complementary Metal
Oxide Semiconductor). One must be careful
mixing the two types, there logic low and logic
high are different voltages. A CMOS might take a
TTL high as a LOW and a TTL will accept a CMOS
low as a high. Because of this they are generally
incompatible, but there are a few CMOS that can
accept TTL inputs and vice versa.
The buffer and NOT gates are the simplest of the
logic gates. The buffer would be used as a digital
signal booster, if a logic signal was to travel for
some distance voltage drop from wire resistance
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would lower a logic high voltage so low that when
it reaches its destination its read as a logic low,
putting this in between would solve that problem.
The buffer's algebra function is B = A.
NOT gates simply change the input from a 1 to a 0
or vice versa. It is also called an inverter and has
many uses in logic circuits. For example, you
have 2 lights, but you only want 1 on at any one
time, you would put a NOT gate between one light
so when there is a logic high input 1 light is onand the other connected to the NOT gate is off
and when there is a logic low input the second
comes on because of the NOT gate. The circle on
the end of the triangle indicates that its an
inverting gate and you can recognize any
inverting logic device by this circle.
The equivalent binary algebra function is B = A',
where B is the output and A is an input value.
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The AND gate most commonly come in IC
packages of 2 and 3 input versions. The outputonly produces a logical 1 when all of the inputs
are 1. An AND gate could be used in an alarm
circuit, where input A would be a reed switch
input and B would be an armed control, So the
alarm would only be activated if the alarm was
active AND the reed switch was circuit was
opened (opened door ect.).
The equivalent binary algebra function is C = A * B
* ... * N, where C is the output and A and B are two
of N total inputs.
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The OR gate has a minimum of two inputs and
produces an output of 1 if at least one of theinputs has a value of 1. An OR gate could be used
to expand the number of reed switches in the
previous example.
The equivalent binary algebra function is C = A +
B + ... + N, where C is the output and A and B are
two of N total inputs.
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The NAND gate has a minimum of two inputs and
is the equivalent of an AND gate with a NOT gateon the output. It produces a 0 only if all of the
inputs are 1. A NAND gate could be used to
switch a device off if it gets too hot or a cooling
fan stops working, so a temperature sensor would
be connected to input A and a tachometer output
filtered so its a logical high when there is rotation
and logical low when there is no rotation, is
connected to input B. The equivalent binary
algebra function is C = (A * B * ... * N)', where C is
the output and A and B are two of N total inputs.
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The NOR gate has a minimum of two inputs and is
the same as an OR gate with a NOT gate on theoutput. The NOR gate produces a 1 only if all of
the inputs are 0. The NOR gate can be used to
shutdown a device if either of 2 temperature
sensors measure a temperature too high for the
circuit.
The equivalent binary algebra function isF=(A+B+...+N)', where F is the output and A and B
are two of N total inputs.
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The XOR aka EOR gate has a minimum of two
inputs. The NOR gate produces a 1 only if one ofthe inputs is a 1. The equivalent binary algebra
function is C = AB'+ A'B, where C is the output
and A and B are two inputs.
The XNOR aka ENOR gate has a minimum of twoinputs. The XNOR gate produces an output of 1 if
inputs A and B match. The most obvious
application for this logic gate is a comparator, this
could be used as error checking in data
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transmission or it can form the basis of a
combination keypad. The equivalent binary
algebra function is C = AB + A'B', where C is the
output and A and B are two inputs.
Discrete logic gates
I often use discrete component implementations of logic gates in my projects,often consisting of AND and OR gates made from diodes and resistors. I"invented" or "discovered" how to make these gates when I was about 14 andhave been happily using them ever since.
The reason I use these gates is often to save space: if all I need is one ANDgate, for example to reset a counter, then I can easily implement it with 2 diodesand a resistor. It saves using a whole additional IC such as a 74HC08 containingfour AND gates, of which I only need one. At other times, it can be convenient tobe able to build a logic gate with as many inputs as I like. For example, a 7 inputOR-gate is easily implemented just by adding more diodes to the basic 2-input
gate. I might not have the IC's I would need in my junkbox, so I'd have to orderthem and wait for the delivery to arrive. But I'll always have resistors and diodes,so this is sometimes another reason for using logic gates made from discretecomponents.
There are disadvantages to these very simple logic gates too. The mostnoticeable is that the output current ability of the gates is low. It is determined bythe resistor used, and by the current handling ability of the signals driving the
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gate inputs. If the discrete logicgate output is driving a highimpedance IC input such as arefound on the modern 74HC-series IC's, there is no problem.
The 0.7V voltage drop acrossthe diodes means that youcannot cascade the simplediscrete component logic gates,because you may easily violate
the input voltage logic level specifications of the IC's following the gate: thevoltage will fall below what is recognised as a "1" for example. Finally, thediscrete component implementations of logic gates might be slower than their"proper" IC equivalents. I do not currently know how much of a problem this islikely to be.
Now I'll try to explain the various types of gates which I commonly use.
OR gate
I'll start with this one because it is the easiest to understand. The easiest way tounderstand the operation of these gates is to consider the diode as a simpleswitch, which is closed (on) when the voltage on one side (the anode) is higherthan the other (the cathode). The current then flows in the direction of the arrowin the diode's circuit diagram symbol. In the diagram (right) I show the logicsymbol, my discrete component implementation, and the truth table. In an ORgate, the output is "1" (high) if either of the inputs are "1". In this diagram, if either
of the inputs has a "high" voltage, its diode will conduct and current will flow tothe output. A "high" voltage will develop across the resistor, equivalent to theinput voltage minus 0.7V drop, as is usual across silicon diode junctions. If bothof the inputs are low voltage "0", then the diodes don't conduct. In this instancethe gate's output is tied low by the 10K resistor.
AND gate
Now look at my AND gate. It's similar to the OR gate except that the diodes point
in the other direction, and the resistor goes to +5V not ground. The output of anAND gate is "1" only if BOTH the inputs are "1". In my diode-resistorimplementation, if either input is "low" voltage (logic "0") then the diode willconduct and the output is effectively shorted to ground. If both of the inputvoltages are "high" (logic "1") then neither of the diodes will conduct, so theoutput is not shorted to ground: it remains at +5V (logic "1") via the 10K resistor.This gives the desired result. Note that again, due to the silicon junction voltage,the actual "low" output voltage is 0.7V higher than the "low" input voltage.
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NOT gate
(inverter)
You cannot implement an inverting function with diodes and resistors alone. Youalso now need a transistor, to provide the inverting action. There's nothingparticularly special about the transistor to be used, almost any small signal NPNtransistor will suit, since it's driven into saturation (unbiased). If the voltage
presented to the base of the transistor is above 0.7, the transistor will conductwhich drags the output to logic "0", low voltage. If the input voltage is logic "0",then the transistor does not conduct, and the resistor will just tie the output to+5V. You always need that 10K current limiting resistor in the base, or excessivebase-emitter current will destroy the transistor.
NAND and NOR gates
These are easy: just use AND or OR followed by a NOT (inverter). In the case ofthe NAND gate, the transistor's base resistor can be omitted since the maximumcurrent is already limited by the 10K resistor in the diode-resistor AND gate. Thebase resistor is still needed in a NOR gate.
XOR gate
This one is a little more complicated. If we start off imagining an OR gate, thatworks for the first three of the possible four states in the truth table. But not thefinal one, where both inputs are "1". So we need a way of forcing the output to
zero. I accomplised this using a transistor as a switch, with its base driven by anAND gate on the two inputs. So that when both inputs are "1", the AND gate willactivate the transistor, which will force the output to zero. So the XOR gate canbe considered as an OR gate, plus an AND gate, plus a switch to zero theoutput. The additional diode in the base lead of the transistor is to create anadditional 0.7V drop, without it the 0.7V of the AND gate output (even when oneof the inputs is zero) would be enough to put the transistor into partialconduction.
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Signal gate
I like this one for gating incoming pulse trains. I use this circuit sometimes forfrequency counter inputs for example. In the example to the right, the pulse isonly allowed from IN to OUT when both the A and B inputs are "1". It's really justanother kind of AND gate I suppose, effectively having three inputs. But savesone diode compared to an AND-gate implementation as above. (Well, even a 2-input AND gate or OR gate can be reduced to a single resistor and a singlediode, in that case - but somehow I prefer the more balanced approach with twodiodes and a resistor). In the diagram to the right, the diodes could be reversedand the output would then be forced to "1" if either of A or B were a "1". Thiscould be useful to drive a negative edge triggered counter clock input, forexample.
Other notes
1. Gates with more than two inputs: Easy! Just add more diodes, one for eachinput!
2. The 10K resistor: I have shown 10K for the purposes of illustration only. Inreality resistance value needs to be chosen depending on the input resistance ofthe following circuit that needs to be driven by the output. It needs to be relativelysmall in comparison. If the gate is driving a single 4000-series CMOS or 74HC-series logic IC input, then the resistor could easily be made upwards of 100K or
even 1M, depending on the application.