Transcript TTL - UStudy.in
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
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