Comp541 transistors and all that a brief overview
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COMP541 Transistors and all that… a brief overview. Montek Singh Sep 8, 2014. Transistors as switches. At an abstract level, transistors are merely switches 3-ported voltage-controlled switch n-type: conduct when control input is 1 p-type: conduct when control input is 0.

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Comp541 transistors and all that a brief overview

COMP541Transistors and all that…a brief overview

Montek Singh

Sep 8, 2014


Transistors as switches

Transistors as switches

  • At an abstract level, transistors are merely switches

    • 3-ported voltage-controlled switch

      • n-type: conduct when control input is 1

      • p-type: conduct when control input is 0


Silicon as a semiconductor

Silicon as a semiconductor

  • Transistors are built from silicon

  • Pure Si itself does not conduct well

  • Impurities are added to make it conducting

    • As provides free electrons  n-type

    • B provides free “holes”  p-type

Figure 1.26 Silicon lattice and dopant atoms


Mos transistors

MOS Transistors

  • MOS = Metal-oxide semiconductor

  • 3 terminals

    • gate: the voltage here controls whether current flows

    • source and drain: are what the current flows between

Figure 1.29 nMOS and pMOS transistors


Nmos transistors

nMOS Transistors

  • Gate = 0

    • OFF = disconnect

      • no current flows between source & drain

  • Gate = 1

    • ON= connect

      • current can flow between source & drain

      • positive gate voltage draws in electrons to form a channel

Figure 1.30 nMOS transistor operation


Pmos transistors

pMOS Transistors

  • Just the opposite

    • Gate = 1  disconnect

    • Gate = 0  connect

  • Summary:


Cmos topologies

CMOS Topologies

  • There is actually more to it than connect/disconnect

    • nMOS: pass good 0’s, but bad 1’s

      • so connect source to GND

    • pMOS: pass good 1’s, but bad 0’s

      • so connect source to VDD

  • Typically use them incomplementary fashion:

    • nMOS network at bottom

      • pulls output value down to 0

    • pMOS network at top

      • pulls output value up to 1

    • only one of the two networks must conduct at a time!

      • or smoke may be produced

    • if neither network conducts  output will be floating


Inverter

Inverter


Comp541 transistors and all that a brief overview

NAND


3 input nor gate

3-input NOR Gate?


2 input and gate

2-input AND Gate?


Transmission gates

Transmission Gates

  • Transmission gate is a switch:

    • nMOS pass 1’s poorly

    • pMOS pass 0’s poorly

    • Transmission gate is a better switch

      • passes both 0 and 1 well

    • When EN = 1, the switch is ON:

      • Ais connected to B

    • When EN = 0, the switch is OFF:

      • A is not connected to B

  • IMPORTANT: Transmission gates are not drivers

    • will NOT remove input noise to produce clean(er) output

    • simply connect A and B together (current could even flow backward!)

    • use very carefully!


Logic using transmission gates

Logic using Transmission Gates

  • Typically combine two (or more) transmission gates

    • Together form an actual logic gate whose output is always driven 0 or 1

      • Exactly one transmission gate drives the output;all remaining transmission gates float their outputs

  • Example: XOR

    • when C = 0, TG0 conducts

      • F = A

    • when C = 1, TG1 conducts

      • F = A’

    • therefore:

      • F = A xor C

TG0

TG1


Tristate buffer and tristate inverter

Tristate buffer and tristate inverter

  • When enabled: sends input to output

  • When disabled: output is floating (‘Z’)

  • Implementation:

    • Tristate buffer using only a pass gate

      • If on: output  input

      • If off: output is floating

    • Tristate inverter

      • Top half and bottom half are not fullycomplementary

      • Either both conduct: output  NOT(input)

        • will act as a driver!

      • Or both off: output is floating


Power consumption

Power Consumption

  • Power = Energy consumed per unit time

    • Dynamic power consumption

    • Static power consumption


Dynamic power consumption

Dynamic Power Consumption

  • Energy consumed due to switching activity:

    • All wires and transistor gates have capacitance

    • Energy required to charge a capacitance, C, to VDD is CVDD2

    • Circuit running at frequency f: transistors switch (from 1 to 0 or vice versa) at that frequency

    • Capacitor is charged f/2 times per second (discharging from 1 to 0 is free)

      Pdynamic = ½CVDD2f


Static power consumption

Static Power Consumption

  • Power consumed when no gates are switching

    • Caused by the quiescent supply current, IDD(also called the leakage current)

      Pstatic = IDDVDD


Power consumption example

Power Consumption Example

  • Estimate the power consumption of a wireless handheld computer

    • VDD = 1.2 V

    • C = 20 nF

    • f = 1 GHz

    • IDD = 20 mA

      P = ½CVDD2f + IDDVDD

      =½(20 nF)(1.2 V)2(1 GHz) +

      (20 mA)(1.2 V)

      = 14.4 W


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