Saturday, October 2, 2010

Transistors and FETs

Page: 8 of 8

Summary of Bipolar Junction Transistors

  • The Bipolar Junction Transistor (BJT) is a three layer device constructed form two semiconductor diode junctions joined together, one forward biased and one reverse biased.
  • There are two main types of bipolar junction transistors, the NPN and the PNP transistor.
  • Transistors are "Current Operated Devices" where a much smaller Base current causes a larger Emitter to Collector current, which themselves are nearly equal, to flow.
  • The most common transistor connection is the Common-emitter configuration.
  • Requires a Biasing voltage for AC amplifier operation.
  • The Collector or output characteristics curves can be used to find either Ib, Ic or β to which a load line can be constructed to determine a suitable operating point, Q with variations in base current determining the operating range.
  • A transistor can also be used as an electronic switch to control devices such as lamps, motors and solenoids etc.
  • Inductive loads such as DC motors, relays and solenoids require a reverse biased "Flywheel" diode placed across the load. This helps prevent any induced back emf's generated when the load is switched "OFF" from damaging the transistor.
  • The NPN transistor requires the Base to be more positive than the Emitter while the PNP type requires that the Emitter is more positive than the Base.

Summary of Field Effect Transistors

  • Field Effect Transistors, or FET's are "Voltage Operated Devices" and can be divided into two main types: Junction-gate devices called JFET's and Insulated-gate devices called IGFET´s or more commonly known as MOSFET's.
  • Insulated-gate devices can also be sub-divided into Enhancement types and Depletion types. All forms are available in both N-channel and P-channel versions.
  • FET's have very high input resistances so very little or no current (MOSFET types) flows into the input terminal making them ideal for use as electronic switches.
  • The input impedance of the MOSFET is even higher than that of the JFET due to the insulating oxide layer and therefore static electricity can easily damage MOSFET devices so care needs to be taken when handling them.
  • FET's have very large current gain compared to junction transistors.
  • They can be used as ideal switches due to their very high channel "OFF" resistance, low "ON" resistance.

The Field Effect Transistor Family-tree

FET Types

Field Effect Transistors can be used to replace normal Bipolar Junction Transistors in electronic circuits and a simple comparison between FET's and transistors stating both their advantages and their disadvantages is given below.
 Field Effect Transistor (FET) Bipolar Junction Transistor (BJT)
1Low voltage gainHigh voltage gain
2High current gainLow current gain
3Very input impedanceLow input impedance
4High output impedanceLow output impedance
5Low noise generationMedium noise generation
6Fast switching timeMedium switching time
7Easily damaged by staticRobust
8Some require an input to turn it "OFF"Requires zero input to turn it "OFF"
9Voltage controlled deviceCurrent controlled device
10Exhibits the properties of a Resistor 
11More expensive than bipolarCheap
12Difficult to biasEasy to bias

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