From analog to digital circuits

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From analog to digital circuits

• A phenomenological overview

Bogdan Roman
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Outline

• Insulators, conductors and semiconductors
• Semiconductor diodes the p-n junction
• The Field Effect Transistor (FET)
• The Junction FET (JFET)
• The Metal Oxide Semiconductor FET (MOSFET)
• The MOS Inverter
• Resistive and same MOS type
• Complementary MOS (CMOS) technology
• Elementary gates
• Flip-flops
• Examples

Loosely based on the IA and 3B Engineering dept.
courses (Linear Circuits and Devices, Digital
Circuits,Information Processing, Integrated
Digital Electronics)
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Insulators and conductors
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Semiconductors Intrinsic silicon
At room temperature, the thermal energy kT 1/40
eV is enough to break a few covalent bonds to
produce free electrons. This also leaves holes
(i.e. positive net charges left by the broken
covalent bond). Both electrons and holes
contribute to current flow.
At low temperatures, silicon is an insulator
since there is not enough thermal energy to break
the covalent bonds.
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Semiconductors Extrinsic silicon
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The p-n junction The diode
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The Diode (contd.)
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The Junction Field Effect Transistor (JFET)
- Proposed by Shockley in 1951 - First made by
Teszner in 1958 in France
JFET Interactive (opens browser)
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Metal Oxide Semiconductor FET (MOSFET)

• - First made in 1960 at Bell Laboratories in the
  USA by Atalla and Kahng.
• Offers extremely high component density in
  integrated circuits.
• - Very high input resistance, low noise, simpler
  fabrication than bipolar transistors.

MOSFET Interactive (opens browser)
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The NMOS inverter
Resistive load
NMOS load

• When input is low (0) then T1 is off, hence
  output goes high (1) (i.e. VDD)
• When VIN high (1) then T1 conducts (linear
  region) and brings the output low (0), depending
  on RL
• High RL low logic zero and low power
  consumption but large area on silicon and slow
  switching gt compromise

• T2 has the gate tied to its drain and is always
  on (and in saturation). Acts as a pseudo-resistor
  load.
• Similar operation to the resistive load inverter
• Smaller area on silicon (so easier to
  manufacture) and faster switching but has a lower
  high logic voltage (VDD VT), and high power
  consumption when input high.

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Complementary MOS (CMOS) inverter
In CMOS technology, the output is clamped to one
of the power rails by a conductive (on) device,
while the other device serves as a load of
effectively infinite resistance (off). This leads
to static properties that approximate those of
the ideal inverter. - The PMOS devices is slower
(lower mobility of holes) so it has to be larger
to compensate. It is also more complex to
manufacture.
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NOR and NAND gates
The 74HC02 IC has four 2-input NOR gates
The 74HC00 IC has four 2-input NAND gates
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Flip-flops and clocked flip-flops
?

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Multiplexer
A crucial circuit, vital for implementing
functions
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Binary Counter
Current state (ABCD) Next state (ABCD)
0 0 0 0 0 0 0 1
0 0 0 1 0 0 1 0
0 0 1 0 0 0 1 1
. .
1 1 1 1 0 0 0 0
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Real stuff