Bipolar Junction Transistors BJT

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Bipolar Junction Transistors (BJT)

• Transistor consists of two n- and one p- (npn),
  or two p- and one n-type (pnp) materials. The
  three terminals of a transistor are named as
  emitter E, collector C, and base B.
• The emitter is heavily doped, base lightly doped
  and the collector doping level is in between that
  of the emitter and the base. The base layer width
  is much smaller.
• Transistor is an important example of an active
  component.

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• NPN is more widely used. Majority carriers are
  electrons, so it operates more quickly. PNP is
  used for special applications.

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BJTs can be viewed as two back to back diodes.
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Transistor operation

• Let us consider an npn transistor
• In normal operation, the EB junction is forward
  biased and the BC junction is reverse biased.
• Forward biasing reduces the depletion region.
  This reduction results in a heavy flow of
  majority carriers from n- (emitter) to p-type
  (base).

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• The reverse-bias between base and collector
  increases the depletion region size, resulting in
  minority carrier flow only
• Most of the majority carriers from the emitter
  layer will pass directly into the collector
  layer.
• The collector current has two components
• Majority carriers
• Minority carriers leakage current

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Transistor operation

• IE IC IB
• IC IC majority ICO minority
• For general purpose diodes, leakage current is in
  the order of micro- or nanoampers, where the
  majority carrier current is in the order of
  miliampers.
• Hence, leakage current is generally ignored.
• Though, it is temperature sensitive and must be
  considered for wide temperature range
  applications.

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Collector Current

• The ratio of collector current to emitter current
  is expressed by a quantity called alpha
• a IC / IE
• a ranges from 0.90 to 0.998
• IC b IB , b a/(a-1)
• b (DC current gain) is not a highly reliable
  parameter, it depends on temperature and VBE
• For AC situations, ac alpha is defined by
• a ac dIC / dIE

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• A transistor in a circuit will be in one of three
  conditions
• Cut off (no collector current), useful for switch
  operation.
• In the active region (some collector current,
  more than a few tenths of a volt above the
  emitter), useful for amplifier applications
• In saturation (collector a few tenths of a volt
  above emitter), large current useful for "switch
  on" applications.

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Characteristics curves
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Common-Base configuration

• Base is common to both input and output
  terminals. In addition, it is closest to or at
  ground potential.
• This configuration is used for high frequency
  applications because the base separates the input
  and output, minimizing oscillations at high
  frequency. It has a high voltage gain, relatively
  low input impedance and high output impedance
  compared to the common collector.

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Common-Emitter configuration

• This is the most common configuration for
  transistor amplifiers.
• By using different values of VBB and/or RB, base
  current is controlled.

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Common Emitter Amplifier
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