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 totempole 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 700C
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…..