2. Different logic families :
• RTL(Resistor Transistor Logic)
• DTL(Diode Transistor Logic)
• TTL(Transistor Transistor Logic)
• ECL(Emitter Coupled Logic)
• CMOS(Complementary Metal Oxide
Semiconductor Logic)
Out of these RTL and DTL are rarely used.
3. • It is a logic family consisting completely of
bipolar junction transistors (BJT) and
resistors.
• the logic gating function and the
amplifying function are performed
What is TTL ?
4. History
• It was build in 1961 by James L Bui.
• Commercially used in logic design in
1963.
• Commercially it starts with the 74 series
like the 7404, 74S86 etc.
5. Features of TTL Family:
• Logic low level : 0 or 0.2V.
• Logic high level : 5V.
• Typical fan out of 10. It means it can
support at most 10 gates at its output.
• A basic TTL device draws a power of
almost 10mW.
• Average propagation delay : 9ns.
• Noise margin : 0.4V.
6. TTL Applications:
• Used in controller application for providing
0 to 5Vs
• Used as switching device in driving lamps
and relays
• Used in processors of mini computers like
DEC VAX
• Used in printers and video display
terminals
7. Sub- families
• 7 series of TTL 54-/74-logic family
• different speed and lower dissipation
characteristics :
Bipolar Prefix
Standard TTL. 7400 series
Low - power TTL 74L00 series
High -speed TTL 74H00 series
Schottky TTL 74S00 series
Low power Schottky TTL (LS) 74LS00 series
Fast (F) and Advanced Schottky (AS) 74AS00 series
Advanced Low power Schottky TTL 74ALS00 series
Fast TTL 74F00 series
9. Fundamental TTL gate
Inputs both logical ones : multiple emitter transistor – reverse-active mode.
An approximately constant current flows from the positive rail, through the
resistor and into the base of the current passes through the base-emitter
junction of the output transistor, allowing it to conduct and pulling the output
voltage low (logical zero).
One input logical zero : current flows out of this input . no current flows
through the base of the output transistor, and the output voltage becomes
high (logical one).
10. Classification of TTL:
• Based on output
• Open Collector Output :
With any of the input at logic low, voltage drop across
the base of Q1 is around 0.9V, not enough for the
transistors Q2 and Q3 to conduct. Thus output is either
floating or Vcc, i.e. High level.
when all inputs are high, transistor
Q2 and Q3 get enough base current
and are in saturation mode.
Output is at logic low.
11. APPLICATIONS
• It is used in 3 major applications
– In driving lamps or relays
– In performing wired logic
– In construction of a common bus system
12. Totem Pole Output :
• adds to this a "totem-pole" (“push-pull") output.
• 2 n-p-n transistors Q3 and Q4,
• lifting" diode D1 and
• resistor R3=130 ohm ,
• Advantage :
- low output resistance at output logical "1“
- results in reduction of propagation delay.
• Disadvantage :
- no more than 3.5 V of the output logical "1"
13. • When V2 is off, V4 is off, V3 operates in active region
as a voltage follower producing high output voltage
(logical "1").
• When V2 is "on", it activates V4, driving low voltage
(logical "0") to the output.
14. Three state Gate:
• It provides 3 state output.
• "enable" input is high -
normal operation of the
gate.
• "enable" is low -both
output transistors are
cutoff.
