Transcript Preface - Hainan University

3.4 Bipolar Logic
1. Diode Logic
 Diode logic level: VCC 5V, LOW 
0~2V, HIGH  3~5V
 Noise margin: 1V
D1
VCC
R
A
Z
B
D2
Diode AND gate
A
B
Z
0
0
1
1
0
1
0
1
0
0
0
1
Z  AB
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3.4 Bipolar Logic
2. Transistor Logic Inverter
VCC
R2
VCC
Vout
VOUT
VIN
R1
Q1
VCE(sat)
LOW undefined HIGH
Vin
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3.4 Bipolar Logic
3. Schottky Transistors
- R2
collector
base
VBCWhy
=0.4V
propagation
+
+
can be reduced
emitter
Vdelay
IN
VCE=0.2V
+ a Schottky
by using
+ 0.25V VBE=0.6V -? transistor
VCC
R2
VBC=0.25V
+
+
VOUT V
IN
VCE=0.35V
+
VIN
Q1
VBE=0.6V - R1
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3.4 Bipolar Logic
4. Transistor-Transistor Logic
Diode AND gate
and input protection
What the
purpose of
the 120-
resistor R5?
VA
Output
stage
Phase
splitter
Active
load
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3.4 Bipolar Logic

Function Table
X Y
L
L
H
H
X
0
0
1
1
VA
Q2 Q3
L 1.05 off
H 1.05 off
L 1.05 off
H 1.2 on
Y
0
1
0
1
Z
1
1
1
0
Q4 Q5 Q6 VZ
on on
on on
on on
off off
X
Y
off off 2.7
off off 2.7
off off 2.7
on on 0.35
Z
Z
H
H
H
L
Z  XY
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3.4 Bipolar Logic

Logic levels and noise margins
VOHmin: The minimum output voltage in the HIGH
state, 2.7V for most TTL families.
VIHmin: The minimum input voltage guaranteed to
be recognized as a HIGH, 2.0V for all TTL families.
VILmax: The maximum input voltage guaranteed to be
recognized as a LOW, 0.8V for most TTL families.
VOLmax: The maximum output voltage in the LOW
state, 0.5V for most TTL families.
High-state DC noise margin=2.7-2.0=0.7V
Low-state DC noise margin=0.8-0.5=0.3V
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3.4 Bipolar Logic
TheFanout
currentisflow
in a TTLofinput
or
Fanout:
a measure
the number
of
defined
to be to
positive
if the
gateoutput
inputslead
thatisare
connected
(and driven
flows into the lead, and
by) current
a singleactually
gate output.
negative if current flows out of the lead.
 IILmax: The maximum current that an input
requires to pull it LOW. (-0.4mA)
Since current flows out of a TTL input in the
LOW state, IILmax has a negative value.


IIHmax: The maximum current that an input
requires to pull it HIGH. (20 μA)
Since current flows into of a TTL input in the
HIGH state, IIHmax has a positive value.
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3.4 Bipolar Logic



IOLmax: The maximum current an output can
sink in the LOW state while maintaining an
output voltage no more than VOLmax. (8mA)
IOHmax: The maximum current an output can
source in the HIGH state while maintaining
an output voltage no less than VOHmin. (-400 μA)
LOW state fanout = IOLmax/IILmax = 8/0.4 = 20
HIGH state fanout = IOHmax/IIHmax = 400/20 =20
 Overall fanout: The overall fanout is the lesser
of the LOW- and HIGH-state fanouts.

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3.4 Bipolar Logic
Confirm an output is not being overloaded
Loading a TTL output with more than its rated
fanout has the same deleterious effects that were
described for CMOS devices. That is, DC noise
margins may be reduced or eliminated, transition
times and delays may increase, and the device
may overheat.

If
HIGH state I OH max   I IH max
LOW state I OL max   I IL max
Then the output is not being overloaded.
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3.4 Bipolar Logic

Unused Inputs

An unused input can be tied to another.
An unused AND or NAND input can be tied
to logic 1(pull-up resistor, VCC, or floating).

An unused OR or NOR input can be tied to
logic 0(pull-down resistor, ground).

Why an unused TTL input can be floating?
Why use a pull-down or pull-up resistor?
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3.4 Bipolar Logic

Pull-down and pull-up resistor calculation
R pd
VIL max

nIIL max
IILmax
VILmax
nIILmax
RPd
IILmax
IILmax
VCC
RPu
VIHmin
nIIHmax
IIHmax
IIHmax
VCC  VIH min
R pu 
nIIH max
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3.4 Bipolar Logic
5. Additional TTL Gate Types

NOR gate

NAND gate

AND gate

Three-state outputs gate

Open-collector outputs
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3.4 Bipolar Logic
6. TTL Families
74 FAM nn
prefix
Alphabetic
family
mnemonic
Numeric
function
designator
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3.4 Bipolar Logic



74S(Schottky TTL): with schottky transistors
and low resistor, this family has much higher
speed,but higher power consumption,than the
original 74-series TTL.
74LS(Low-power Schottky TTL): matches the
speed of 74-series TTL but has about one-fifth
of its power consumption.
74AS(Advanced Schottky TTL): offers speeds
approximately twice as fast as 74S with
approximately the same power consumption.
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3.4 Bipolar Logic


74ALS(Advanced Low-power Schottky TTL):
offers both lower power and higher speed than
74LS.
74F(Fast TTL): is positioned between 74AS
and 74ALS in the speed/power tradeoff, and is
probably the most popular choice for highspeed requirements in new TTL designs.
Electrical characteristics of the
TTL families are summarized on
page 167-167 in the text-book.
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