Standard TTL: Digital Electronics Transistor - Transistor Logic
Standard TTL: Digital Electronics Transistor - Transistor Logic
Standard TTL: Digital Electronics Transistor - Transistor Logic
Standard TTL
The
above figure shows the internal structure and characteristics of a
standard TTL NAND gate. The NAND gate of it is a quad two input
type. And it has four circuits of 5400/740. In plain ways the
circuit of this type of TTL operates as follows. The Q1 showed in
the figure is a two emitter NPN transistor.
This type is NAND gate is analogous to two transistors whose
base and emitter terminals are joined together. The diodes
named as D2 and D3 are used to limit the input voltages which are
negative in nature.
This is a
subfamily under the main family. This is named so because lower
power consumption and dissipation is achieved. Though the speed
at which the operation is done is somewhat reduced. The above
figure is of a low power TTL which is made using AND gates. The
NAND gate used in this is of 74L00 or 54L00 type and is of quad
two input type. The construction of this type of TTL is almost
similar to that of standard TTL except the resistance which is of a
higher value. For this increased value of the resistance the power
dissipation of the circuit is lowered.
Schottky TTL
A
nother TTL subfamily is Schottky TTL. This design was used to
speed up the time of operation. The speed offered by this type of
TTL is twice the speed that is offered by the high power TTL. The
power dissipation for both the TTLs are same and there is no
extra power consumption. The figure above represents the basic
NAND based diagram of Schottky TTL. The circuit diagram is
pretty much similar to that of a high power TTL, here the Q
transistor of high power TTL is missing. The Schottky transistor
which is used for this type TTL is nothing but a bipolar transistor
which has its base and collector connected by a schottky diode.
This Schottky TTL is further devided in many parts like low power
Schottky, advanced low power schottky and advanced Schottky
whose discussion has been avoided due to complexities.
Supply Voltage +5V (+/- 0.5V) +5V (+/- 0.5V) +5V (+/- 0.25V)
TTL is still used a lot when building ‘one off’ logic circuits as
the gates are cheap and fairly robust (i.e. you aren't likely to
damage them when building the circuit!). However, most
modern large scale commercial and industrial systems use
CMOS logic as it is cheaper/better for integrated systems.
The main disadvantage of CMOS is that it is static sensitive,
hence it can be depressingly easy to destroy CMOS logic
simply by taking it out of its package carelessly!!