101 Introduction to Digital Integrated Circuits

1
10-1 Introduction to Digital Integrated Circuits

• Several different families (were concentrating
  on TTL)
• Basic gate is the NAND
• first need to understand RTL and DTL (historical
  value)
• npn and pnp emitters, bases and collectors
• difference between bipolar and unipolar
• fundamental fact vacuum tubes and transistors
  are active circuits which electronically control
  the flow of electrons

2
10-2 Special Characteristics

• Fan-out how many gates can you hook to the
  output of another gate
• Power dissipation how much heat can a transistor
  tolerate before it melts down
• Propagation delay time it takes a signal to
  propagate from input to output (note could be
  different for HIGN and LOW)
• Noise margin how much unwanted voltage can a
  signal absorb without causing confusion as to its
  identity

3
10-3 Bipolar Characteristics

• Concentrate on npn type
• electron flow vs. conventional current flow
  (Franklin)
• arrow points in direction of conventional current
  flow
• need to have a load resistor to prevent burn up
• Kirchoff law base current emitter current
  collector current
• pumping one electron out of base allows 50
  electrons to flow from emitter to collector
  (Beta, or hfe
• base current will flow when forward bias gets
  over 0.6 volts

4
10-3 (contd)

• Transistors can operate in one of three
  conditions (1) cutoff, (2) saturation, or
  (3) active
• if deep into saturation, there may be a delay is
  going to cutoff
• go over example on page 408 (this describes the
  procedures for determinine in which condition the
  transitor is operating)

5
10-4 RTL and DTL circuits

• Basic gate of RTL is NOR low is 0.2v high is
  1.0 to 3.6v 12 mW 25nsec no longer used
• discuss Fig. 10-8
• Basic gate of DTL is NAND low is 0.2v high is
  4.0 to 5.0v 12 mW 30nsec fanout 8 NM of 1v
• discuss Fig. 10-9 (note two diodes)
• replace one diode with a transistor for Fig.
  10-10 this improves fanout

6
10-5 TTL Logic

• TTL has gradually evolved to Advanced Low Power
  Schottky TTL
• speed-power product (in pico Joules)
• for fast speed, you need to reduce storage time
  and also reduce the RC time (but lowering R
  increases P)

7
10-5 (contd)

• Three types of output configuration (1) open
  collector, (2) totem pole, and (3) tristate
• discuss Figs. 10-11, 12, 13 showing how open
  collector can be formed into a common bus
• Totem pole improves propagation delay (from 35 to
  10nsec) it replaces the load resistor with a
  transistor and a diode which sit on top of the
  output transistor (and acts as a pull up
  circuit) this greatly lowers the effective R,
  thus lowering the RC time constant.
• Schottky prevents the diode (or transistor) from
  going into saturation thus it is not delayed in
  going to cutoff it is constructed by putting a
  semiconductor next to metal note the distinctive
  symbol

8
10-5 (contd)

• See Fig. 10-15 Q4 qne Q5 form a Darlington pair
  this gives high current gain with low resistance
  and decreases low to high propagation delay
• Tristate allows totem poles to be connected in
  wired output logic (normally forbidden) has a
  control input (C) when C goes low then the
  output is at high impedance (and is effectively
  out of the circuit)