Università degli Studi di Roma Tor Vergata
Dipartimento di Ingegneria Elettronica
Analogue Electronics
Paolo Colantonio
A.A. 2015-16
Bipolar transistors
• 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
Construction
• Two polarities:
• npn
• pnp
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Notation
• 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
• 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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Construction
Realization by growth
• The device is growth by strengthening a semiconductor. During the operation different doping materials
(acceptor or donor) are added to realize the p or n regions.
Realization by alloy
• On the two faces of a semiconductor (e.g. n‐Ge) are placed two globe of doping material (e.g. Indium) which
melt in the semiconductor when the temperature is increased. During the cooling the regions where the
Indium was melt becomes p‐type
Planar realization
• In the semiconductor substrate, by using photolithographic windows, a doping material is diffused to realize
the base and the emitter. The external contacts are realized by metal deposition
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Bipolar transistor operation
• Relationship between the collector and the base currents 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
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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Bipolar transistor operation
• 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 lightly doped and very thin
• Because it is lightly doped, the current produced is mainly electrons flowing from the emitter to the base
• Because the base region is thin, while the base collector is reverse biased, 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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Currents in a BJT
• Assuming the current and voltage references as
reported in the figure, the current in the collector
can be expressed as:
• Being  the large signal current amplification factor
• IC0 the reverse bias current of the Collector‐Base junction
• In general, assuming the base‐emitter junction forward biased, the collector current can be expressed as:
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Currents in a BJT
• By using the Kirchhoff current law, it is possible to write:
• Combining the two relationships, it is possible to relate the output current IC to the
controlling current IB:
• Being
The reverse saturation current when the base is open (IB=0)
The large signal current amplification gain
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Currents in a BJT
• Thus the behaviour can be described by the current gain,  (hfe) or by the transconductance, gm of the device (accounting for the “diode” behavior of the base‐
emitter junction)
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A simple amplifier
• The circuit shows a simple 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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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 (CE) 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 forward‐biased pn junction
• The input characteristics are therefore similar to those of a semiconductor diode
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Output characteristics
• The output characteristics can be divided in three regions
• Region near to the origin is the saturation region, which is normally avoided in linear circuits
• Slope of lines in the active region 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 dc is given by
• AC current gain hfe is given by
• Assuming
1
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Saturation region
+VCC
Ic[mA]
200μA
RL
C
rsat
IC
160μA
120μA
B
80μA
VCE
VBE
rsat≈nΩ
20
E
40μA
IE
10
20μA
0.1
0.2
0.3
0.5
Vce[v]
• In saturation both emitter and collector junctions are forward biased
• Increasing the bias current, the collector current is practically unaffected
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Active region
+VCC
Ic[mA]
200μA
RL
160μA
C
IC
120μA
B
80μA
VCE
VBE
20
E
40μA
IE
10
20μA
0.1
0.2
0.3
0.5
Vce[v]
• In the active region
• emitter‐base junction is forward biased
• collector‐base junction is reverse biased
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Interdiction
• The interdicted region is defined as the region for which IE=0
+VCC
RL
R1
V1
VBE
IE=0
• In order to have IE=0, it is required a small positive voltage VBE (0.1V for Ge, 0 for Si)
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Phototransistor
• The phototransistor is quite similar to the photodiode
IC
C
IC
IL>
n
Radiation
JC
VCE
JE
n
E
VCE
• Usually it is used in CE configuration with open base (IB=0)
• The incident radiation increase the saturation reverse current ICO+IL
• There is an advantage with respect to the photodiode, due to the multiplying factor (1+>>1)
• If the base is not open
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Analysis of bipolar amplifier circuits
• The analysis of a circuit containing a BJT typically requires to study two mixed regime, due to the presence of both DC and AC signals
• Assuming a linear behavior (i.e. small signal), we can adopt the superposition principle
• It is convenient to look at its DC (or quiescent) behavior separately from its AC (or small signal) behavior
VCC
DC Analysis
• Only DC sources (I or V) are considered
• It is adopted to determine the quiescent
(device) bias point
AC Analysis
• Only AC sources (I or V) are considered
• It is adopted to determine the variation of the
electrical parameters (I and V) in the
neighborhood of the operating point (i.e.
Taylor approximation)
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RL
iB
iC
R1
+
vI
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vBE
-
vCE
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Analysis of bipolar amplifier circuits
• Applying the Kirchhoff voltage law at the input and output mesh
VCC
RL
iB
iC
RB
+
vI
vBE
-
vCE
• Thus separating the DC from the AC components, it follows
DC
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AC
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Graphical analysis
• Consider the following circuit
• C1 and C2 are two DC blocking capacitance
• RL is the loading impedance
DC analysis
• The capacitances are considered OPEN circuits
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AC analysis
• The capacitances are considered SHORT circuits
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Graphical analysis: DC
• Applying the Kirchhoff voltage law at the input mesh
• Applying the Kirchhoff voltage law at the output mesh
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Graphical analysis: AC
• The DC block capacitance are assumed short circuit
• The DC bias VDD is not varying thus it is equivalent (AC) as a virtual GND
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