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TTL Logic Family

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Introduction

• Uses bipolar technology including NPN transistors, diodes and resistors.

• The NAND gate is the basic building block

• Contains different subfamilies with varying characteristics such as speed and power consumption

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Standard TTL NAND Gate

A B Q1 Q2 Q3 Q4 Y

L L sat off off Off H

L H sat off off Off H

H L sat off off Off H

H H iam sat sat On L

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NAND Output Current Sinking & Sourcing

Sinking Sourcing

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What is a totem pole?

In art and history….

Totem poles are carved and painted vertical logs, constructed by many northwest coast native American Indian people. The poles display mythological images, animal spirits whose significance is their association with the lineage.

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Totem pole in TTL ?

• What is totem pole?– addition of an active pull up circuit in the

output of a gate is called totem pole. Using Q3 and Q4 to achieve this purpose

• Why totem pole?– To increase the switching speed of the

gate which is limited due to the parasitic capacitance at the output.

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NOT Gate 7404

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NOR Gate 7402

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AND Gate 7408

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OR Gate 7432

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XOR Gate

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Open Collector Output

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Tristate Gate

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Low Power TTL (74L)

Same as standard TTLExcept that the value of the resistance is increased to lower power dissipation. However, this is at the expense of reduced speed.

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High Speed High Power TTL (74H)

Reduced Resistors increase power dissipation.Removed output diode and added a Darlington pair transistor to reduce output resistance

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Shottky Diode

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Standard TTL Transistor Switching Problem

• Trs are driven into deep saturation to fully conduct, or cutoff to switch off.

• The result of deep saturation is that the two junctions are now forward biased.

• The forward biasing of the BC junction forces a large number of minority carriers to the collector region.

• When the Tr switches off, these minority carriers needs to be removed. This takes a finite amount of time called the storage time (major component of the propagation delay) and thus increases the switch off time.

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Solution 1• Prevent the Tr from going deep in

saturation. This accomplished by preventing the BC junction from becoming forward biased.

• The Shottky diode is used to do the above by placing it across the BC junction. Because of its lower barrier potential, it will conduct current from the base directly to the collector before the BC is forward biased. Thus less carriers are stored in the collector area and the switching becomes much faster.

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SOLUTION 2

Normal Transistor in saturation

Transistor with Shottky diode in saturation

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Schottky TTL (74S)

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Low Power Shottky 74LS

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Advanced Low Power Shottky 74ALS

• All saturating transistors are clamped by using Schottky diodes.

• Inputs and outputs are clamped by Schottky diodes

• New manufacturing and materials reduce stray capacitance to increase speed.

• active turn-off of the LOW-level output transistor, producing a better HIGH-level output voltage and thus a higher HIGH-level noise immunity.

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74ALS00