TTL (Transistor Transistor Logic)

Introduction

 Transistor Transistor logic or just TTL, logic gates are built around only transistors.

 TTL was developed in 1965. Through the years basic TTL has been improved to meet the performance requirements.

 There are many versions or families of TTL  Standard TTL.

 High Speed TTL  Low Power TTL.

  Schhottky TTL .

As such all the families of TTL have three configuration for outputs.

 Totem - Pole output.

 Open Collector Output.

 Tristate Output.

TTL circuit

Cont.,

 Thus it has

two emitter-base junctions

that can be used to turn

Q1 ON

.

 The transistors Q3 and Q4 are connected in a

totem pole arrangement

.

Basic TTL NAND gate

Cont.,

 In normal operation, either

Q3

or

Q4

will be conducting, depending on the logic state of the output.

 The transistor

Q2 acts as a phase splitter

.

TTL NAND gate 3 input circuit

Cont.,

 When both the inputs

A and B are at high level

, the diodes

D1 and D2

will conduct in

reverse bias (OFF)

and the diode

D3

will conduct in

forward bias (ON),

hence Q1 goes in to cut off.

Cont.,

 Now the current flow through the

emitter of Q2

will turn On the transistor Q4.

 At the same time the collector current of

Q2 produces a voltage drop

across R2 that reduces Q2 collector voltage to a low value.

 Now the

output is low

because

Q4 is conducting

Diode equivalent for Q1

Cont.,

 When either or both inputs are low, the transistor

Q1

conducts because the either, or both diodes

D1

and

D2

are conducting in forward biasing.

 Now the diode

D3

is

not conducting properly

, so not a sufficient

current is flowing through the base of Q2

.

 Hence

Q2 goes to cut-off

.

TTL NAND Gate with a Totem Pole Output Stage

Cont.,

 There is

no emitter current of Q2

,

no base current of Q4

, and it turns OFF.

 The

high collector voltage of Q2

, turns On the transistor.

 Actually

Q3 acts as an emitter

follower.

 Now

output is high

, because Q4 is in cut-off.

 The function of diode D is to prevent both

Q3 and Q4 from being On

simultaneously.

Improved TTL Series

 74 Series  Schottky TTL, 74S Series: higher speed  Low-Power Schottky TTL, 74LS series  Advanced Schottky TTL, 74AS Series  Advanced Low-Power Schottky TTL, 74ALS Series  74F-Fast TTL

Comparison of TTL Series

Characteristics of TTL gates

1.

Operates reliably over the range from

4.75V to 5.25V

2.

Operates in ambient temperature range from

0 to 70 0 C

3.

One NAND gate requires an average

power of 10mW

.

4.

Propagation delay range from

7ns to 11ns 5.

Fan-out

: It can drive standard TTL inputs.

Advantages

 Compatible with other logic circuits  Low output impedance  High speed operation  Good noise immunity  Low power dissipation  High fanout  Low cost

Disadvantages

1.

TTL ICs generate switching transistors 2.

Used special techniques for circuit preparation 3.

Wired output capability is not possible

The End

…..Thank you…..