Electronics III by Engr. James
LECTURE NOTE ON ELECTRONICS III (EEE314)
COMPILED
BY
ENGR. JOHN ADUMA JAMES
Electronics III by Engr. James
SECTION 1 THYRISTOR, SCR
Thyristors or silicon controlled rectifiers (SCR) as they are sometimes known may appear to be
unusual electronics components in many ways, but they are particularly useful for controlling power
circuits.
As such these electronics components are used for many power control applications, often where
current and voltage levels are relatively high. Thyristors may also be used in lower power
applications including light control, as well as for power supply protection and many other
applications. Thyristors are simple to use and cheap to buy, making them an ideal option for many
circuits.
The idea for the thyristor is not new. The idea for the device was first put forward in 1950 by
William Shockley, one of the inventors of the transistor. Although some later investigation of the
device was undertaken by others a couple of years later, it was not until the early 1960s when they
became available. After the introduction of the thyristor, they soon became popular for electronic
switching and power supply circuits.
A high current thyristor / SCR
The thyristor may be considered a rather an unusual form of electronics component because it
consists of four layers of differently doped silicon rather than the three layers of the conventional
bipolar transistors.
Whereas conventional bipolar transistors may have a p-n-p or n-p-n structure with the electrodes
named collector, base and emitter, the thyristor has a p-n-p-n structure with the outer layers with
their electrodes referred to as the anode (n-type) and the cathode (p-type). The control terminal of
the SCR is named the gate and it is connected to the p-type layer that adjoins the cathode layer.
Basic structure of a thyristor / SCR
Thyristors are usually manufactured from silicon, although, in theory other types of semiconductor
could be used. The first reason for using silicon for thyistors is that silicon is the ideal choice
because of its overall properties. It is able to handle the voltage and currents required for high
power applications. Additionally it has good thermal properties. The second major reason is that
silicon technology is well established and it is widely used for a variety of semiconductor devices.
Electronics III by Engr. James
As a result it is very cheap and easy for semiconductor manufacturers to use for their electronic
components.
Thyristor applications
Thyristors, or silicon controlled rectifiers, SCRs are used in many areas of electronics where they
find uses in a variety of different applications. Some of the more common applications for them are
outlined below:
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AC power control (including lights, motors,etc).
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AC power electronic switching.
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Overvoltage protection crowbar for power supplies.
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Control elements in phase angle triggered controllers.
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Within photographic flash lights where they act as the electronic switch to discharge a stored
voltage through the flash lamp, and then cut it off at the required time.
Thyristors are able to switch high voltages and withstand reverse voltages making them ideal for
electronic switching applications, especially within AC scenarios.
Thyristor discovery
The idea for the thyristor was first described by Shockley in 1950. It was referred to as a bipolar
transistor with a p-n hook-collector. The mechanism for the operation was analysed further in 1952
by Ebers.
Then in 1956 Moll investigated the switching mechanism of the thyristor. Development continued
and more was learned about the device such that the first silicon controlled rectifiers became
available in the early 1960s where it started to gain a significant level of popularity for power
switching.
When GE launched their devices, they used the term silicon controlled rectifier, or SCR, because it
only conducted in one direction and was controllable. They used the name SCR as a trade mark for
their products.
Working principle of thyristor
Electronics III by Engr. James
The way in which a thyristor operates is different to other devices. Normally no current flows
across the device. However if a supply is connected across the device, and a small amount of
current is injected into the gate, then the device will "fire" and conduct. It will remain in the
conducting state until the supply is removed.
To see how the thyristor operates, it is worth looking at a thyristor equivalent circuit. For the sake of
an explanation, the thyristor circuit can be considered as two back to back transistors. The first
transistor with its emitter connected to the cathode of the thyristor is an NPN transistor, whereas a
second transistor with its emitter connected to the anode of the thyristor, SCR is a PNP transistor.
The gate is connected to the base of the NPN transistor as shown below.
Thyristor symbols & basics
The thyristor or silicon controlled rectifier, SCR is a semiconductor device that has a number of
unusual characteristics. It has three terminals: Anode, cathode and gate, reflecting thermionic valve
/ vacuum tube technology as shown above. As might be expected the gate is the control terminal
while the main current flows between the anode and cathode.
As can be imagined from its circuit symbol shown below, the device is a "one way device" giving
rise to the GE name of silicon controlled rectifier. Therefore when the device is used with AC, it
will only conduct for a maximum of half the cycle.
In operation, the thyristor or SCR will not conduct initially. It requires a certain level of current to
flow in the gate to "fire" it. Once fired, the thyristor will remain in conduction until the voltage
across the anode and cathode is removed - this obviously happens at the end of the half cycle over
which the thyristor conducts. The next half cycle will be blocked as a result of the rectifier action. It
will then require current in the gate circuit to fire the SCR again. In this way the thyristor can be
used as an electronic switch.
The silicon controlled rectifier, SCR or thyristor symbol used for circuit diagrams or circuit seeks to
emphasis its rectifier characteristics while also showing the control gate. As a result the thyristor
symbol shows the traditional diode symbol with a control gate entering near the junction.
Thyristor Circuits & Design:
Thyristors or SCRs have the characteristic that when the gate receives a triggering current, it fires
the thyristor enabling current to flow until the voltage between anode and cathode is removed. This
enables the thyristor to switch high voltages and currents, although it only over one half of the
cycle. Circuits can dim lights, control motors and generally switch high voltages and currents.
Other types of thyristor or SCR
There is a number of different types of thyristor - these are variants of the basic component, but
they offer different capabilities that can be used in various instances and may be useful for certain
circuits.
Electronics III by Engr. James
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Reverse conducting thyristor, RCT: Although thyristors normally block current in the reverse
direction, there is one form called a reverse conducting thyristor which has an integrated reverse
diode to provide conduction in the reverse direction, although there is no control in this
direction.
Within a reverse conducting thyristor, the device itself and the diode do not conduct at the same
time. This means that they do not produce heat simultaneously. As a result they can be integrated
and cooled together.
The RCT can be used where a reverse or freewheel diode would otherwise be needed. Reverse
conducting thyristors are often used in frequency changers and inverters.
Gate Assisted Turn-Off Thyristor, GATT: The GATT is used in circumstances where a fast
turn-off is needed. To assist in this process a negative gate voltage can sometimes be applied. In
addition to reducing the anode cathode voltage. This reverse gate voltage helps in draining the
minority carriers stored on the n-type base region and it ensures that the gate-cathode junction is
not forward biased
The structure of the GATT is similar to that of the standard thyristor, except that the narrow
cathode strips are often used to enable the gate to have more control because it is closer to the
centre of the cathode.
Gate Turn-Off Thyristor, GTO: The GTO is sometimes also referred to as the gate turn off
switch. This device is unusual in the thyristor family because it can be turned off by simply
applying a negative voltage to the gate - there is no requirement to remove the anode cathode
voltage. See further page in this series more fully describing the GTO.
Asymmetric Thyristor: This device is used in circuits where the thyristor does not see a reverse
voltage and therefore the rectifier capability is not needed. As a result it is possible to make the
second junction, often referred to as J2 (see page on the device structure) can be made much
thinner. The resulting n-base region provides a reduced Von as well as improved turn on time and
turn off time.
Application of Thyristor
Thyristors are widely used in many areas of electronics acting as electronic switches. Thyristor
circuits can be used for many power applications as these electronics components are able to switch
high currents very easily. In addition to this these they are very cheap and they are widely available.
Basic thyristor / SCR structure
The thyristor consists of a four layer PNPN structure with the outer layers are referred to as the
anode (P-type) and cathode (N-type). The control terminal of the thyristor is named the gate and it
is connected to the P-type layer located next to the cathode.
Electronics III by Engr. James
As a result the thyristor has three junctions rather than the one junction of a diode, and two within
transistors.
The three junctions are normally denoted as J1, J2, and J3. They are numbered serially with J1 being
nearest to the anode.
Thyristor / SCR materials
Although it is possible to use a variety of different materials for thyristors, silicon is the most
popular. The trade name for this type of device - silicon controlled rectifier - also indicates that
silicon is the most popular material.
Silicon provides good thermal conductivity as well as a high voltage and current capability. Another
advantage is that the processes for silicon are more mature, and hence cheaper to run, than those for
other materials.
However, other materials including silicon carbide, SiC; gallium nitride, GaN; diamond, C; and
semi-wide-gap semiconductor material gallium arsenide, GaAs as well, have been investigated and
according to the research they demonstrated promising properties under extreme conditions of high
power, high temperature and high frequency. Nevertheless silicon still remains the most popular
substance.
Thyristor semiconductor structure and fabrication
The level of doping varies between the different layers of the thyristor. The cathode is the most
heavily doped. The gate and anode are the next heavily doped. The lowest doping level is within the
central N type layer. This is also thicker than the other layers and these two factors enable a large
blocking voltage to be supported. Thinner layers would mean that the device would break down at
lower voltages.
Fabrication structure of a thyristor / SCR
In view of the very high currents and power levels that some thyristors are used to switch, thermal
considerations are of paramount importance. The anode of the SCR or silicon controlled rectifier is
usually bonded to the package since the gate terminal is near the cathode and needs to be connected
separately. This is accomplished in such a way that heat is removed from the silicon to the package.
Apart from the internal considerations, the external heat-sinking considerations for the thyristor
must be carefully implemented otherwise the device may overheat and fail.
Electronics III by Engr. James
Many smaller thyristors are mounted in the standard TO cans, and larger thyristors have much
larger outlines, but are designed to bolt to a heatsink to remove the power. Despite their efficiency,
power is still dissipated and needs to be removed.
Asymmetric thyristor / SCR structure
The asymmetric thyristor is characterised by what is termed a cathode short and an anode short. It
can be seen from the diagram that both the cathode and anode connections connect to N+ and the P
regions in the case of the cathode and the P+ and N regions on the case of the anode.
The "short" between the P and N regions has the effect of adding a resistor between the junctions,
i.e. cathode to gate in the case of the cathode connection. This has a variety of effects including
reducing carrier lifetime and improving the transient response time.
Different structures are used by different manufacturers for different applications. Also silicon is by
far the most widely used material for thyristors.
Thyristors or SCRs, silicon controlled rectifiers are well established and the technology is very
stable. The devices are widely used for power switching, an application to which they are very
suited.
When designing and using thyristor or SCR circuits it helps to understand how the thyristor works.
Essentially the operation of the thyristor / SCR can be explained in terms of a latching switch. Once
tuned on by a current at the gate, it requires the voltage across the cathode and anode to be removed
before it stops conducting.
Thyristor operation: the basics
In operation, the thyristor / SCR has three states in which it can be at any given time:

Reverse blocking: In this mode or state the thyristor blocks the current in the same way as that
of a reverse biased diode. The thyristor / SCR can only conduct in one direction and blocks in
the reverse direction.
Electronics III by Engr. James

Forward blocking: In this mode or state the thyristor operation is such that it blocks forward
current conduction that would normally be carried by a forward biased diode. In this state the
thyristor / SCR is not in its “turn-on” state as the gate has not fired.
 Forward conducting: In this mode the thyristor / SCR has been triggered into conduction by a
current on the gate. It will remain conducting regardless of the state of the gate. Current only
needs to be applied to the gate to fire the thyristor / SCR, and it will remain conducting. The
device will stop conducting when the forward current drops below a threshold value known as
the "holding current."
The thyristor consists of four semiconductor regions: P N P N. The outer P region forms the anode,
and the outer n region forms the cathode as shown below.
Equivalent circuit of a thyristor
This arrangement forms a positive feedback loop within the thyristor. The output of one transistor
fed to the input of the second. In turn the output of the second transistor is fed back to the input of
the first. As a result it can be seen that the total current gain of the device exceeds one. This means
that when a current starts to flow, it quickly builds up until both transistors are fully turned on or
saturated.
When a voltage is applied across a thyristor no current flows because neither transistor is
conducting. As a result there is no complete path across the device. If a small current is passed
through the gate electrode, this will turn "on" the transistor TR2. When this occurs it will cause the
collector of TR2 to fall towards the voltage on the emitter, i.e. the cathode of the whole device.
When this occurs it will cause current to flow through the base of TR1 and turn this transistor "on".
Again this will now try to pull the voltage on the collector of TR1 towards its emitter voltage. This
will cause current to flow in the emitter of TR2, causing its "on" state to be maintained. In this way
it only requires a small trigger pulse on the gate to turn the thyristor on. Once switched on, the
thyristor can only be turned off by removing the supply voltage.
The operation of the thyristor considered in this way is relatively straightforward to understand
The Gate Turn-Off Thyristor, GTO is a variant of the more standard form of thyristor. Rather than
the gate being used to turn the thyristor on, within a gate turn-off thyristor, GTO, the gate pulse
turns the device off.
The additional capabilities of the gate-turn-gg thyristor enable it to be used in applications where a
standard thyristor would not be suitable. Although its use is more limited, it can be used in a
number of specialist applications.
These gate turn-off thyristors are useful in a number of areas, particularly within variable speed
motor drives, high power, inverters and similar areas. Although they are not nearly as well known
as the more standard forms of thyristor, the gate turn off thyristor, is now widely used as it is able to
overcome many of the disadvantages of the traditional thyristor. As a result the gate turn-off
thyristor is used in virtually all DC to AC and DC to DC high voltage conversion units
Gate turn-off thyristor
Electronics III by Engr. James
The capability for the gate turn off thyristor to be turned-on by a gate signal, and also turned-off by
a gate signal of negative polarity give it a unique capability within the thyristor family of devices.
The device turn on is accomplished by a "positive current" pulse between the gate and cathode
terminals. As the gate-cathode behaves like PN junction, there is a relatively small voltage between
the terminals.
The turn on phenomenon in GTO is however, not as reliable as that of a standard thyristor and small
positive gate current must be maintained even after turn on to improve reliability.
Gate turn-off thyristor structure
Like the standard thyristor, the gate turn-off thyristor is a four layer device having three junctions.
Again the layers are P N P N with the outside p layer providing the anode connection, and the
outside n layer providing he cathode connection.
To attain high emitter efficiency, the cathode layer is highly doped to give an n+ region. This has
the drawback that it renders the junction nearest to the cathode (normally referred to as J3) with a
low breakdown voltage - typically 20- 40 volts.
The doping level of the p region for the gate is graded. This is to provide good emitter efficiency for
which the doping level should be low, while providing a good turn off characteristic for which a
high doping level is needed.
The gate electrode is often inter-digitated to optimise the current turn=off capability. High current
devices, i.e. 1000A and above may have several thousand segments which are all connected to the
common gate contact.
Another key parameter for a gate turn-off thyristor is the maximum forward blocking voltage. This
is determined by the doping level and thickness of the n type base region. As many devices may
need to block voltages of several kilovolts, the doping level of this region needs to be kept relatively
low.
Gate turn off thyristor operation
Many aspects of the Gate turnoff thyristor, GTO are very similar to that of the ordinary thyristor. It
can be thought of as being one PNP and one NPN transistor being connected in a regenerative
configuration whereby once turned on the system maintains itself in this state.
Equivalent circuit of a thyristor including a gate turn off thyristor
When a potential is applied across the gate turn-off thyristor between the anode and cathode, no
current will flow because neither device is turned on. Current would only flow if the voltage
exceeded the breakdown voltage and current would flow as a result of avalanche action, but this
mode would not be wanted for normal operation. In this non-conducting state the gate turn-off
thyristor is said to be in its forward blocking mode.
Electronics III by Engr. James
To turn the device on it is necessary to inject current into gate circuit of the device. When this is
done, it turns on TR2 in the diagram. This pulls the collector of this transistor down towards the
emitter voltage and in turn this turns on the other transistor - TR1.
The fact that TR1 is now switched on ensures current flows into the base of TR2, and thus this
feedback process ensures that once the gate turn-off thyristor like any other thyristor is turned on it
remains on.
The key capability of the gate turn-off thyristor is its ability to be turned off by the use of the gate
electrode on the device. The device turn off is achieved by applying a negative bias to the gate with
respect to the cathode. This extracts current from the base region of TR2. The resulting voltage drop
in the base starts to reverse bias the junction and thereby stopping the current flow in this transistor
- TR2.
This then stops the injection into the base region of TR1 and this prevents current flow in this
transistor.
In terms of the physics of the turn off phase, it is found that during the turn off phase of the GTO,
current is crowded into higher and higher density current filaments in areas that are most remote
from the gate region. These high current density areas become hot, and can cause device failure if
the current is not extinguished quickly.
When the current filaments are extinguished, the overall current flow stops and the depletion layers
around the junctions grow - the gate turn-off thyristor enters its forward blocking state again.
Although the gate turn off thyristor has any similarities to the standard thyristor , its chief difference
is that it has the capability of being able to be turned off by voltages on the gate. This provides more
capability for the device and enables the gate turn off thyristor to be used in areas where the
standard thyristor cannot be used. Accordingly the gate turn off thyristor is a useful tool for many
applications.
Triacs are electronic components that are widely used in AC power control applications. They are
able to switch high voltages and high levels of current, and over both parts of an AC waveform.
This makes triac circuits ideal for use in a variety of applications where power switching is needed.
One particular use of triac circuits is in light dimmers for domestic lighting, and they are also used
in many other power control situations including motor control and electronic switches.
As a result of their performance, triacs tend to be used for low to medium power electronic
switching applications, leaving thyristors to be used for the very heat duty AC power switching
applications.
Electronics III by Engr. James
A medium current triac
Triac basics
The triac is a development of the thyristor. While the thyristor can only control current over one
half of the cycle, the triac controls it over two halves of an AC waveform.
As such the triac can be considered as a pair of parallel but opposite thyristors with the two gates
connected together and the anode of one device connected to the cathode of the other, etc..
Triac switching waveform
The fact that the triac switching action occurs on both halves of an AC waveform means that for AC
electronic switching applications, the complete cycle can be used. For basic thyristor circuits, only
half the waveform is used and this means that basic circuits using thyristors will not utilise both
halves of the cycle. Two devices are required to utilise both halves. However the triac only requires
one device to control both halves of the AC waveform and in many respects it is an ideal solution
for an electronic switch for AC.
Triac symbol
Like other electronic components, the triac has its own circuit symbol for use on circuit diagrams
and this indicates its bi-directional properties. The triac symbol can be seen to be a couple of
thyristor symbols in opposite senses merged together.
Triac circuit symbol
Electronics III by Engr. James
Like a thyristor, a triac has three terminals. However the names of these are a little more difficult to
assign, because the main current carrying terminals are connected to what is effectively a cathode of
one thyristor, and the anode of another within the overall device.
There is a gate which acts as a trigger to turn the device on. In addition to this the other terminals
are both called Anodes, or Main Terminals These are usually designated Anode 1 and Anode 2 or
Main Terminal 1 and Main Terminal 2 (MT1 and MT2). When using triacs it is both MT1 and MT2
have very similar properties.
Working principle of traic
Before looking at how a triac works, it helps to have an understanding of how a thyristor works. In
this way the basic concepts can be grasped for the simpler semiconductor device and then applied to
a triac which is more complicated.
For the operation of the triac, it can be imagined from the circuit symbol that the triac consists of
two thyristors in parallel but around different ways. The operation of the triac can be looked on in
this fashion, although the actual operation at the semiconductor level is rather more complicated.
Triac equivalent circuit
The triac structure is shown below and it can be seen that there are several areas of N-type and Ptype material that form what is effectively a pair of back to back thyristors. Triac equivalent circuit
Triac equivalent circuit
Triac equivalent circuit
Triac basic structure
The triac is able to conduct in a number of ways - more than the thyristor. It can conduct current
irrespective of the voltage polarity of terminals MT1 and MT2. It can also be triggered by either
positive or negative gate currents, irrespective of the polarity of the MT2 current. This means that
there are four triggering modes or quadrants:
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I+ Mode MT2 current is +ve, gate current is +ve
I- Mode MT2 current is +ve, gate current is -ve
III+ Mode: MT2 current is -ve, gate current is +ve
III- Mode: MT2 current is -ve, gate current is -ve
Electronics III by Engr. James
It is found that the triac trigger current sensitivity is greatest when the MT2 and gate currents are
both of the same polarity, i.e. both positive or both negative. If the gate and MT2 currents are of the
opposite polarity then the sensitivity is typically about half the value of when they are the same.
The typical IV characteristic of a triac can be seen in the diagram below with the four different
quadrants labelled.
Triac applications
Triacs are used in many applications. These electronic components are often used in low to medium
power AC switching requirements. Where large levels of power need to be switched, two thyristors
/ SCRs tend to be used as they can be controlled more easily.
Never the less triacs are widely used in many applications:
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Lighting control - especially domestic dimmmers.
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Control of fans and small motors.
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Electronic switches for general AC switching and control
There are naturally many other triac applications, but these are some of the most common.
In one specific application, triacs can be included in modules called solid state relays. Here an
optical version of this semiconductor device is activated by an LED light source turning the solid
state relay on according to the input signal.
Typically within solid state relays, the LED light or infrared source and the optical triac are
contained within the same package, sufficient isolation being provided to withstand high voltages
which may extend to hundreds of volts or possibly even more.
Solid state relays come in many forms, but those used for AC switching may use a triac.
Using triacs
There are a number of points to note when using triacs. Although these semiconductor devices
operate very well, to get the best performance out of them it is necessary to understand a few hints
on tips on using triacs.
It is found that because of their internal construction and the slight differences between the two
halves, these electronic components do not fire symmetrically. This results in harmonics being
generated: the less symmetrical the triac fires, the greater the level of harmonics that are produced.
It is not normally desirable to have high levels of harmonics in a power system and as a result triacs
are not favoured for high power systems. Instead for these systems two thyristors may be used as it
is easier to control their firing.
To help in overcoming the problem of the triac non-symmetrical firing, and the resulting harmonics,
another semiconductor device known as a diac (diode AC switch) is often placed in series with the
Electronics III by Engr. James
gate of the triac. The inclusion of this semiconductor device helps make the switching more even
for both halves of the cycle and thereby creating a more effective electronic switch.
This results from the fact that the diac switching characteristic is far more even than that of the
triac. Since the diac prevents any gate current flowing until the trigger voltage has reached a certain
voltage in either direction, this makes the firing point of the triac more even in both directions.
Internal circuitry of triac light dimmer
Triac circuit examples
There are many ways in which triacs can be used. The two examples below give a taste of what can
be done with these semiconductor devices.
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Simple triac electronic switch circuit: The triac can function as an electronic switch - it could
enable a trigger pulse of a low power switch to turn the triac on to control a much higher power
levels that might be possible with a simple switch. Simple triac switch circuit
Triac variable power or dimmer circuit: One of the most popular triac circuits varies the phase
on the input of the triac to control the power that can be dissipated into load.
A basic triac circuit using phase of input waveform to control dissipated power in the load
There are many more triac circuits that can be used. The device is very versatile and can be used in
a variety of circuits, typically to provide various forms of AC switching.
Triacs are ideal devices for use in many AC small power applications. Triac circuits for use as
dimmers and small electronic switches are widespread and they are simple and easy to implement.
When using triacs, diacs are often included in the circuit as mentioned above to help reduce the
level of harmonics produced.
Diac
A DIAC is a full-wave or bi-directional semiconductor switch that can be turned on in both forward
and reverse polarities.
The name DIAC comes from the words DIode AC switch. The DIAC is an electronics component
that is widely used to assist even triggering of a TRIAC when used in AC switches and as a result
they are often found in light dimmers such as those used in domestic lighting. These electronic
components are also widely used in starter circuits for fluorescent lamps.
Electronics III by Engr. James
Although the term is not often seen, DIACs may also be called symmetrical trigger diodes - a term
resulting from the symmetry of their characteristic curve.
DIACs come in a variety of formats. As discrete components they may be contained in small leaded
packages, they can be obtained in surface mount packages, in large packages that bolt to a chassis,
or a variety of other packages. As they are often used as a DIAC TRIAC combination, they are
often integrated into the same die as a TRIAC.
DIAC symbol
The DIAC symbol used to depict this electronic component in circuit diagrams can be remembered
as a combination of what may appear to be two diodes in parallel with each other but connected in
opposite directions.
DIAC circuit symbol
Owing to the fact that DIACs are bi-direction devices the terminals cannot be labelled as anode and
cathode as they are for a diode. Instead they may be labelled as A1 and A2 or MT1 and MT2, where
MT stands for "Main Terminal."
DIAC operation
DIAC circuits use the fact that a DIAC only conducts current only after a certain breakdown voltage
has been exceeded. The actual breakdown voltage will depend upon the specification for the
particular component type.
When the DIAC breakdown voltage occurs, the resistance of the component decreases abruptly and
this leads to a sharp decrease in the voltage drop across the DIAC, and a corresponding increase in
current. The DIAC will remain in its conducing state until the current flow through it drops below a
particular value known as the holding current. When the current falls below the holding current, the
DIAC switches back to its high resistance, or non-conducting state.
DIACs are widely used in AC applications and it is found that the device is "reset" to its nonconducting state, each time the voltage on the cycle falls so that the current falls below the holding
current. As the behaviour of the device is approximately equal in both directions, it can provide a
method of providing equal switching for both halves of an AC cycle, e.g. for TRIACs.
Most DIACs have a breakdown voltage of around 30 volts, although the exact specifications will
depend upon the particular type of device. Interestingly their behaviour is somewhat similar to that
of a neon lamp, although they offer a far more precise switch on voltage and thereby provide a far
better degree of switching equalisation.
Electronics III by Engr. James
Diac structure
The DIAC can be fabricated as either a two layer or a five layer structure. In the three layer
structure the switching occurs when the junction that is reverse biased experiences reverse
breakdown. The three layer version of the device is the more common and can have a break-over
voltage of around 30 V. Operation is almost symmetrical owing to the symmetry of the device.
A five layer DIAC structure is also available. This does not act in quite the same manner, although
it produces an I-V curve that is very similar to the three layer version. It can be considered as two
break-over diodes connected back to back.
The structure of a DIAC
For most applications a three layer version of the DIAC is used. It provides sufficient improvement
in switching characteristics. For some applications the five layer device may be used.
DIAC applications
One of the major uses of DIACs within TRIAC circuits. TRIACs do not fire symmetrically as a
result of slight differences between the two halves of the device.
The non-symmetrical firing and resulting waveforms give rise to the generation of unwanted
harmonics – the less symmetrical the waveform the greater the level of harmonic generation.
DIAC TRIAC COMBINATION
To resolve the issues resulting from the non-symmetrical operation, a DIAC is often placed in series
with the gate. This device helps make the switching more even for both halves of the cycle. This
results from the fact that the DIAC switching characteristic is far more even than that of the TRIAC.
Since the DIAC prevents any gate current flowing until the trigger voltage has reached a certain
voltage in either direction, this makes the firing point of the TRIAC more even in both directions. In
view of their usefulness, DIACs may often be built into the gate terminal of a TRIAC.
DIACs are a widely used electronic component. The chief application of DIACs is for use in
conjunction with TRIACs to equalise their switching characteristics. By equalising the switching
characteristics of these TRIACs, the level of harmonics generated when switching AC signals can
be reduced. Despite this, for large applications, two thyristors are generally used. Nevertheless the
DIAC / TRIAC combination is very useful for lower power applications including light dimmers,
etc
Electronics III by Engr. James
SECTION II UNIJUNCTION TRANSISTOR
Unijunction transistor: Although a unijunction transistor is not a thyristor, this device can trigger
larger thyristors with a pulse at base B1. A unijunction transistor is composed of a bar of N-type
silicon having a P-type connection in the middle. See Figure (a). The connections at the ends of the
bar are known as bases B1 and B2; the P-type mid-point is the emitter. With the emitter
disconnected, the total resistance RBBO, a datasheet item, is the sum of RB1 and RB2 as shown in
Figure (b). RBBO ranges from 4-12kΩ for different device types. The intrinsic standoff ratio η is the
ratio of RB1 to RBBO. It varies from 0.4 to 0.8 for different devices. The schematic symbol is Figure
(c)
Unijunction transistor: (a) Construction, (b) Model, (c) Symbol
The Unijunction emitter current vs voltage characteristic curve (Figure(a) below ) shows that as
VE increases, current IE increases up IP at the peak point. Beyond the peak point, current increases
as voltage decreases in the negative resistance region. The voltage reaches a minimum at the valley
point. The resistance of RB1, the saturation resistance is lowest at the valley point.
IP and IV, are datasheet parameters; For a 2n2647, IP and IV are 2µA and 4mA, respectively. [AMS]
VP is the voltage drop across RB1 plus a 0.7V diode drop; see Figure(b) below. VV is estimated to
be approximately 10% of VBB.
Electronics III by Engr. James
Unijunction transistor: (a) emitter characteristic curve, (b) model for VP .
The relaxation oscillator is an application of the unijunction oscillator. R E charges CE until the peak
point. The unijunction emitter terminal has no effect on the capacitor until this point is reached.
Once the capacitor voltage, VE, reaches the peak voltage point VP, the lower emitter-base1 E-B1
resistance quickly discharges the capacitor. Once the capacitor discharges below the valley point
VV, the E-RB1 resistance reverts back to high resistance, and the capacitor is free to charge again.
Unijunction transistor relaxation oscillator and waveforms. Oscillator drives SCR.
During the capacitor discharge through the E-B1 saturation resistance, a pulse may be seen on the
external B1 and B2 load resistors, Figure above. The load resistor at B1 needs to be low to not
affect the discharge time. The external resistor at B2 is optional. It may be replaced by a short
circuit. The approximate frequency is given by 1/f = T = RC. A more accurate expression for
frequency is given in the Figure above.
Electronics III by Engr. James
The charging resistor RE must fall within certain limits. It must be small enough to allow IP to flow
based on the VBB supply less VP. It must be large enough to supply IV based on the VBB supply less
VV. [MHW] The equations and an example for a 2n2647:
Programmable Unijunction Transistor (PUT): Although the unijunction transistor is listed as
obsolete (read expensive if obtainable), the programmable unijunction transistor is alive and well. It
is inexpensive and in production. Though it serves a function similar to the unijunction transistor,
the PUT is a three terminal thyristor. The PUT shares the four-layer structure typical of thyristors
shown in Figure below. Note that the gate, an N-type layer near the anode, is known as an “anode
gate”. Moreover, the gate lead on the schematic symbol is attached to the anode end of the symbol.
Programmable unijunction transistor: Characteristic curve, internal construction, schematic
symbol.
The characteristic curve for the programmable unijunction transistor in Figure above is similar to
that of the unijunction transistor. This is a plot of anode current IA versus anode voltage V A. The
gate lead voltage sets, programs, the peak anode voltage V P. As anode current increases, voltage
increases up to the peak point. Thereafter, increasing current results in decreasing voltage, down to
the valley point.
Electronics III by Engr. James
The PUT equivalent of the unijunction transistor is shown in Figure below. External PUT resistors
R1 and R2 replace unijunction transistor internal resistors R B1 and RB2, respectively. These resistors
allow the calculation of the intrinsic standoff ratio η.
PUT equivalent of unijunction transistor
Figure below shows the PUT version of the unijunction relaxation oscillator. Resistor R charges the
capacitor until the peak point, then heavy conduction moves the operating point down the negative
resistance slope to the valley point. A current spike flows through the cathode during capacitor
discharge, developing a voltage spike across the cathode resistors. After capacitor discharge, the
operating point resets back to the slope up to the peak point.
PUT relaxation oscillator
Problem: What is the range of suitable values for R in Figure above, a relaxation oscillator? The
charging resistor must be small enough to supply enough current to raise the anode to V P the peak
Electronics III by Engr. James
point while charging the capacitor. Once VP is reached, anode voltage decreases as current
increases (negative resistance), which moves the operating point to the valley. It is the job of the
capacitor to supply the valley current IV. Once it is discharged, the operating point resets back to the
upward slope to the peak point. The resistor must be large enough so that it will never supply the
high valley current IP. If the charging resistor ever could supply that much current, the resistor
would supply the valley current after the capacitor was discharged and the operating point would
never reset back to the high resistance condition to the left of the peak point.
We select the same VBB=10V used for the unijunction transistor example. We select values of R1
and R2 so that η is about 2/3. We calculate η and VS. The parallel equivalent of R1, R2 is R G,
which is only used to make selections from Table below. Along with VS=10, the closest value to
our 6.3, we find VT=0.6V and calculate VP.
We also find IP and IV, the peak and valley currents, respectively in the Table. We still need V V, the
valley voltage. We used 10% of VBB= 1V, in the previous unijunction example. Consulting the
datasheet, we find the forward voltage V F=0.8V at IF=50mA. The valley current IV=70µA is much
less than IF=50mA. Therefore, VV must be less than VF=0.8V. How much less? To be safe we set
VV=0V. This will raise the lower limit on the resistor range a little.
Choosing R > 143k guarantees that the operating point can reset from the valley point after
capacitor discharges. R < 755k allows charging up to V P at the peak point.
Electronics III by Engr. James
Selected 2n6027 PUT parameters, adapted from 2n6027 datasheet. [ON1]
Parameter
Conditions
min
typical
max
VT
V
VS=10V, R G=1Meg
0.2
0.7
1.6
VS=10V, R G=10k
0.2
0.35
0.6
µA
IP
VS=10V, R G=1Meg
-
1.25
2.0
VS=10V, R G=10k
-
4.0
5.0
µA
IV
VF
units
VS=10V, R G=1Meg
-
18
50
VS=10V, R G=10k
70
150
-
VS=10V, R G=200Ω
1500
-
-
IF=50mA
-
0.8
1.5
V
Figure below shows the PUT relaxation oscillator with the final resistor values. A practical
application of a PUT triggering an SCR is also shown. This circuit needs a V BB unfiltered supply
(not shown) divided down from the bridge rectifier to reset the relaxation oscillator after each
power zero crossing. The variable resistor should have a minimum resistor in series with it to
prevent a low pot setting from hanging at the valley point.
PUT relaxation oscillator with component values. PUT drives SCR lamp dimmer.
PUT timing circuits are said to be usable to 10kHz. If a linear ramp is required instead of an
exponential ramp, replace the charging resistor with a constant current source such as a FET based
constant current diode. A substitute PUT may be built from a PNP and NPN silicon transistor by
omitting the cathode gate and using the anode gate.
Electronics III by Engr. James
SECTION III FIELD EFFECT TRANSISTOR – THE BASICS
Electronics III by Engr. James
The concept of the field effect transistor is based around the concept that charge on a nearby object
can attract charges within a semiconductor channel. It essentially operates using an electric field
effect - hence the name.
The FET consists of a semiconductor channel with electrodes at either end referred to as the drain
and the source.
A control electrode called the gate is placed in very close proximity to the channel so that its
electric charge is able to affect the channel.
In this way, the gate of the FET controls the flow of carriers (electrons or holes) flowing from the
source to drain. It does this by controlling the size and shape of the conductive channel.
The semiconductor channel where the current flow occurs may be either P-type or N-type. This
gives rise to two types or categories of FET known as P-Channel and N-Channel FETs.
In addition to this, there are two further categories. Increasing the voltage on the gate can either
deplete or enhance the number of charge carriers available in the channel. As a result there are
enhancement mode FET and depletion mode FETs.
Junction FET circuit symbol
As it is only the electric field that controls the current flowing in the channel, the device is said to
be voltage operated and it has a high input impedance, usually many megohms. This can be a
distinct advantage over the bipolar transistor that is current operated and has a much lower input
impedance.
FET circuits
Field effect transistors are widely used in all forms of electronic circuit designs from those used in
circuits with discrete electronic components, to those employed in integrated circuits.
As the field effect transistor is a voltage operated semiconductor device rather than a current device
like the bipolar transistor, this means that some aspects of the circuit are very different: the bias
arrangements in particular. However electronic circuit design with FETs is relatively easy - it is just
a bit different to that using bipolar transistors.
Using FETs, circuits like voltage amplifiers, buffers or current followers, oscillators, filters and
many more can all be designed, and the circuit designs are very similar to those for bipolar
transistors and even thermionic valves / vacuum tubes, although the bias arrangements are different.
Interestingly valves / tubes are also voltage operated devices, and therefore their circuit designs are
very similar, even in terms of the bias arrangements.
Field Effect Transistor types
Electronics III by Engr. James
There are many ways to define the different types of FET that are available. The different types
mean that during the electronic circuit design, there is a choice of the right electronic component for
the circuit. By selecting the right device it is possible to obtain the best performance for the given
circuit.
FETs may be categorised in a number of ways, but some of the major types of FET can be covered
in the tree diagram below.
Types of field effect transistor
There are many different types of FET on the market for which there are various names. Some of
the major categories are delayed below.

Junction FET, JFET: The junction FET, or JFET uses a reverse biased diode junction to
provide the gate connection. The structure consists of a semiconductor channel which can be
either N-type or P-type. A semiconductor diode is then fabricated onto the channel in such a way
that the voltage on the diode affects the FET channel.
In operation this is reverse biased and this means that it is effectively isolated from the channel only the diode reverse current can flow between the two. The JFET is the most basic type of
FET, and the one that was first developed. However it still provides excellent service in many
areas of electronics.

Insulated Gate FET / Metal Oxide Silicon FET MOSFET: The MOSFET uses an insulated
layer between the gate and the channel. Typically this is formed from a layer of oxide of the
semiconductor.
The name IGFET refers to any type of FET that has an insulated gate. The most common form
of IGFET is the silicon MOSFET - Metal Oxide Silicon FET. Here, the gate is made of a layer of
metal set down on the silicon oxide which in turn is on the silicon channel. MOSFETs are
widely used in many areas of electronics and particularly within integrated circuits.
The key factor of the IGFET / MOSFET is the exceedingly gate high impedance these FETs are
able to provide. That said, there will be an associated capacitance and this will reduce the input
impedance as the frequency rises.

Dual Gate MOSFET: This is a specialised form of MOSFET that has two gates in series along
the channel. This enables some considerable performance improvements to be made, especially
at RF, when compared to single gate devices.
The second gate of the MOSFET provides additional isolation between the input and output, and
in addition to this it can be used in applications like mixing / multiplication.

MESFET: The MEtal Silicon FET is normally fabricated using Gallium Arsenide and is often
referred to as a GaAs FET. Often GaAsFETs are used for RF applications where they can
provide high gain low noise performance. One of the drawbacks of GaAsFET technology results
from the very small gate structure, and this makes its very sensitive to damage from static, ESD.
Great care must be taken when handling these devices.
HEMT / PHEMT: The High Electron Mobility Transistor and Pseudomorphic High Electron
Mobility Transistor are developments of the basic FET concept, but developed to enable very

Electronics III by Engr. James
high frequency operation. Although expensive, they enable very high frequencies and high levels
of performance to be achieved.

FinFET: FinFET technology is now being used within integrated circuits to enable higher
levels of integration to be achieved by allowing smaller feature sizes. As higher density levels
are needed and it becomes increasingly difficult to realise ever smaller feature sizes, FinFET
technology is being used more widely.
 VMOS: VMOS standard for vertical MOS. It is a type of FET that uses a vertical current flow
to improve the switching and current carrying performance. VMOS FETs are widely used for
power applications.
Although there are some other types of field effect transistor that may be seen in the literature, often
these types are trade names for a particular technology and they are variants of some of the FET
types listed above.
Application of FET
Field affect transistor technology can be used in a number of areas where bipolar transistors are not
as suitable: each of these semiconductor devices has its own advantages and disadvantages, and can
be used to great effect in many circuits. The field effect transistor has a very high input impedance
and is a voltage driven device and this opens it up to being used in many areas.
Electronics III by Engr. James
SECTION IV PHOTODIODE
A photodiode is a semiconductor device with a P-N junction that converts photons (or light) into
electrical current. The P layer has an abundance of holes (positive), and the N layer has an
abundance of electrons (negative). Photodiodes can be manufactured from a variety of materials
including, but not limited to, Silicon, Germanium, and Indium Gallium Arsenide. Each material
uses different properties for cost benefits, increased sensitivity, wavelength range, low noise levels,
or even response speed.
Figure 1 shows a cross section of a typical photodiode. A Depletion Region is formed from
diffusion of electrons from the N layer to the P layer and the diffusion of holes from the P layer to
the N layer. This creates a region between the two layers where no free carriers exist. This develops
a built-in voltage to create an electric field across the depletion region. This allows for current to
flow only in one direction (Anode to Cathode). The photodiode can be forward biased, but current
generated will flow in the opposite direction. This is why most photodiodes are reversed biased or
not biased at all. Some photodiodes cannot be forward biased without
damage.
A photon can strike an atom within the device and release an electron if the photon has enough
energy. This creates an electron-hole pair (e- and h+) where a hole is simply an “empty space” for
an electron. If photons are absorbed in either the P or N layers, the electron hole pairs will be
recombined in the materials as heat if they are far enough away (at least one diffusion length) from
the depletion region. Photons absorbed in the depletion region (or close to it) will create electron
hole pairs that will move to opposite ends due to the electric field. Electrons will move toward the
positive potential on the Cathode, and the holes will move toward the negative potential on the
Anode. These moving charge carriers form the current (photocurrent) in the photodiode. Figure
1 shows the different layers of a photodiode (P-N Junction) as well as multiple connection points on
top and bottom.
Figure 1. P-N Photodiode Cross-section
The depletion region creates a capacitance in the photodiode where the boundaries of the region act
SiO2
Anode (+)
AR Coating
P+ Active Area
Depletion Region
N- Type Substrate
N+
Contact Metal
Cathode (-)
Fig1
as the plates of a parallel plate capacitor. Capacitance is inversely proportional to the width of the
Electronics III by Engr. James
depletion region. Reverse bias voltage also influences the capacitance of the region.
KEY PERFORMANCE SPECIFICATIONS
There are four major parameters used in choosing the right photodiode and whether or not to
reverse bias the photodiode.

Response (speed/time) of the photodiode is determined by the capacitance of the P-N
junction. It is the time needed for charge carriers to cross the P-N junction. This is directly
affected by the width of the depletion region.
 Responsivity is the ratio of photocurrent generated from incident light, to that incident light
power. This is usually expressed in units of A/W (current over power). A typical responsivity
curve of a photodiode will show A/W as a function of wavelength. This is called Quantum
Efficiency.
 Dark current is the current in the photodiode when there is no incident light. This can be one
of the main sources of noise in the photodiode system. Photocurrent from background
radiation can also be included in this measurement. Photodiodes are usually put into
an enclosure that does not allow any light to hit the photodiode to measure the dark current.
Because the current generated by the photodiode can be very small, dark current levels can
obscure the current produced by incident light at low light levels. Dark current increases with
temperature. Without biasing, the dark current can be very low. The ideal photodiode would
have no dark current.
 Breakdown Voltage is the largest reverse voltage that can be applied to the photodiode before
there is an exponential increase in leakage current or dark current. Photodiodes should be
operated below this maximum applied reverse bias or damage to the photodiode may occur.
Breakdown voltage decreases with an increase in temperature.
Other important parameters include material, size of the photodiode and active area, and cost.
Careful consideration is needed when browsing for photodiodes for your research or application.
Photodiodes made from different materials (silicon, germanium, indium gallium arsenide
phosphide, or indium gallium arsenide) have varying levels of sensitivity as well as differing speeds
and dark current. Silicon, for example, provides sensitivity for wavelengths between ~400 and 1000
nm. However, it has the highest sensitivity at higher wavelengths (~900 nm). Germanium, on the
other hand, provides sensitivity for wavelengths between ~800 to 1600 nm (with the peak ~1400
nm). The material of the photodiode is critical when finding the right photodiode to incorporate into
your laser diode system.
TYPES OF PHOTODIODES
P-N JUNCTION
This is the most basic photodiode. The physics of how the P-N junction photodiode operates was
reviewed earlier. The PIN and APD photodiode are variations from the P-N junction.
The depletion region contains few free charge carriers, and the width of the depletion region can be
manipulated by adding voltage bias.
Electronics III by Engr. James
Current passing through the photodiode can only flow in one direction based on the P and N doped
materials. If reverse biased, current will not flow through a photodiode without incident light
creating photocurrent.
PIN PHOTODIODE
The PIN photodiode is similar to the P-N Junction with one major difference. Instead of placing the
P and N layers together to create a depletion region, an intrinsic layer is placed between the two
doped layers. This layer is shown in Figure 2. This intrinsic layer is highly resistive and increases
the electric field strength in the photodiode. There are many benefits to the added intrinsic layer
because the depletion region is greatly increased.
The capacitance of the junction is decreased, and so the speed of the photodiode increased. The
increased layer also allows for a larger volume of photon to electron-hole conversion and higher
Quantum Efficiency.
SiO2
Anode (+)
AR Coating
P+ Active Area
I Region
N- Type Substrate
N+
Fig 2
Contact Metal
Cathode (-)
PIN photodiodes also have high frequency response. The major advantage of the PIN photodiode,
compared to the P-N junction, is the high response speed from the increased depletion region.
AVALANCHE PHOTODIODE
Avalanche photodiodes (APD) use impact ionization (avalanche effect) to create an internal gain in
the material. APDs require high reverse bias operation (near reverse breakdown voltage). Each
photo-generated carrier creates more pairs and so is multiplied by avalanche breakdown. This
creates internal gain within the photodiode, which in turn increases the effective responsivity (larger
current
generated per photon). Figure 3 shows the cross section of the APD.
The typical spectral response range is around 300 – 1100 nm. Current noise in an APD is higher
than in a PIN photodiode, but the increased signal gain is much greater making signalto-noise ratio
greater in APDs. APDs generally have a higher response speed and the ability to detect or measure
light in lower levels.
MODES OF OPERATION
“PHOTOVOLTAIC” MODE UNBIASED
Photodiodes can be operated without any voltage bias. APDs are designed to be reversed biased, so
this section will be relevant to the P-N and PIN photodiodes. Without added voltage across the
junction, dark current can be extremely low (near zero). This reduces the overall noise current of the
system. Thus unbiased P-N or PIN photodiodes are better suited for low light level applications
Electronics III by Engr. James
compared to operation with reverse voltage bias. (The reverse biased APD will still provide a higher
sensitivity than P-N or PIN photodiodes for low light applications.) Unbiased photodiodes can also
work well for low frequency applications (up to 350 kHz). Unbiased mode (where V = 0) can be
seen in Figure 4 in between the forward bias mode (in green) and the reverse bias mode (in blue).
The plot shows very little, if any, dark current when unbiased, which can be seen by the lack of
current at the intersection of the I-V curve at V=0.
When the photodiode is illuminated, the electric field in the depletion region increases. This
produces the photocurrent which increases with increasing photon flux. This is most commonly
seen in solar cells where the generated voltage is measured between the two terminals.
Compared to biased mode, photovoltaic mode has less variation of photocurrent responsivity with
temperature.
The major downfall with unbiased photodiodes is the slow response speed. Without bias to the
system, the capacitance of the photodiode is at a maximum, leading to a slower speed.
“PHOTOCONDUCTIVE” MODE REVERSE BIASED
When the photodiode is reverse biased, an external voltage is applied to the P-N junction. The
negative terminal is connected to the positive P layer, and the positive terminal is connected to the
negative N layer. This causes the free electrons in the N layer to pull toward the positive terminal,
and the holes in the P layer to pull toward the negative terminal. When the external voltage is
applied to the photodiode, the free electrons start at the negative terminal and immediately fill the
holes in the P layer with electrons. This creates negative ions in the atoms with extra electrons. The
charged atoms then oppose the flow of free electrons to the P layer. Similarly, holes go about the
same process to create positive ions but in the opposite direction. When reverse biased, current will
only flow through the photodiode with incident light creating photocurrent.
The reverse bias causes the potential across the depletion region to increase and the width of the
depletion region to increase. This is ideal for creating a large area to absorb the maximum amount
of photons.
The response time is reduced by the reverse bias by increasing the size of the depletion layer. This
increased width reduces the junction capacity and increases the drift velocity of the carriers in the
photodiode. The transit time of the carriers is reduced, improving the response time.
Unfortunately, increasing the bias current increases the dark current as well. This noise can be a
problem for very sensitive systems using P-N or PIN photodiodes. This hinders the performance in
low light situations. If using APDs, the signal to noise ratio will be large regardless because of the
gain of the photodiode. Because a photon is ideally absorbed in the depletion region, the P layer can
be constructed to be extremely thin. This can be balanced with the reverse bias to create an optimal
photodiode with a faster response time while maintaining as low as noise as possible.
Electronics III by Engr. James
INTEGRATION WITH LASER DIODE
A monitor photodiode is often integrated into a laser diode package by the laser diode manufacturer.
It produces a current partially proportional to the output laser diode optical power. If photodiode
current is used as feedback, a control system will try to keep the photodiode current (and therefore
the laser diode optical power) constant. The output of the adjustable current source will vary to keep
the optical power level the same (this is called Constant Power (CP) Mode). Photodiode current and
laser diode output power are related by a transfer function given in the laser diode datasheet.
Not only can photodiodes monitor the DC or CW output of a laser by providing current back to the
laser system, they can also test a laser pulse shape and record peak powers of a laser pulse.
The information in datasheets for photodiodes include the four major components discussed earlier,
the type of photodiode, peak sensitivity wavelengths, and most importantly size and cost.
Photodiodes that are already incorporated into the laser diode system can be limited in options and
information. Laser datasheets usually give the maximum reverse voltage and sometimes the
sensitivity of the photodiode.
If the specifications of the photodiode are extremely important to your laser design, custom builds
or assemblies may be required to meet your needs.
SUMMARY
When deciding to reverse bias your photodiode, or not, it all comes down to balancing speed and
noise and deciding what is most important. If your application depends on extremely low noise and
low dark current, you should choose to not bias your photodiode. If speed is your main concern, you
should choose to reverse bias your photodiode as the response time is improved. In other words, if
your
application is precision based, photovoltaic mode will better fit your needs. If your application is
speed (high) based, photoconductive mode or reversed biased mode will better fit this area.
Reverse biasing the photodiode will be much more responsive than unbiased mode. If operating in
photovoltaic mode, the response may need to be amplified.
The type of photodiode may also affect your decision of bias. Certain types of photodiodes can only
be reversed biased, and others may have amplification of the response internal to the system. APDs
will be effective in low light situations where sensitivity is critical but are expensive, P-N
photodiodes are the most basic design and not widely used, and PIN photodiodes are the most
common photodiode and the cheapest while having very low noise. As discussed earlier, the
materials, size, and cost also affect the type of photodiode needed for the application
The working principle of LED works on the quantum principle.
According to quantum theory, electrons move from high energy levels to low energy levels. When
energy is released from a photon, the distance between the two levels of photon energy is the same
Electronics III by Engr. James
If the PN-junction diode is in the forward bias, current flows through the diode. The flow of current
in semiconductors is due to two currents in the opposite direction of the current and the flow of
electrons in the current direction.
Hence there will be recombination due to the flow of these charge carriers. The recombination
indicates that the electrons in the conduction band jump below the valence band. When electrons
jump from one band to another the electrons will emit electromagnetic energy in the form of
photons and the photon energy is equal to the forbidden energy interval.
Applications of Light Emitting Diodes:
Light Emitting Diodes have many uses but some of their uses are as follows:

LEDs are used in household LED lamps.

LED fluid is used as an industrial light.

Roads are used in street lights and traffic signals.

Used as a display on mobile.

Used in digital clocks and in calculators.
Advantages of LED:
The advantages of LED are as follows:
1. Temperature Range: Its operating range is found in a wide range of 00C -700C.
2. Switching time: LED switching time is in the order of 1ns.Thus, they are useful in dynamic
operations where a large number of arrays are used.
3. Low power consumption: It uses very little power and can run even if the given power is low.
4. Better control: The radiant power of the LED is the function of the current flowing through it.
Thus, the light intensity of the LED can be easily controlled.
5. Economical and reliable: LEDs are cheap in price and have a high degree of reliability.
6. Small size and portability: They are small in size and can be stacked together to form an
alphanumeric display.
Disadvantages of LED:
Disadvantages of LED are as follows:
Electronics III by Engr. James
1. Overvoltage or Overcurrent: There is a possibility of loss when the power exceeds a certain
limit. The same is possible even when the power is low.
2. Overheating due to radiant power: It heats up with an excessive increase in radiant power.
This can damage the LED.
LCD DISPLAY UNIT:
Colour and monochrome liquid crystal displays (LCDs) of various sizes have been created for TV
receivers. LCDs in video display devices are currently available in a variety of sizes and colours,
ranging from a few inches to several inches. LCD TVs are television sets that create pictures using
LCD technology.
When voltage is applied to an LCD panel, it shines a backlight through a layer of liquid crystals,
twisting the changing amounts of light passing through colour filters to form a picture on the screen.
By carefully filtering white light, LCD televisions generate a black and colourful image. A series of
cold cathode fluorescent lamps (CCFLs) are generally used to give illumination at the back of the
panel. A grid of millions of individual LCD shutters is used. It will open and close to enable a
specific amount of white light to pass through.
Each shutter is coupled with a coloured filter that removes everything from the original white
source except the red, green, and blue (RGB) components. A single sub-pixel is formed by each
shutter filter combination. Because the sub-pixels are so small, the three sub-pixels with red, green,
and blue colour filters are combined to create a single colour spot known as Pixel.
The relative strength of the light flowing through the sub-pixels is used to adjust the colour shade.
The front and rear of a typical shutter assembly are formed by a sandwich of many layers placed on
two thin glass sheets. The polarising film, glass sheet, active matrix components, and addressing
electrodes appear first on the back sheet, followed by the director.
Fig: General concept of the LCD unit.
The active matrix components are replaced by patterned colour filters on the front sheet, which is
comparable to the back sheet. A patterned plastic sheet is sandwiched between the two sheets,
Electronics III by Engr. James
dividing the liquid crystal into separate shutters. The shutter assembly is coupled with control
electronics and a backlight to form a full television.
A single bulb or a group of lights can be used to create a backlight. To disperse the light, a diffuser
or frosted mirror is utilised. A column of liquid crystal molecules is suspended between two
transparent electrodes and two polarising filters in each pixel. The polarizer's axes are perpendicular
to one another. Light flowing through one would be blocked by the other if the liquid crystals were
not there.
The polarization of light entering one filter is twisted by the liquid crystal, allowing it to pass
through the other. The liquid crystal molecule is relaxed before an electrical charge is applied. The
helical shape or twisting of these molecules is caused by charges on the molecules.
The light that passes through one polarised filter is rotated as it travels through the liquid crystal,
allowing it to pass through the second polarised filter and making the assembly transparent. The
molecules of the liquid crystal align themselves parallel to the electric field when an electric charge
is given to the electrodes, thus restricting the rotation of incoming light. The light travelling through
the liquid crystals will be polarised perpendicular to the second filter if they are entirely untwisted,
and therefore completely blocked.
The pixel will appear to be dark. Light may be permitted to flow through in various amounts by
regulating the twist of the liquid crystal in each pixel, lighting the pixel accordingly. The LCD panel
provides excellent colour reproduction and contrast. CRT displays are limited in size, therefore they
may be manufactured in bigger proportions. Despite their size, they are light in weight and maybe
readily placed, especially if mounted on a wall.
Advantages of LCD are:
1. LCD's have great Compactness.
2. LCD's are thinner and lightweight devices.
3. It is inexpensive.
4. It uses a few microwatts for display, compared to a few milliwatts for LEDs.
5. When compared to CRT and LED, it uses less electricity.
Disadvantages of LCD:
1. Contrast and brightness are lacking.
2. Additional light sources are required.
3. The operating temperature range is restricted.
4. Lack of trustworthiness.
5. The speed is really slow.
6. AC drives are required for LCDs.
Electronics III by Engr. James
Applications of Liquid Crystal Display (LCD)
1. Thermometer with liquid crystals.
2. Optical imaging is a type of imaging that uses light to create images.
3. The liquid crystal display technology may also be used to visualise radiofrequency waves in
waveguides.
4. It's used in medical settings.
Electronics III by Engr. James
SECTION V OPTOCOUPLER DEVICES AND APPLICATION
An optocoupler (or an optoelectronic coupler) is basically an interface between two circuits
which operate at (usually) different voltage levels. The key advantage of an optocoupler is the
electrical isolation between the input and output circuits. With an optocoupler, the only contact
between the input and the output is a beam of light. Because of this it is possible to have an
insulation resistance between the two circuits in the thousands of megohms. Isolation like this is
useful in high voltage applications where the potentials of two circuits may differ by several
thousand volts.
The most common industrial use of the optocouplers (or optically-coupled isolators) is as a
signal converter between high-voltage pitot devices (limit switches etc.) and low voltage solidstate logic circuits. Optical isolators can be employed in any situationwhere a signal must be
passed between two circuits which are isolated from each other. Complete electrical isolation
between two circuits (i.e.the two circuits have no conductors in common) is often necessary to
prevent noise generated in on ecircuit from being passed to the other circuit. This is especially
necessary for the coupling
between high-voltage information-gathering circuits and lowvoltage digital logic circuits.The information circuits are almost badly exposed to noise sources
and the logic circuitscannot toleratenoisesignals.
In many applications SCR and triac power circuits are under the control of sensitiveelectronic
systems. For example, it is not unusual to have a microprocessor system pro-grammed to turn
motors, lights, and heaters on and off. To reduce the possibility of power-line noise being
induced into the control electronics, and to protect it in the even to fan SCR or triac failure, it is
highly desirable to provide isolation.
The ideal isolation scheme should only allow signal flow in one direction, should respond to dc
levels, and should offer an extremely large resistance between the inputand output circuits. These
features are available in a class of optoelectronic devices called optocouplers or optoisolators.
The optical coupling method eliminates the need for a relay-controlled contact or anisolating
transformer, which are traditional methods of providing electrical isolation be-tween circuits.
The optical coupling method is superior in many applications, because itgetsridofsome
ofthelessdesirable featuresof relaysand transformers.
The optocouplers works well on either ac or dc high-voltage signals. For this reason,signal
converters employing optical coupling are sometimes referred totheuniversal signal
converters.
Electronics III by Engr. James
The optocoupler is a device that contains an infra-red LED and a photo detector (such as a
photodiode, photo transistor, Darlington pair, SCR or triac) combined in one package.
An autocoupler combining a LED and a photodiode in a single package is shown in figure. It has
a LED on the input side and a photodiode on the output side.The left source voltage and the
optocoupler the. Light from the LED impinges on the
series resistor setup a current through the LED.Then
photo-diode, and this setsup a reverse current in the output circuit. This reverse current develops
a voltage across the output resistor
R. The output voltage then equals the output supply voltage V2 minus the voltage drop across the
load resistor R. When the input voltage is varied, the amount of light fluctuates.
Types of Optocouplers:
1. Slotted Optocoupler- A slotted optocoupler has a slot moulded into the package between the
LED light source and the phototransistor light sensor; the slot houses transparent windows, so
that the LED light can normally freely
reach the face of transistor , but can be interrupted or
reflective-slotted-optocoupler
blocked via opaque object placed within the slot.The slotted optocoupler can thus be employed in
a variety of presence detecting applications, including end-of-tape detection, limit switching, and
liquid level detection.
2. Reflective Optocoupler- Here the LED and phototransistor are optically screened from each
other within the package, and both face outwards (in same direction) from the
Electronics III by Engr. James
package. The construction is such that an optocoupled link can be set up by a reflectiveobject
(such as metallic paint or tape, or even smoke particles) placed a short distance outside the
package, in line with both the LED . The reflective coupler can thus be employed in applications
such as tape-position detection, engine-shaft revolution counting or speed measurement, or
smoke or fog detection etc.
Characteristics of an Optocoupler:
Current Transfer Ratio (CTR). One of the most important parameters of an optocoupler device
is its optocoupling efficiency. This parameter is maximized by closely matching spectrally the
optocoupler-characteristics
LED and the photo transistor (which usually
operate in the infra-red range). The optocoupling
efficiency of an optocoupler may be conveniently specified by the output-to-input current
transfer ratio (CTR) i.e., the ratio of the output current Ic (measured at the collector terminal of
the phototransistor), to the input current IF flowing into the LED.
Input-to-Output Isolation Voltage (Viso). This is the maximum potential difference (dc) that
can be allowed to exist between the input and output terminals. Typical values range from 500V
to 4kV.
Maximum Collector-Emitter Voltage,VCE(max). This is the maximum allowable dc voltage that
can be applied across the output transistor. Typical values may vary from20 to 80volts.
Bandwidth. This is the typical maximum signal frequency (in kHz) that can be usefully passed
through the optocoupler when the device is operated in its normal mode. Typical values vary
from 20 to 500 kHz, depending on the type of device construction.
Response Time. Divided into rise time trand fall time t*. For a phototransistor output stages,
tr and tr are usually around 2 to 5𝜇s.
Electronics III by Engr. James
A simple isolating optocoupler uses a single photo transistor output stage and is usually
housed in a six-pin package, with the base terminal of the photo transistor externally
available. In normal use the base is left open circuit, and under such a condition the
optocoupler has a minimum CTR value of 20 % and a useful bandwidth of300kHz
Electronics III by Engr. James
SECTION VI TRANSISTOR
A Transistor is an Electric device that regulates the flow of Electric Current and Voltage. It
acts as a switch or gate of Electric signals. A Transistor is usually composed of three layers of
semiconductor components that carry Current. Most of the Transistors are composed of pure
silicon, with some made of germanium however, sometimes other semiconductor materials
are used.
Transistors can be used for a wide range of digital and analogue functions, including
amplifiers, switches, Voltage stabilizers, signal modulation, and oscillators, because of their
high response and accuracy. Transistors can be packaged individually or in a tiny space,
allowing for integrating up to 100 million Transistor integrated circuits.
Parts of Transistor
A Transistor is made up of three layers of semiconductor materials, or terminals that help to
link the Transistor to an external circuit and carry Current. The Current that is applied
through one pair of terminals of a Transistor is controlled by the Current applied to any other
pair of terminals of the Transistor. For a Transistor, there are three terminals. They are as
follows:
1. Base: The base is used to activate the Transistor.
2. Collector: The Transistor's positive lead is known as the collector.
3. Emitter: The Transistor's emitter is the negative lead.
Characteristics of Transistor
Transistor Characteristics is the basis that represents the relationship between the Electric
Current and Electric Voltage of a circuit. There are three types of Transistor characteristic
curves based on the configuration of the circuit.
1. Input Characteristic - The Input characteristics describe any changes that occur in
the Input Current because of the variation of the Input Voltage by keeping the Output
Voltage constant.
2. Output Characteristic - This is a graph of Output Current on one axis and Output
Voltage on another, at a constant Input Current.
3. Current Transfer Characteristic - This is a characteristic curve that points to the
fluctuation of the Output Current to that of the Input Current. Here, the Output
Voltage is kept constant.
Transistor Configuration
Any type of Transistor circuit can be designed by using the above mentioned three Transistor
characteristics. The configuration of the Transistors is based on the Transistor terminals.
There are three types of Transistor circuit configuration, these are:

Common Emitter Transistor

Common Base Transistor
Electronics III by Engr. James

Common Collector Transistor (emitter follower).
Each circuit configuration has a different characteristic curve. Based on the requirement of
the circuit, the Transistor configuration is chosen accordingly.
Few things are considered while using the correct Transistor for the circuit. These are the
maximum Voltage rating between the emitter and the collector (UCEmax), maximum power
to build a circuit, and maximum collector Current (ICEmax). An Electric circuit must not
exceed these maximum values to function properly. Permanent damage to the circuit may
occur if it exceeds the value. It is also important to maintain proper Current amplification and
frequency.
Common Emitter Configuration
In this kind of configuration, an Emitter is used as a common terminal for both Input and
Output. It works as an Inverting Amplifier Circuit. Here, the Input is applied in the region of
the base Emitter and the Output is obtained from between the Terminals of the Collector and
Emitter.
In this case,
VBE is the Input Voltage,
IB is the Input Current,
VCE is the Output Voltage, and
IC is the Output Current.
The Common-Emitter Configuration is usually based on Transistor-Based Amplifiers. Under
this condition, the Emitter Current is equivalent to the sum of base Current and collector
Current.
Hence,
IE = IC + IB
(the image will be uploaded soon)
This equation is the Transistor equation for the CE configuration. The ratio of the collector
Current to that of emitter Current gives Current gain alpha in the Common Base
Configuration. Similarly, the ratio of the Collector Current to that of the base Current gives
Current gain beta in the Common-Emitter Configuration.
The Relationship Between the Two Current Gains is:
Current gain (α) = IC/IE
Current gain (β) = IC/IB
Collector Current IC =αIE = βIB
This configuration uses one of the three circuit configurations. It has average Input and
Output values of impedance. It also has an average Current and Voltage gains. The Output
signal of this configuration has a phase shift of 180⁰, so the Input and the Output are inversely
proportional to each other.
( image will be uploaded soon)
Input and Output Characteristics of Common-Emitter Configuration
1. Input Characteristics of Transistor
Electronics III by Engr. James
The Input characteristic of a Transistor is obtained between the Input's Current IB and the
Input Voltage VB by having a constant Output Voltage VCE. By keeping the Output Voltage
VCE constant and changing the Input Voltage VBE of different points, we can examine the
values of the Input Current of each of the points. Now, using the values obtained from
different points, a graph is drawn by plotting the values of IB and VBE at constant VCE.
Rin = VBE/IB (at a constant VCE)
This is the required equation to calculate the Input resistance Rin.
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2. Output Characteristics
The Output characteristic of a common emitter is obtained between the Output Voltage VCE
and Output Current IC at a constant Input Current IB. By keeping the base Current IB constant
and changing the value of Output Voltage VCE at different points, we can calculate the value
of collector IC for each point. Now, if we plot a graph between IC and VCE, we get the Output
characteristics of a common-emitter configuration.
Rout = VCE/IC (at a constant IB)
This is the equation to calculate Output resistance.
(the image will be uploaded soon)
The Output characteristics can be divided into three regions:



The active region of the Transistor
Saturation region of the Transistor
Cut-off region of the Transistor
The Active Region of the Transistor
The active region of the Transistor is the area on the Output curve where the Output Current
is almost constant and independent of the Output Voltage. The Transistor operates in the
Active region if the base resistance is greater than the maximum value allowed. A Transistor
can only be used as an amplifier if it is in the active region. In addition, the emitter junction
should be in forwarding bias, and the collector junction should be in reverse bias for
operation in the active region.
Saturation Region of the Transistor
The saturation region of the Transistor is the area where the collector Current rapidly
increases with a little increase in Output Voltage. The base resistance should be less than the
maximum allowed value to run the Transistor in the saturation region. Both emitter and
collector junctions should be in forwarding bias for operation in the saturation region. The
Transistor works like the ON stage of a switch in the saturation zone.
Cut-off Region of the Transistor
The base Current is effectively zero in the Cut-Off region. As a result, even at higher Output
Voltage, collector Current becomes zero. To operate a Transistor in the cut-off region, both
the emitter and collector junctions must be in reverse-biased condition. A Transistor operates
like the OFF stage of a switch in the cut-off region
Electronics III by Engr. James
SECTION VII TRANSISTOR AS AN AMPLIFIER
For a transistor to act as an amplifier, it should be properly biased. We will discuss the need
for proper biasing in the next chapter. Here, let us focus how a transistor works as an
amplifier.
Transistor Amplifier
A transistor acts as an amplifier by raising the strength of a weak signal. The DC bias
voltage applied to the emitter base junction, makes it remain in forward biased condition.
This forward bias is maintained regardless of the polarity of the signal. The below figure
shows how a transistor looks like when connected as an amplifier.
The low resistance in input circuit, lets any small change in input signal to result in an
appreciable change in the output. The emitter current caused by the input signal contributes
the collector current, which when flows through the load resistor RL, results in a large
voltage drop across it. Thus a small input voltage results in a large output voltage, which
shows that the transistor works as an amplifier.
Example
Let there be a change of 0.1v in the input voltage being applied, which further produces a
change of 1mA in the emitter current. This emitter current will obviously produce a change
in collector current, which would also be 1mA.
A load resistance of 5kΩ placed in the collector would produce a voltage of
5 kΩ × 1 mA = 5V
Hence it is observed that a change of 0.1v in the input gives a change of 5v in the output,
which means the voltage level of the signal is amplified.
Performance of Amplifier
As the common emitter mode of connection is mostly adopted, let us first understand a few
important terms with reference to this mode of connection.
Input Resistance
As the input circuit is forward biased, the input resistance will be low. The input resistance
is the opposition offered by the base-emitter junction to the signal flow.
By definition, it is the ratio of small change in base-emitter voltage (ΔVBE) to the resulting
change in base current (ΔIB) at constant collector-emitter voltage.
Input resistance, Ri=ΔVBEΔIBRi=ΔVBEΔIB
Electronics III by Engr. James
Where Ri = input resistance, VBE = base-emitter voltage, and IB = base current.
Output Resistance
The output resistance of a transistor amplifier is very high. The collector current changes
very slightly with the change in collector-emitter voltage.
By definition, it is the ratio of change in collector-emitter voltage (ΔVCE) to the resulting
change in collector current (ΔIC) at constant base current.
Output resistance = Ro=ΔVCEΔICRo=ΔVCEΔIC
Where Ro = Output resistance, VCE = Collector-emitter voltage, and IC = Collector-emitter
voltage.
Effective Collector Load
The load is connected at the collector of a transistor and for a single-stage amplifier, the
output voltage is taken from the collector of the transistor and for a multi-stage amplifier,
the same is collected from a cascaded stages of transistor circuit.
By definition, it is the total load as seen by the a.c. collector current. In case of single stage
amplifiers, the effective collector load is a parallel combination of RC and Ro.
Effective Collector Load, RAC=RC//RoRAC=RC//Ro
=RC×RoRC+Ro=RAC=RC×RoRC+Ro=RAC
Hence for a single stage amplifier, effective load is equal to collector load RC.
In a multi-stage amplifier (i.e. having more than one amplification stage), the input
resistance Ri of the next stage also comes into picture.
Effective collector load becomes parallel combination of RC, Ro and Ri i.e,
Effective Collector Load, RAC=RC//Ro//RiRAC=RC//Ro//Ri
RC//Ri=RCRiRC+RiRC//Ri=RCRiRC+Ri
As input resistance Ri is quite small, therefore effective load is reduced.
Current Gain
The gain in terms of current when the changes in input and output currents are observed, is
called as Current gain. By definition, it is the ratio of change in collector current (ΔIC) to
the change in base current (ΔIB).
Current gain, β=ΔICΔIBβ=ΔICΔIB
The value of β ranges from 20 to 500. The current gain indicates that input current becomes
β times in the collector current.
Voltage Gain
Electronics III by Engr. James
The gain in terms of voltage when the changes in input and output currents are observed, is
called as Voltage gain. By definition, it is the ratio of change in output voltage (ΔVCE) to
the change in input voltage (ΔVBE).
Voltage gain, AV=ΔVCEΔVBEAV=ΔVCEΔVBE
=Change in output current×effective load Change in input current×input resistance
=ΔIC×RACΔIB×Ri=ΔICΔIB×RACRi=β×RACRi=ΔIC×RACΔIB×Ri=ΔICΔIB×RACRi=β×
RACRi
For a single stage, RAC = RC.
However, for Multistage,
RAC=RC×RiRC+RiRAC=RC×RiRC+Ri
Where Ri is the input resistance of the next stage.
Power Gain
The gain in terms of power when the changes in input and output currents are observed, is
called as Power gain.
By definition, it is the ratio of output signal power to the input signal power.
Power gain, AP=(ΔIC)2×RAC(ΔIB)2×RiAP=(ΔIC)2×RAC(ΔIB)2×Ri
=(ΔICΔIB)×ΔIC×RACΔIB×Ri=(ΔICΔIB)×ΔIC×RACΔIB×Ri
= Current gain × Voltage gain
Amplifier Classes
Amplifiers are classified into classes according to their construction and operating
characteristics
Not all amplifiers are the same and there is a clear distinction made between the way their
output stages are configured and operate. The main operating characteristics of an ideal
amplifier are linearity, signal gain, efficiency and power output but in real world amplifiers
there is always a trade off between these different characteristics.
Generally, large signal or power amplifiers are used in the output stages of audio amplifier
systems to drive a loudspeaker load. A typical loudspeaker has an impedance of between 4Ω
and 8Ω, thus a power amplifier must be able to supply the high peak currents required to
drive the low impedance speaker.
One method used to distinguish the electrical characteristics of different types of amplifiers is
by “class”, and as such amplifiers are classified according to their circuit configuration and
method of operation. Then Amplifier Classes is the term used to differentiate between the
different amplifier types.
Electronics III by Engr. James
Amplifier Classes represent the amount of the output signal which varies within the amplifier
circuit over one cycle of operation when excited by a sinusoidal input signal. The
classification of amplifiers range from entirely linear operation (for use in high-fidelity signal
amplification) with very low efficiency, to entirely non-linear (where a faithful signal
reproduction is not so important) operation but with a much higher efficiency, while others
are a compromise between the two.
Amplifier classes are mainly lumped into two basic groups. The first are the classically
controlled conduction angle amplifiers forming the more common amplifier classes of A, B,
AB and C, which are defined by the length of their conduction state over some portion of the
output waveform, such that the output stage transistor operation lies somewhere between
being “fully-ON” and “fully-OFF”.
The second set of amplifiers are the newer so-called “switching” amplifier classes of D, E, F,
G, S, T etc, which use digital circuits and pulse width modulation (PWM) to constantly
switch the signal between “fully-ON” and “fully-OFF” driving the output hard into the
transistors saturation and cut-off regions.
The most commonly constructed amplifier classes are those that are used as audio amplifiers,
mainly class A, B, AB and C and to keep things simple, it is these types of amplifier
classes we will look at here in more detail.
Class A Amplifier
Class A Amplifiers are the most common type of amplifier topology as they use just one
output switching transistor (Bipolar, FET, IGBT, etc) within their amplifier design. This
single output transistor is biased around the Q-point within the middle of its load line and so
is never driven into its cut-off or saturation regions thus allowing it to conduct current over
the full 360 degrees of the input cycle. Then the output transistor of a class-A topology never
turns “OFF” which is one of its main disadvantages.
Class “A” amplifiers are considered the best class of amplifier design due mainly to their
excellent linearity, high gain and low signal distortion levels when designed correctly.
Although seldom used in high power amplifier applications due to thermal power supply
considerations, class-A amplifiers are probably the best sounding of all the amplifier classes
mentioned here and as such are used in high-fidelity audio amplifier designs.
Class A Amplifier
To achieve high linearity and gain, the output stage of a class A amplifier is biased “ON”
(conducting) all the time. Then for an amplifier to be classified as “Class A” the zero signal
idle current in the output stage must be equal to or greater than the maximum load current
(usually a loudspeaker) required to produce the largest output signal.
Electronics III by Engr. James
As a class A amplifier operates in the linear portion of its characteristic curves, the single
output device conducts through a full 360 degrees of the output waveform. Then the class A
amplifier is equivalent to a current source.
Since a class A amplifier operates in the linear region, the transistors base (or gate) DC
biasing voltage should by chosen properly to ensure correct operation and low distortion.
However, as the output device is “ON” at all times, it is constantly carrying current, which
represents a continuous loss of power in the amplifier.
Due to this continuous loss of power class A amplifiers create tremendous amounts of heat
adding to their very low efficiency at around 30%, making them impractical for high-power
amplifications. Also due to the high idling current of the amplifier, the power supply must be
sized accordingly and be well filtered to avoid any amplifier hum and noise. Therefore, due
to the low efficiency and over heating problems of Class A amplifiers, more efficient
amplifier classes have been developed.
Class B Amplifier
Class B amplifiers were invented as a solution to the efficiency and heating problems
associated with the previous class A amplifier. The basic class B amplifier uses two
complimentary transistors either bipolar of FET for each half of the waveform with its output
stage configured in a “push-pull” type arrangement, so that each transistor device amplifies
only half of the output waveform.
In the class B amplifier, there is no DC base bias current as its quiescent current is zero, so
that the dc power is small and therefore its efficiency is much higher than that of the class A
amplifier. However, the price paid for the improvement in the efficiency is in the linearity of
the switching device.
Class B Amplifier
When the input signal goes positive, the positive biased transistor conducts while the negative
transistor is switched “OFF”. Likewise, when the input signal goes negative, the positive
transistor switches “OFF” while the negative biased transistor turns “ON” and conducts the
negative portion of the signal. Thus the transistor conducts only half of the time, either on
positive or negative half cycle of the input signal.
Then we can see that each transistor device of the class B amplifier only conducts through
one half or 180 degrees of the output waveform in strict time alternation, but as the output
stage has devices for both halves of the signal waveform the two halves are combined
together to produce the full linear output waveform.
This push-pull design of amplifier is obviously more efficient than Class A, at about 50%, but
the problem with the class B amplifier design is that it can create distortion at the zerocrossing point of the waveform due to the transistors dead band of input base voltages from 0.7V to +0.7.
Electronics III by Engr. James
We remember from the Transistor tutorial that it takes a base-emitter voltage of about 0.7
volts to get a bipolar transistor to start conducting. Then in a class B amplifier, the output
transistor is not “biased” to an “ON” state of operation until this voltage is exceeded.
This means that the the part of the waveform which falls within this 0.7 volt window will not
be reproduced accurately making the class B amplifier unsuitable for precision audio
amplifier applications.
To overcome this zero-crossing distortion (also known as Crossover Distortion) class AB
amplifiers were developed.
Class AB Amplifier
As its name suggests, the Class AB Amplifier is a combination of the “Class A” and the
“Class B” type amplifiers we have looked at above. The AB classification of amplifier is
currently one of the most common used types of audio power amplifier design. The class AB
amplifier is a variation of a class B amplifier as described above, except that both devices are
allowed to conduct at the same time around the waveforms crossover point eliminating the
crossover distortion problems of the previous class B amplifier.
The two transistors have a very small bias voltage, typically at 5 to 10% of the quiescent
current to bias the transistors just above its cut-off point. Then the conducting device, either
bipolar of FET, will be “ON” for more than one half cycle, but much less than one full cycle
of the input signal. Therefore, in a class AB amplifier design each of the push-pull transistors
is conducting for slightly more than the half cycle of conduction in class B, but much less
than the full cycle of conduction of class A.
In other words, the conduction angle of a class AB amplifier is somewhere between 180o and
360o depending upon the chosen bias point as shown.
Class AB Amplifier
The advantage of this small bias voltage, provided by series diodes or resistors, is that the
crossover distortion created by the class B amplifier characteristics is overcome, without the
inefficiencies of the class A amplifier design. So the class AB amplifier is a good
compromise between class A and class B in terms of efficiency and linearity, with conversion
efficiencies reaching about 50% to 60%.
Class C Amplifier
The Class C Amplifier design has the greatest efficiency but the poorest linearity of the
classes of amplifiers mentioned here. The previous classes, A, B and AB are considered
linear amplifiers, as the output signals amplitude and phase are linearly related to the input
signals amplitude and phase.
However, the class C amplifier is heavily biased so that the output current is zero for more
than one half of an input sinusoidal signal cycle with the transistor idling at its cut-off point.
Electronics III by Engr. James
In other words, the conduction angle for the transistor is significantly less than 180 degrees,
and is generally around the 90 degrees area.
While this form of transistor biasing gives a much improved efficiency of around 80% to the
amplifier, it introduces a very heavy distortion of the output signal. Therefore, class C
amplifiers are not suitable for use as audio amplifiers.
Class C Amplifier
Due to its heavy audio distortion, class C amplifiers are commonly used in high frequency
sine wave oscillators and certain types of radio frequency amplifiers, where the pulses of
current produced at the amplifiers output can be converted to complete sine waves of a
particular frequency by the use of LC resonant circuits in its collector circuit.
Amplifier Classes Summary
Then we have seen that the quiescent DC operating point (Q-point) of an amplifier
determines the amplifier classification. By setting the position of the Q-point at half way on
the load line of the amplifiers characteristics curve, the amplifier will operate as a class A
amplifier. By moving the Q-point lower down the load line changes the amplifier into a
class AB, B or C amplifier.
Then the class of operation of the amplifier with regards to its DC operating point can be
given as:
Amplifier Classes and Efficiency
As well as audio amplifiers there are a number of high efficiency Amplifier Classes relating
to switching amplifier designs that use different switching techniques to reduce power loss
and increase efficiency. Some amplifier class designs listed below use RLC resonators or
multiple power-supply voltages to reduce power loss, or are digital DSP (digital signal
processing) type amplifiers which use pulse width modulation (PWM) switching techniques.
Other Common Amplifier Classes

Class D Amplifier – A Class D audio amplifier is basically a non-linear switching amplifier
or PWM amplifier. Class-D amplifiers theoretically can reach 100% efficiency, as there is
no period during a cycle were the voltage and current waveforms overlap as current is
drawn only through the transistor that is on.

Class F Amplifier – Class-F amplifiers boost both efficiency and output by using harmonic
resonators in the output network to shape the output waveform into a square wave. Class-F
amplifiers are capable of high efficiencies of more than 90% if infinite harmonic tuning is
used.

Class G Amplifier – Class G offers enhancements to the basic class AB amplifier design.
Class G uses multiple power supply rails of various voltages and automatically switches
between these supply rails as the input signal changes. This constant switching reduces the
average power consumption, and therefore power loss caused by wasted heat.
Electronics III by Engr. James

Class I Amplifier – The class I amplifier has two sets of complementary output switching
devices arranged in a parallel push-pull configuration with both sets of switching devices
sampling the same input waveform. One device switches the positive half of the waveform,
while the other switches the negative half similar to a class B amplifier. With no input
signal applied, or when a signal reaches the zero crossing point, the switching devices are
both turned ON and OFF simultaneously with a 50% PWM duty cycle cancelling out any
high frequency signals.
To produce the positive half of the output signal, the output of the positive switching device
is increased in duty cycle while the negative switching device is decreased by the same and
vice versa. The two switching signal currents are said to be interleaved at the output, giving
the class I amplifier the named of: “interleaved PWM amplifier” operating at switching
frequencies in excess of 250kHz.

Class S Amplifier – A class S power amplifier is a non-linear switching mode amplifier
similar in operation to the class D amplifier. The class S amplifier converts analogue input
signals into digital square wave pulses by a delta-sigma modulator, and amplifies them to
increases the output power before finally being demodulated by a band pass filter. As the
digital signal of this switching amplifier is always either fully “ON” or “OFF” (theoretically
zero power dissipation), efficiencies reaching 100% are possible.

Class T Amplifier – The class T amplifier is another type of digital switching amplifier
design. Class T amplifiers are starting to become more popular these days as an audio
amplifier design due to the existence of digital signal processing (DSP) chips and multichannel surround sound amplifiers as it converts analogue signals into digital pulse width
modulated (PWM) signals for amplification increasing the amplifiers efficiency. Class T
amplifier designs combine both the low distortion signal levels of class AB amplifier and
the power efficiency of a class D amplifier.
We have seen here a number of classification of amplifiers ranging from linear power
amplifiers to non-linear switching amplifiers, and have seen how an amplifier class differs
along the amplifiers load line. The class AB, B and C amplifiers can be defined in terms of
the conduction angle, θ as follows:
Amplifier Class by Conduction Angle
Amplifier Class
Description
Conduction Angle
Class-A
Full cycle 360o of Conduction
θ = 2π
Electronics III by Engr. James
Class-B
Half cycle 180o of Conduction
θ=π
Class-AB
Slightly more than 180o of conduction
π < θ < 2π
Class-C
Slightly less than 180o of conduction
θ<π
Class-D to T
ON-OFF non-linear switching
θ=0
Electronics III by Engr. James
SECTION VI11 SOLVED PROBLEMS ON TRANSISTOR
Q1. A common base transistor amplifier has an input resistance of 20 Ω and output
resistance of 100 kΩ. The collector load is 1 kΩ. If a signal of 500 mV is applied
between emitter and base, find the voltage amplification. Assume αac to be nearly one.
Solution :
Fig.1 shows the conditions of the problem. Here the output resistance is very high as
compared to input resistance, since the input junction (base to emitter) of the transistor is
forward biased while the output junction (base to collector) is reverse biased.
Fig. 1
Q2. In a common base connection, IE = 1mA, IC = 0.95mA. Calculate the value of IB .
Solution :
Q3. In a common base connection, current amplification factor is 0.9. If the emitter
current is 1mA, determine the value of base current.
Solution :
Electronics III by Engr. James
Q4. In a common base connection, IC = 0.95 mA and IB = 0.05 mA. Find the value of α.
Solution :
Q5. In a common base connection, the emitter current is 1mA. If the emitter circuit is
open, the collector current is 50 μA. Find the total collector current. Given that α =
0.92.
Solution :
Q6. In a common base connection, α = 0.95. The voltage drop across 2 kΩ resistance
which is connected in the collector is 2V. Find the base current.
Solution :
Electronics III by Engr. James
Fig. 2
Fig. 2 shows the required common base connection.
The voltage drop across RC (= 2 kΩ) is 2V.
Q7. For the common base circuit shown in Fig. 3, determine IC and VCB . Assume the
transistor to be of silicon.
Fig. 3
Solution :
Since the transistor is of silicon, VBE = 0.7V.
Electronics III by Engr. James
Applying Kirchhoff’s voltage law to the emitter-side loop,we get,
Applying Kirchhoff’s voltage law to the collector-side loop, we have,
Q8. Find the value of β if (i) α = 0.9 (ii) α = 0.98 (iii) α = 0.99.
Solution :
(i) α = 0.9
(ii) α = 0.98
(iii) α = 0.99
Q9. Calculate IE in a transistor for which β = 50 and IB = 20 μA.
Solution :
Electronics III by Engr. James
Q10. Find the α rating of the transistor shown in Fig. 4. Hence determine the value of
IC using both α and β rating of the transistor.
Fig. 4
Solution :
Fig. 8.20 shows the conditions of the problem.
Electronics III by Engr. James
Q11. For a transistor, β = 45 and voltage drop across 1kΩ which is connected in the
collector circuit is 1 volt. Find the base current for common emitter connection.
Solution :
Fig. 5
Fig. 5 shows the required common emitter connection. The voltage drop across RC (= 1 kΩ)
is 1 volt.
Q12. A transistor is connected in common emitter (CE) configuration in which collector
supply is 8 V and the voltage drop across resistance RC connected in the collector circuit
is 0.5 V. The value of RC = 800 Ω. If α = 0.96, determine : (i) collector-emitter voltage (ii)
base current.
Electronics III by Engr. James
Solution :
Fig.6
Fig. 6 shows the required common emitter connection with various values.
(i)
(ii)
Q13. An n-p-n transistor at room temperature has its emitter disconnected. A voltage of
5 V is applied between collector and base. With collector positive, a current of 0.2 μA
flows. When the base is disconnected and the same voltage is applied between collector
Electronics III by Engr. James
and emitter, the current is found to be 20 μA. Find α, IE and IB when collector current is
1 mA.
Solution :
Fig. 7
When the emitter circuit is open as shown in Fig.7 (i) , the collector-base junction is reverse
biased. A small leakage current ICBO flows due to minority carriers.
Electronics III by Engr. James
Q14. Determine VCB in the transistor circuit shown in Fig. 10 (i). The transistor is of
silicon and has β = 150.
Solution :
Fig.10
Fig. 10 (i) shows the transistor circuit while Fig. 10 (ii) shows the various currents and
voltages along with polarities.
Q15. In a transistor, IB = 68 μA, IE = 30 mA and β = 440. Determine the α rating of the
transistor. Then determine the value of IC using both the α rating and β rating of the
transistor.
Solution :
Electronics III by Engr. James
Q16. A transistor has the following ratings : IC (max) = 500 mA and βmax = 300.
Determine the maximum allowable value of IB for the device.
Solution :
For this transistor, if the base current is allowed to exceed 1.67 mA, the collector current will
exceed its maximum rating of 500 mA and the transistor will probably be destroyed.
Q17. Fig. 11 shows the open circuit failures in a transistor. What will be the circuit
behaviour in each case ?
Solution :
Fig. 11
Fig 11 shows the open circuit failures in a transistor. We shall discuss the circuit behaviour in
each case.
Electronics III by Engr. James
(i) Open emitter :
Fig. 11 (i) shows an open emitter failure in a transistor. Since the collector diode is not
forward biased, it is OFF and there can be neither collector current nor base current.
Therefore, there will be no voltage drops across either resistor and the voltage at the base and
at the
collector leads of the transistor will be 12V.
(ii) Open-base :
Fig. 11 (ii) shows an open base failure in a transistor. Since the base is open, there can be no
base current so that the transistor is in cut-off. Therefore, all the transistor currents are 0A. In
this case, the base and collector voltages will both be at 12V.
(iii) Open collector :
Fig. 11 (iii) shows an open collector failure in a transistor. In this case, the emitter diode is
still ON, so we expect to see 0.7V at the base. However, we will see 12V at the collector
because there is no collector current.
Q18. For the circuit shown in Fig. 12 , draw the d.c. load line.
Fig.12
Solution :
Electronics III by Engr. James
The collector-emitter voltage VCE is given by ;
This locates the point A of the load line on the collector current axis. By joining these two
points, we get the d.c. load line AB as shown in Fig. 13.
Hybrid Parameters
The hybrid parameters are generally used to determine amplifier characteristic parameters
such as voltage gain, input and output resistance etc. we have already known that amplifier
characteristic parameters may be determined by using current gain(β) and values of other
circuit components. Hybrid methods had the following advantages:
1. Value of circuit components are easily available, and
2. The procedure followed is quite simple and easy to understand.
H-PARAMETERS OF LINEAR CIRCUIT
Every linear circuit having input and output can be analyzed as two port networks. In these
networks there are four parameters called hybrid or h-parameters. Out of these four
Electronics III by Engr. James
parameters, one is measured in ohm, one in mho and other two are dimension less. Since
these parameters have mixed dimension, so they are called hybrid parameters.
Consider a linear circuit shown in Figure (a). in this circuit when input voltage 1 is applied,
input current i1 flows. Then output voltage V2 and current i2 appears. Both currents i1 and i2
are assumed to flow inside the box of the linear circuit. Both voltages V1 and V2 are assumed
to be positive from the upper to lower terminals.
The linear circuit may be replaced by an equivalent circuit as shown in Figure (a). the
equivalent circuit is called hybrid model of a linear circuit. In such as circuit, the input and
the output voltages and currents (called variables) may be related by the set of the following
two equations:
V1 = h11.i1 + h12.V2
I2 = h21.i + h22.V2
Where
V1 = input voltage
V2 = Output voltage
I1 = input current
I2 = output current
And h11, h12, h21 and h22 are hybrid parameters.
Electronics III by Engr. James
DETERMINATION OF H-PARAMETERS
The parameters h11 and h21 may be determined by short circuiting the output terminals of a
given circuit. On the other hand, h12 and h22 may be determined by open circuiting the input
terminals of the given circuit.
Determination of h11 and h21
These are determined by short circuiting the output terminals of a given circuit as shown in
Figure (b). a shot circuit at the output terminal makes the voltage v2 equal to zero. Input
voltage is given by relation,
VI = h11.i1 + h12.V2
Putting the value of V2 (equal to zero) in the above equation, the input voltage,
V1 = h11.i1 or h11=V1/i1
Thus h11 may be determined from the ratio V1/i1. The value of i1 obtained by applying a
voltage at the input and then measuring the value of input current (i1). Since h11 is the ratio of
voltage to current, therefore it has the units of ohms i.e. the same unit as that of a resistance.
Because of this fact, h11 is called input resistance of the circuit with output short circuited.
Similarly, output current is given by the relation.
I2 = h21.i1 + h22.V2
Again putting the value of V2 the output current,
I2 = h21. i1 or h21 = i2/i1
Thus h21 may be determined from the ratio i2/i1, the values of i2 and i1 may be obtained by
applying a voltage at the input and then measuring the input current (i1) and output current
(i2). Since h21 is the ratio of currents, therefore it has no units. The parameter h21 is called the
forward current gain of the circuit with output short circuited.
Electronics III by Engr. James
Determination of h12 and h22
These are determined by open circuiting the input terminals of the given circuit as shown in
Figure (b). an open circuit at the input terminals, makes the current (i1) equal to zero. We also
know that the input voltage is given by the relation.
V1 = h11.i1 + h12.V2
Putting the value of i1 in this equation, the output voltage will be
V1 = h12. V2 or h12 = V1/V2
Thus h12 may be determined from the ratio V1/V2. The value of V1 may be obtained by
applying a voltage (V2) and the measuring the input voltage (V1). Since, h12 is a ratio of
voltages, therefore it has no units. As h12 is the ratio of input voltage (V1) to the output
voltage (V2), therefore, its value known as the reverse voltage gain.
Similarly, the output current is given by the relation.
I2 = h21.i1 + h22.V2
Again putting the value of i1 (equal to zero) in this equation. The output currents will be,
I2 = h22. V2 or h22 = i2/V2
No.
Parameter
Meaning
Condition
1
2
3
4
h11
h12
h21
h22
Input resistance
Reverse voltage gain
Forward current gain
Output conductance
Output shorted
Input open
Output shorted
Input open
hi = h11 = Input resistance with output shorted,
hr = h12 = Reverse voltage gain with input open,
hf = h21 = Forward current gain with output shorted,
ho = h22 = Output conductance with input open.
The h-parameters of a transistor depend upon the type of the configuration used i.e. common
emitter (CE), common collector (CC) or common base (CB). Because of this, each of the four
h-parameters carries a second subscript letter e, b or c. the letter ‘e’ is used to represent
common emitter; ‘c’ for common collector and ‘b’ for common base configuration.
Electronics III by Engr. James
Transistor configuration
General
S.No parameter Common emitter Common base Common collector
1
2
3
4
h11
h12
h21
h22
hie
hre
hic
hoe
hib
hrb
hfb
hob
hie
hre
hfe
hoe
Examples
CE Stage with RE Bypassed
The h-parameter model will be applied to a single common emitter (CE) stage with the
emitter resistor (RE) bypassed. The model will be used to build equations for voltage gain,
current gain, input and output impedance. The circuit is shown below:
The small signal parameter hreVce is often too small to be considered so the input resistance is
just hie. Often the output resistance hoe is often large compared wi the the collector resistor
RC and its effects can be ignored. The h-parameter equivalent model is now simplified and
drawn below:
Electronics III by Engr. James
Input Impedance Zi
The input impedance is the parallel combination of bias resistors RB1 and RB2. As the power
supply is considered short circuit at small signal levels then RB1 and RB2 are in parallel.
RBB will represent the parallel combination:
RB1 RB2
RBB = RB1 || RB2 =
RB1 + RB2
As RBB is in parallel with hie then:
Zi = RBB || hie
Output Impedance Zo
As hfeIb is an ideal current generator with infinite output impedance, then output impedance
looking into the circuit is:
Zo = RC
Voltage Gain Av
Note the − sign in the equation, this indicates phase inversion of the output waveform.
Vo = -Io RC = -hfe Ib RC
as Ib = Vi / hie then:
Vi
= -hfe
RC
hie
-hfe
=
RC Vi
hie
Vo
Av =
-hfe RC
=
Vi
hie
Current Gain Ai
The current gain is the ratio Io / Ii. At the input the current is split between the parallel branch
RBB and hie. So looking at the equivalent h-parameter model again (shown below):
Electronics III by Engr. James
The current divider rule can be used for Ib:
RBB Ii
Ib =
RBB + hie
Ib
RBB
=
Ii
RBB + hie
At the output side, Io = hfe Ib
re-arranging Io / Ib = hfe
Io
Ai =
Io Ib
=
Ii
RBB
= hfe
Ib Ii
RBB + hie
RBB hfe
Ai =
RBB + hie
If RBB >> hie then,
RBB hfe
Ai ≈
= hfe
RBB
CE Stage with RE Unbypassed
The h-parameter model of a common emitter stage with the emitter resistor unbypassed is
Electronics III by Engr. James
now shown. The model will be used to build equations for voltage gain, current gain, input
and output impedance. The circuit is shown below:
As in the previous example, RB1 and RB2 are in parallel, the bias resistors are replaced by
resistance RBB, but as RE is now unbypassed this resistor appears in series with the emitter
terminal. The hybrid small signal model is shown below, once again effects of small signal
parameters hreVce and hoe have been omitted.
Input Impedance Zi
The input impedance Zi is the bias resistors RBB in parallel with the impedance of the base,
Zb.
Zb = hie + (1 + hfe) RE
Since hfe is normally much larger than 1, the equation can be reduced to:
Electronics III by Engr. James
Zb = hie + hfe RE
Zi = RBB || (hie + hfe RE)
Output Impedance Zo
With Vi set to zero, then Ib = 0 and hfeIb can be replaced by an open-circuit. The output
impedance is:
Zo = RC
Voltage Gain Av
Note the − sign in the equation, this indicates phase inversion of the output waveform.
Vi
Ib =
Zb
Vo = -Io RC = -hfe Ib RC
Vi
= -hfe
RC
Zb
Vo
Av =
-hfe RC
=
Vi
Zb
As Zb = hie + hfe RE often the product hfeRE is much larger than hie, so Zb can reduced to the
approximation:
Zb ≈ hfeRE
-hfeRC
∴ Av =
hfeRE
Vo
RC
=−
Av =
Vi
RE
Current Gain Ai
The current gain is the ratio Io / Ii. At the input the current is split between the parallel branch
RBB and Zb. So looking at the equivalent h-parameter model again (shown below):
Electronics III by Engr. James
The current divider rule can be used for Ib:
RBB Ii
Ib =
RBB + Zb
Ib
RBB
=
Ii
RBB + Zb
At the output side, Io = hfe Ib
re-arranging Io / Ib = hfe
Io
Ai =
Io Ib
=
Ii
RBB
= hfe
Ib Ii
RBB + Zb
RBB hfe
Ai =
RBB + Zb
Example CE Stage
Electronics III by Engr. James
The hybrid parameters must be known to use the hybrid model, either from the datasheet or
measured. In the above circuit, Zi, Zo, Av, and Ai will now be calculated. Note that this CE
stage uses a single bias resistor RB1 which is the value RBB.
Zi
Zb = hie + (1 + hfe) RE
= 0.56k + ( 1 + 120) 1.2k = 145.76k
Zi = RB || Zb
Zi = 270k || 145.76k = 94.66k
Zo
Zo ≈ 5.6k
Av
hfe RC
Av = −
Zb
120 x 5.6k
=−
145.76k
Av = − 4.61
Electronics III by Engr. James
Ai
RBB hfe
Ai =
RBB + Zb
270k x 120
=
270k + 145.76k
Ai = 77.93
Summary
The hybrid model is limited to a particular set of operating conditions for accuracy. If the
device is operated at a different collector current, temperature or Vce level from the
manufacturers datasheet then the h parameters will have to be measured for these new
conditions. The hybrid model has parameters for output impedance and reverse voltage ratio
which can be important in some circuits.