ICS3M

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chapter 2 transistors (BJT)

• 2.1 Transistor classification
• 2.2 Bipolar junction transistors (BJT)
  construction
• 2.3 Transistor action and operating
• 2.4 Quiescent Operating Point
• 2.5 Bipolar transistor characteristics
• 2.6 Transistor parameters
• 2.7 Current gain
• 2.8 Typical BJT characteristics and maximum
  ratings
• 2.9 Transistor operating configurations

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2.1 Transistor classification
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2.2 Bipolar junction transistors (BJT)
construction

• Bipolar transistors generally comprise n-p-n or
  p-n-p junctions of either silicon (Si) or
  germanium (Ge) material.
• N phosphorus or arsenic P boron or gallium
• The junctions are, in fact, produced in a single
  slice of silicon by diffusing impurities through
  a photographically reduced mask.
• Silicon transistors are superior when compared
  with germanium transistors in the vast majority
  of applications

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?The symbols and simplified junction models for
n-p-n and p-n-p transistors are shown in Figure
2.3. It is important to note that the base region
is extremely narrow.
Figure 2.3 The symbols and simplified junction
models for n-p-n and p-n-p transistors
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E Emitter B Base C - Collector
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Electronics-BTEC
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2.3 Transistor action

• ? In the n-p-n transistor, transistor action is
  accounted for as follows
• ? the base-emitter junction is forward biased and
  the base-collector junction is reverse biased
• ? Around 99.5 of the electrons leaving the
  emitter will cross the Base collector junction
  and only 0.5 of the electrons will Recombine
  with holes in the narrow base region.

Figure 2.4 Transistor action of n-p-n transistor
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• ? the base-emitter junction is forward biased and
  the base-collector junction is reverse biased

Figure 2.5 Transistor action of p-n-p transistor
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2.2.2 leakage current

• ? For an n-p-n transistor, the base-collector
  junction is reversed biased for majority
  carriers, but a small leakage current, ICBO ,
  flows from the collector to the base due to
  thermally generated minority carriers (holes in
  the
• collector and electrons in the base), being
  present. The base-collector junction is forward
  biased to these minority carriers.
• ? With modern transistors, leakage current is
  usually very small (typically less than 100nA)
  and in most applications it can be ignored.
• ? The control of current from emitter to
  collector is largely independent of the
  collector-base voltage and almost wholly governed
  by the emitter-base voltage.

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2.2.3 bias and current flow

• ? In normal operation (i.e. for operation as a
  linear amplifier) the base-emitter junction of a
  transistor is forward biased and the
  collector-base junction is reverse biased.
• ?The current flowing in the emitter circuitis
  typically 100 times greater than that flowing in
  the base.

Figure 2.7 bias and current flow
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2.2.3 bias and current flow

• Leakage current ICBO

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2.2.3 bias and current flow
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2.2.4 Transistor operating configurations

• ? Three basic circuit configurations are used for
  transistor amplifiers.
• ? These three circuit configurations depend upon
  which one of the three transistor connections is
  made common to both the input and the output.
• ? In the case of bipolar junction transistors,
  the configurations are known as common emitter,
  common collector (or emitter follower), and
  common base.

Figure 2.8 Transistor operating configurations
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2.2.5 bipolar transistor characteristics

• ? The characteristics of a bipolar junction
  transistor are usually presented in the form of a
  set of graphs relating voltage and current
  present at the transistors terminals.

Figure 2.9 measurement circuit of bipolar
transistor characteristics
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? In this mode, the input current is applied to
the base and the output current appears in
the collector.
Figure 2.10 Typical input characteristic
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• ? Each curve corresponds to a different value of
  base current. Note the knee in the
  characteristic below VCE 2V.
• ? Also note that the curves are quite flat.
• ? For this reason (i.e. since the collector
  current does not change very much as the
  collector-emitter voltagechanges) we often refer
  to this as a constant current characteristic.

Figure 2.11 Output characteristics
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• ? Here IC is plotted against IB for a
  small-signal general-purpose transistor.
• ? The slope of this curve (i.e. the ratio of IC
  to IB) is the common-emitter current gain of the
  transistor.

Figure 2.12 Transfer characteristic
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2.2.6 Bipolar transistor parameters

• ? In particular, the three characteristic graphs
  can be used to determine the following parameters
  for operation in common-emitter mode

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2.2.6 Bipolar transistor parameters

• ? In particular, the three characteristic graphs
  can be used to determine the following parameters
  for operation in common-emitter mode

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2.2.6 Bipolar transistor parameters

• ? In particular, the three characteristic graphs
  can be used to determine the following parameters
  for operation in common-emitter mode

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2.2.6 Bipolar transistor parameters
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2.2.7 Current gain

• ? We use the symbol hFE to represent the static
  value of common-emitter current gain.
• ? Similarly, we use hfe to represent the dynamic
  value of common-emitter current gain.
• ? Note that hFE is found from corresponding
  static values while hfe is found by measuring the
  slope of the graph.
• ? Furthermore, most transistor parameters
  (particularly common-emitter current gain, hfe)
  are liable to wide variation from one device to
  the next.

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2.2.8 Typical BJT characteristics and maximum
ratings
Table 2.2 Transistor characteristics and
maximum ratings
PTOTmax is the maximum device power dissipation.
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2.4 The junction field-effect transistor
Figure 2.13 Conformation of N channel J.F.E.T
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2.4 The junction field-effect transistor
N channel JFET
P channel JFET
Figure 2.14 Symbol of JFET
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2.4 The junction field-effect transistor
Figure 2.15 Operation of N channel JFET
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2.5 Metal-oxide-semiconductor field-effect
transistor
2.5.1 depletion-type MOS FET
N channel
P channel
Construction of N channel depletion-type MOS FET
Figure 2.16 depletion-type MOS FET
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2.5 Metal-oxide-semiconductor field-effect
transistor
2.5.2 Enhancement-type MOS FET
N channel
P channel
Construction of N channel enhancement-type MOS FET
Figure 2.16 depletion-type MOS FET
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2.4 Quiescent Operating Point
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