1. 3.4 Bipolar Logic 1. Diode Logic
Diode logic level: VCC 5V, LOW  0~2V, HIGH  3~5V
Noise margin: 1V
VCC A B Z R
Diode AND gate D1 D2 A B Z 1
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2. 3.4 Bipolar Logic 2. Transistor Logic Inverter VCC R2 VOUT Vout VIN Q1
VCE(sat)
LOW
undefined
HIGH
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3. 3.4 Bipolar Logic 3. Schottky Transistors collector R2 base VBC=0.4V
emitter
collector R2 +
VIN -
VCE=0.2V
VBE=0.6V
VBC=0.4V
Why propagation delay can be reduced by using a Schottky transistor ? +
VIN -
VCE=0.35V
VBE=0.6V
VBC=0.25V
+ 0.25V - R2 R1
VIN
VOUT Q1
VCC
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4. 3.4 Bipolar Logic 4. Transistor-Transistor Logic
What the purpose of the 120- resistor R5?
4. Transistor-Transistor Logic VA
Diode AND gate
and input protection
Output
stage
Phase
splitter
Active load
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5. 3.4 Bipolar Logic VA Q3 Q2 Z X Y Q5 Q4 VZ Q6 Function Table
L L  off on on off off H
L H  off on on off off H
H L  off on on off off H
H H  on off off on on  L X Y Z 1 X Y Z
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6. 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= =0.7V
Low-state DC noise margin= =0.3V
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7. 3.4 Bipolar Logic
The current flow in a TTL input or output lead is defined to be positive if the current actually flows into the lead, and negative if current flows out of the lead.
Fanout: Fanout is a measure of the number of gate inputs that are connected to (and driven by) a single gate output.
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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8. 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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9. 3.4 Bipolar Logic If HIGH state LOW state
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
LOW state
Then the output is not being overloaded.
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10. 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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11. 3.4 Bipolar Logic Pull-down and pull-up resistor calculation VILmax
RPd
nIILmax
IILmax
VIHmin
RPu
nIIHmax
IIHmax
VCC
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12. 3.4 Bipolar Logic 5. Additional TTL Gate Types NOR gate NAND gate
Three-state outputs gate
Open-collector outputs
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13. 74 FAM nn 3.4 Bipolar Logic Alphabetic family mnemonic
6. TTL Families
74 FAM nn
prefix
Alphabetic family mnemonic
Numeric function designator
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14. 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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15. 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 high-speed requirements in new TTL designs.
Electrical characteristics of the TTL families are summarized on page in the text-book.
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