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Digital Logic Structures: Chapter 3

1. Digital Logic Structures: Chapter 3. COMP 2610. Dr. James Money COMP 2610. Transistors. Most microprocessors are constructed out of what are called MOS transistors MOS stands for metal-oxide semiconductor For us, we just assume these work according to their specs. Transistors.

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Digital Logic Structures: Chapter 3

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  1. 1 Digital Logic Structures: Chapter 3 COMP 2610 Dr. James Money COMP 2610

  2. Transistors • Most microprocessors are constructed out of what are called MOS transistors • MOS stands for metal-oxide semiconductor • For us, we just assume these work according to their specs

  3. Transistors • The Pentium IV CPU has over 32 millions MOS transistors • The IBM PowerPC 750 FX has 38 millions MOS transistors • We are going to study how these connect together to get larger units

  4. Transistors • There are two types of transistors • p-type • n-type • These both work logically similar to wall switches in our homes

  5. Transistors • Let’s first consider the wall switch example:

  6. Transistors • Instead of the wall switch, you could insert a n-type or p-type MOS transistor:

  7. Transistors • In (a), the MOS transistor is by itself • In (b), it is combined with the wall switch circuit • Note there are three terminals in the diagram

  8. Transistors • Three terminals: • Source – input voltage • Drain – output voltage • Gate – controls output of transistor, depends on types of MOS

  9. Transistors • What is important is for a n-type transistor, if the gate is supplied with 2.9 volts, the source to drain acts like a closed circuit • That is, the drain is 2.9 volts • If the n-type transistor gate has 0 volts, this is an open circuit and connection between source and drain is broken

  10. Transistors • To conclude, for n-type MOS transistor in figure (b): • If Gate has 2.9 volts or on, then voltage is suppiled and the light comes on • If Gate has 0 volts or off, the voltage is not supplied and the light is off

  11. Transistors • Figure (c) is the shorthand version of the circuit we will normally use • We only show the terminals of the power supply • The triangle represents source and the horizontal line represents drain

  12. Transistors • A p-type transistor works in the opposite fashion: • If Gate has 2.9 volts, the circuit is broken and drain has 0 volts • If Gate has 0 volts, the circuit is closed and the drain has 2.9 volts

  13. Transistors • p-type transistor diagram:

  14. Transistors • Note that n-type and p-type act in complementary ways • Circuits with both types of MOS transistors are called CMOS circuits because of this • CMOS – complementary metal-oxide semiconductor

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