Sensors Technology – MED4
ST11 – Transistor
Transistor
Lecturer:
Smilen Dimitrov
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ST11 – Transistor
Introduction
•
The model that we introduced for ST
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ST11 – Transistor
Introduction
•
We have discussed
– The units of voltage, current and resistance, from both a microscopic
and macroscopic (electric circuits) perspective
– The definition of an elementary electric circuit, Ohm’s law and Kirschoff
Laws
– Solving and measurement of voltage divider circuit and more
complicated circuits - and applications in sensors
– Resistive based sensors
– AC current, capacitors, and capacitive based sensors
– Semiconductor structures – diode (and sensor applications)
•
This time we discuss the transistor as an electronic element, and as a basis
for different types of sensors
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ST11 – Transistor
Transistor
•
Transistor is a semiconductor device as well - however, it has three
terminals - so a bit more complex to understand.
–
the application of voltage to the input terminal, changes the conductivity between the other two terminals,
and hence controls current flow through those terminals.
•
Two basic types of transistors, which differ in construction and usage (but
not in main purpose)
– bipolar junction transistors (BJTs)
– field effect transistors (FETs)
•
Other categorizations - a particular transistor may be described as: silicon,
surface mount, BJT, NPN, low power, high frequency switch.
–
Semiconductor material: germanium, silicon, gallium arsenide, silicon carbide
–
Type: BJT, JFET, IGFET (MOSFET), IGBT, "other types"
–
Polarity: NPN, PNP, N-channel, P-channel
–
Maximum power rating: low, medium, high
–
Maximum operating frequency: low, medium, high, radio frequency (RF), microwave
–
Application: switch, general purpose, audio, high voltage, super-beta, matched pair
–
Physical packaging: through hole metal, through hole plastic, surface mount, ball grid array
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ST11 – Transistor
Bipolar junction transistor (BJT)
•
Called bipolar because conduction channel uses both majority and minority
carriers for main electric current
– first type of transistor to be commercially mass-produced
•
The three terminals are named emitter, base and collector.
– Two p-n junctions exist inside the BJT: collector-base junction and baseemitter junction.
– described as a current operated device because the collector current is
controlled by the current flowing between base and emitter terminals.
•
in a P semiconductor, majority carriers are holes, and minority carriers are
free electrons;
– in a N semiconductor, majority carriers are free electrons, and minority
carriers are holes
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ST11 – Transistor
Bipolar junction transistor (BJT)
•
•
A BJT consists of three differently doped semiconductor regions, the emitter
region, the base region and the collector region.
– These regions are, respectively, p type, n type and p type in a PNP
transistor ( - and n type, p type and n type in a NPN transistor).
– Each semiconductor region is connected to a terminal, appropriately
labelled: emitter (E), base (B) and collector (C)
BJT, unlike other transistors, is not a symmetrical device. This means that the
interchange of the collector and the emitter makes the transistor behave differently
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ST11 – Transistor
Bipolar junction transistor (BJT)
•
Small changes in the voltage applied across the base-emitter terminals,
causes the current that flows between the emitter and the collector to
change significantly.
– This effect can be used to amplify the input current. BJTs can be thought
of as voltage-controlled current sources
•
A Bipolar Transistor essentially consists of a
pair of PN Junction Diodes that are joined
back-to-back.
– This forms a sort of a sandwich where
one kind of semiconductor is placed in
between two others.
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ST11 – Transistor
Bipolar junction transistor (BJT)
•
Principle of work – focus on NPN variety (real current direction)
In the absence of any externally applied electric field, we
find that depletion zones form at both PN-Junctions, so
no charge wants to move from one layer to another
…reverse-biased the B-C diode junction - widens the
depletion zone between C and B and so no current will
flow
We apply E-B
voltage whose
polarity is designed
to forward-bias the
E-B junction. This
'pushes' electrons
from the E into the
B region, ...
... and sets up a current flow across the E-B boundary.
Once the electrons have managed to get into the B region
they can respond to the attractive force from the
positively-biased C region.
We apply a moderate
voltage between C and
B ->
C positive with respect
to B
- polarity of voltage
chosen to increase
force pulling the N-type
electrons and P-type
holes apart….
Most (but not
all!) the
electrons that
get into B
move straight
on into the
C,...
... provided the C voltage is positive enough to draw them
out of the B region. That said, some of the electrons get
'lost' on the way across the Base.
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ST11 – Transistor
Bipolar junction transistor (BJT)
•
•
•
The precise value of the Base-Collector voltage we choose doesn't really
matter to what happens provided we don't make it too big and blow up the
transistor! Example = 10 Volt Base-Collector voltage.
In practice, with a Bipolar transistor made using Silicon we can expect to
have to use an Emitter-Base voltage in the range from around a half volt up
to almost one volt. Higher voltages tend to produce so much current that
they can destroy the transistor!
Some of the free electrons crossing the Base encounter a hole and 'drop
into it'. As a result, the Base region loses one of its positive charges (holes)
each time this happens.
– the Base potential would become more negative (i.e. 'less positive'
because of the removal of the holes) until it was negative enough to repel any more
electrons from crossing the E-B junction. The current flow would then stop.
– To prevent this happening we use the applied E-B voltage to remove the
captured electrons from the Base and maintain the number of holes it
contains.
– only about 1% of the free electrons get lost in Base => we see a Base
Current, IB, which is typically around 100 smaller than the Emitter
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Current, IE
ST11 – Transistor
Bipolar junction transistor (BJT)
•
•
Transistor - as an current amplifier. When a small current signal is applied to
the base terminal, it is amplified in the collector circuit. This current
amplification is referred to as hFE or beta (β) and equals Ic/Ib.
Circuit theory – use technical direction of current
Schematic symbols: The
direction of the emitter
arrow defines the type of
transistor.
For proper polarization (of NPN), we say:
– the base is directly polarised (forward biased P base potential is more positive than N emitter potential )
– the collector is inversely polarised (reverse
biased - N collector potential is more positive than P base
potential)
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ST11 – Transistor
Bipolar junction transistor (BJT)
•
The transistor is a complex element, and can be used in a variety of ways.
•
We will mostly discuss the configuration known as a 'common emitter',
(described previously):
– base-emitter PN junction is directly polarised,
– and collector-base PN junction is inversely polarised.
•
For this mode of operation, it is important we remember the following points
in analysis (of an NPN transistor)
– The base-emitter junction behaves like a diode - that is, it has a turn-on
voltage (as in a diode, approx 0.7V)
– When the base-emitter junction is turned on, and base current IB flows,
the collector current IC is related to IB, and amplified by a factor of hFE or
beta (β) - that is, IC = hFE* IB.
– hFE is as parameter which is usually 20 or 30, and in high gain transistor
this factor can be all the way up to 100 or 200
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ST11 – Transistor
Hydraulic analogy of a transistor
•
Analogy of technical direction of current in an NPN transistor – to a water
flow in a pipe system
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ST11 – Transistor
Hydraulic analogy of a transistor
•
Analogy of technical direction of current in an NPN transistor – to a water
flow in a pipe system
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ST11 – Transistor
Transistor (BJT) as an electronic element
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Variety of shapes and sizes
•
We need a set of equations – a model of the transistor – to be used in circuit
theory
– We have three terminals (collector, base, emitter), and
correspondingly we have
– three possible voltages (VBE, VCB, VCE - but only two are
independent due to KVL); and
– three currents (IB, IC, IE - but only two are independent
due to KCL)
that can exist for this element.
•
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ST11 – Transistor
Transistor (BJT) as an electronic element
•
Common large-signal transistor models
– Gummel–Poon model
– Ebers–Moll model
– H-Parameter model

V BE
I E  I ES  e VT


•
Ebers–Moll model

 1


–
IES is the reverse saturation current of
the base-emitter diode
–
IC is the collector current
–
αF is the common base forward short
circuit current gain (0.98 to 0.998)
 VBE

VT

I C   T  I ES e  1




F 
T
1  T
Simpler model - A group of a transistor’s parameters sufficient to predict
circuit gain, input impedance, and output impedance is also referred to as its
small-signal model.
– Simplest small-signal model:
• Can model BE junction with a resistor,
but we also modeled the diode as a switch
• Dependent current source to model
IC = hFE* IB
• Should be made more accurate....
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ST11 – Transistor
Transistor (BJT) as an electronic element
In a real transistor:
•
•
•
yellow shaded area represents the
'cut-off' region:
–
zero input base current, zero output collector current; and
–
maximum (supply rail) collector voltage.
red shaded area is
'saturation' region:
–
Max base current is applied, resulting in max
collector current flow; and
–
minimum collector emitter voltage. (VCE
~ 0)
Our model:
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ST11 – Transistor
Transistor (BJT) as an electronic element
Regions of BJT operation
•
Cut-off region: The transistor is off. There is no conduction between the
collector and the emitter. (IB = 0 therefore IC = 0)
•
Active region: The transistor is on. The collector current is proportional to
and controlled by the base current (IC = βIB) and relatively insensitive to
VCE. In this region the transistor can be an amplifier.
•
Saturation region: The transistor is on. The collector current varies very little
with a change in the base current in the saturation region. The VCE is small,
a few tenths of volt. The collector current is strongly dependent on VCE
unlike in the active region. It is desirable to operate transistor switches will
be in or near the saturation region when in their on state.
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ST11 – Transistor
Transistor (BJT) as an electronic element
Rules for Bipolar Junction Transistors (BJTs):
• For an npn transistor, the potential at the collector VC must be greater than
the potential at the emitter VE by at least a few tenths of a volt; otherwise, current
will not flow through the C-E junction, no matter what the applied voltage at the base.
•
For the npn transistor, there is a voltage drop from the base to the emitter of
0.6 V. In terms of operation, this means that the base voltage VB of an npn transistor must be at
least 0.6 V greater that the emitter voltage VE; otherwise, the transistor will not pass
emitter-to-collector current.
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ST11 – Transistor
Measuring (testing) transistors
For some transistors, the pin function can be identified from packaging:
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ST11 – Transistor
Measuring (testing) transistors
But sometimes, we have to measure: Set a digital multimeter to diode test and
an analogue multimeter to a low resistance range such as × 10, as
described above for testing a diode.
• Test each pair of leads both ways (six tests in total):
• The base-emitter (BE) junction should behave like a diode and conduct one
way only.
•
The base-collector (BC) junction should behave like a diode and conduct
one way only.
•
The collector-emitter (CE) should not conduct either way.
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ST11 – Transistor
FET transistors
•
•
•
•
•
The FET is a three terminal device like the BJT, but operates by a different
principle.
The three terminals are called the source, drain, and gate.
The voltage applied to the gate controls the current flowing in the sourcedrain channel.
No current flows through the gate electrode, thus the gate is essentially
insulated from the source-drain channel.
Because no current flows through the gate, the input impedance of the FET
is extremely large (in the range of 1010–1015 Ω). The large input impedance of the
FET makes them an excellent choice for amplifier inputs.
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ST11 – Transistor
Transistor construction
•
Transistors, like diodes, are produced using a variety of chemical processes.
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ST11 – Transistor
Basic circuits
We used a circuit like this to discuss
transistor properties
– but there are no resistances in neither look
A nor loop B – short circuit!
So, this is not a practical example....
•
Using a single resistor in the emitter is a
practical example, but not interesting
for us:
- since it cannot give any voltage
amplification:
Vi - Vbe = Vo
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ST11 – Transistor
Common emitter amplifier
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A minimal configuration that can provide voltage amplification
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ST11 – Transistor
Common emitter amplifier
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The ranges of our analysis:
If values are replaced:
Amplification factor:
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ST11 – Transistor
Common emitter amplifier
•
For practical applications, this simplest form of the common emitter amplifier
is rarely used; something like below is more common:
•
common emitter amplifier also has equivalent input resistance (looking into
the base):
– Multiplied by factor beta
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ST11 – Transistor
Current source and current mirror
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Current source – simply polarize CE amplifier
Current mirror
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ST11 – Transistor
Astable multivibrator
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•
•
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an oscillator circuit, that generates a pair of
complementary square-wave voltages. As such,
it also be seen as a digital clock - or more
generally, switching, or logic circuit
Only one transistor can conduct at a time
When supply voltage, VCC is applied, one
transistor will conduct more than the other
due to some circuit imbalance.
Q1 is conducting (ON) and Q2 is cut-off (OFF).
– C1 charges exponentially with t=R1C1 towards the supply voltage
through R1 → VB2 also increases exponentially towards VCC.(VC1=0.2)
– When VB2 crosses the coupling voltage, Q2 starts conducting and VC2
falls to VCESAT → VB1 falls due to capacitive coupling between collector
of Q2 and base of Q1 → driving Q1 into OFF state.
– Now Q1 OFF and Q2 ON – the process repeats
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ST11 – Transistor
Differential amplifier
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Important – the basis of a differential amplifier
•
•
Amplifies differential signals, rejects common signals
When differential signal is applied to the inputs: this will incrementally
increase and decrease the base voltages to VB1 + ΔV and VB2 - ΔV
– Because Q1 conducts a little more and Q2 a little less, IE now splits
unevenly creating IC1 > IC2 → forces the voltage at VC1 to decrease
and VC2 to increase. The result: a voltage change at each output
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ST11 – Transistor
Differential amplifier
•
suppose a common-mode input signal is applied:
it incrementally increases both inputs to
VB1 + ΔV and VB2 + ΔV
– Because the conduction level of neither
transistor has changed (both bases and
emitters moved by the same amount),
the collector currents did not change.
IC1 = IC2 ≈ IE / 2.
– Subsequently, the voltages at VC1 and VC2 remain the same!
Therefore, the circuit has rejected a signal common to both inputs.
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ST11 – Transistor
Sensing application - Phototransistor
•
A phototransistor is in essence nothing more than a bipolar transistor that is
encased in a transparent case, so that light can reach the base-collector
junction.
– The phototransistor works like a photodiode, but with a much higher
sensitivity for light, because the electrons that are generated by photons
in the base-collector junction are injected into the base, and this current
is then amplified by the transistor operation.
– However, a phototransistor has a slower response time than a
photodiode.
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