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Bipolar Transistors
Chapter 21
 Introduction
 An Overview of Bipolar Transistors
 Bipolar Transistor Operation
 Bipolar Transistor Characteristics
 Summary of Bipolar Transistor Characteristics
 Bipolar Transistor Amplifiers
 Other Bipolar Transistor Applications
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Introduction
21.1
 Bipolar transistors are one of the main
‘building-blocks’ in electronic systems
 They are used in both analogue and digital circuits
 They incorporate two pn junctions and are
sometimes known as bipolar junction transistors
or BJTs
 Here will refer to them simply as bipolar transistors
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An Overview of Bipolar Transistors
21.2
 While control in a FET is due to an electric field,
control in a bipolar transistor is generally considered
to be due to an electric current
– current into one terminal
determines the current
between two others
– as with a FET, a
bipolar transistor
can be used as a
‘control device’
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 Notation
– bipolar transistors are 3
terminal devices
 collector (c)
 base (b)
 emitter (e)
– the base is the control input
– diagram illustrates the
notation used for labelling
voltages and currents
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 Relationship between the collector current and the
base current in a bipolar transistor
– characteristic is
approximately linear
– magnitude of collector
current is generally
many times that of the
base current
– the device provides
current gain
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 Construction
– two polarities:
npn and pnp
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Bipolar Transistor Operation
21.3
 We will consider npn transistors
– pnp devices are similar but with different polarities of
voltage and currents
– when using npn transistors
 collector is normally more positive than the emitter
 VCE might be a few volts
 device resembles two back-to-back diodes – but has very
different characteristics
 with the base open-circuit negligible current flows from the
collector to the emitter
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 Now consider what happens when a positive voltage
is applied to the base (with respect to the emitter)
– this forward biases the base-emitter junction
– the base region is light doped and very thin
– because it is likely doped, the current produced is
mainly electrons flowing from the emitter to the base
– because the base region is thin, most of the electrons
entering the base get swept across the base-collector
junction into the collector
– this produces a collector current that is much larger than
the base current – this gives current amplification
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 Transistor action
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Bipolar Transistor Characteristics
21.4
 Behaviour can be described by the current gain, hfe
or by the transconductance, gm of the device
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 Transistor configurations
– transistors can be used in a
number of configurations
– most common is as shown
– emitter terminal is common
to input and output circuits
– this is a common-emitter
configuration
– we will look at the
characteristics of the device
in this configuration
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 Input characteristics
– the input takes the
form of a forwardbiased pn junction
– the input
characteristics are
therefore similar to
those of a
semiconductor diode
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 Output characteristics
– region near to the
origin is the
saturation region
– this is normally
avoided in linear
circuits
– slope of lines
represents the
output resistance
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 Transfer characteristics
– can be described by either the current gain or by the
transconductance
– DC current gain hFE or  is given by IC / IB
– AC current gain hfe is given by ic / ib
– transconductance gm is given approximately by
gm  40IC  40 IE siemens
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 Equivalent circuits for a bipolar transistor
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Summary of Bipolar Transistor Characteristics
21.5
 Bipolar transistors have three terminals: collector,
base and emitter
 The base is the control input
 Two polarities of device: npn and pnp
 The collector current is controlled by the base
voltage/current IC = hFEIB
 Behaviour is characterised by the current gain or the
transconductance
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Bipolar Transistor Amplifiers
21.6
 A simple transistor amplifier
– RB is used to ‘bias’ the
transistor by injecting an
appropriate base current
– C is a coupling capacitor
and is used to couple the
AC signal while preventing
external circuits from
affecting the bias
– this is an AC-coupled amplifier
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 AC-coupled amplifier
– VB is set by the conduction voltage of the base-emitter
junction and so is about 0.7 V
– voltage across RB is thus VCC – 0.7
– this voltage divided by RB gives the base current IB
– the collector current is then given by IC = hFEIB
– the voltage drop across RC is given by IC RC
– the quiescent output voltage is therefore
Vo = VCC - IC RC
– output is determined by hFE which is very variable
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 Negative feedback amplifiers
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Example – see Example 21.2 from course text
Determine the
quiescent output
voltage of this
circuit
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Base current is small, so
VB  VCC
R2
10 k
 10
 2. 7 V
R1  R2
27 k  10 k
Emitter voltage
VE = VB – VBE = 2.7 – 0.7 = 2.0 V
Emitter current
IE 
VE 2.0 V

 2 mA
RE 1 k
Since IB is small, collector current IC  IE = 2 mA
Output voltage = VCC – ICRC = 10 - 2 mA 2.2 k = 5.6 V
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 A common-collector amplifier
–
–
–
–
unity gain
high input resistance
low output resistance
a very good
buffer amplifier
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Other Bipolar Transistor Applications
21.7
 A phase splitter
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 A voltage regulator
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 A logical switch
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Key Points
 Bipolar transistors are widely used in both analogue and
digital circuits
 They can be considered as either voltage-controlled or
current-controlled devices
 Their characteristics may be described by their gain or by
their transconductance
 Feedback can be used to overcome problems of variability
 The majority of circuits use transistors in a common-emitter
configuration where the input is applied to the base and the
output is taken from the collector
 Common-collector circuits make good buffer amplifiers
 Bipolar transistors are used in a wide range of applications
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