Disadvantages of half wave rectifier  
•
Power loss. This is because it allows either a positive half cycle or negative half  
cycle. So the remaining half cycle is wasted  
•
•
Pulsating direct current. It produces impure direct current which is not much useful  
Produces low output voltage  
Full – Wave Rectification  
•
There are two ways of achieving full – wave rectification:  
(a) Using two diodes and center – tapped transformer  
(b) Using four diodes (the bridge rectifier)  
(a) Using two diodes  
•
In this circuit both halves of the a.c. cycles are transmitted but in the same  
direction. One way of achieving this is to have a transformer whose output has  
a Centre tap, that is, its output can be taken at two points one being half the other  
Mechanism  
•
In the positive half-cycle, point A is positive with respect to O. Diode D1 conducts but  
diode D2 is reverse-biased. The current passes through D1, C, R and back to O  
•
In the negative half-cycle, point B is positive with respect to O. Diode D2 conducts  
but diode D1 is reverse- biased. The current passes through D2, C, R and back to O  
Alternatively (other way of expressing this)  
Mechanism  
•
During the first half cycle, diode D1 is forward biased while D2 is reverse biased.  
Hence current flows through AD1BCA  
•
During the second half cycle, diode D2 will now be forward biased while D1  
reversed biased .Thus current flows through DD2BCD  
.
N.B  
•
In both half cycles, the direction of flow of current through the resistor is the  
same. The resultant output will therefore take the form shown below  
(b) Using four diodes (bridge rectifier  
)
•
A bridge rectifier uses four diodes such that in each half cycle two diodes are  
forward biased and the remaining two are reversed biased (see the fig. below)  
Mechanism  
•
During the first half cycle, the diodes D1 and D3 are forward biased while D2 and D4  
are reversed biased. Current thus flows through diode D1 and D3 via the resistor R.  
During the second half cycle, diodes D1 and D3 are now reversed biased while D2  
and D4 are forward biased. Current thus flows through D2 and D4 via the resistor R  
•
N.B  
•
In both half cycles current flows through the resistor R in one direction only.ie  
from end A to B. This kind of rectifier can be used with very high voltage  
If a smooth rectified wave is needed, then a capacitor is connected across the resistor  
•
Advantages of the bridge rectification  
•
•
A smaller transformer can be used, because there is no need for centre – tapping  
It is suitable for high voltage regulation  
Disadvantage of the bridge rectifiers  
•
•
High cost of making. Since it uses four diodes  
The value of the diodes used should be precise, else there will be an error in  
rectification  
N.B  
•
In general, the full wave rectifier gives a stronger and smoothened output than  
the half – wave rectifier. If a capacitor is connected across the resistor, the  
rectified output is smoothened. The capacitor is therefore used in this case to  
smoothen the output of the transformer  
Advantages of center tapped full wave rectifier  
•
Output and efficiency are high because an AC supply delivers power during the  
both half cycles  
Disadvantages of center tapped full wave rectifier  
•
•
It is difficult to locate the center on the secondary winding for the tapping  
The diode used must be capable of bearing high peak inverse voltage (PIV).  
This is because PIV coming across each diode is twice the maximum voltage  
across the half of the secondary winding  
Transistors  
Transistor  
Is a semiconductor device for amplifying, controlling, and generating  
electrical signals.  
OR  
Is a semiconductor device used to amplify or switch electronic signals  
and electrical power  
Terminals of Transistor  
•
There are three terminals namely emitter (E), collector (C) and base (B)  
Emitter (E)  
•
•
Is a terminal used to supplies (remove /push) charge carriers to Collector  
It is the negative lead (region)  
Collector (C)  
•
•
•
Is a terminal used to receive charge carriers repelled from emitter terminal  
It is the positive lead (region)  
The maximum possible current obtained at the output (collector) is called  
Saturation current  
Base (B)  
•
•
B:  
Base is a terminal between emitter terminal and collector terminal  
It is used to control the flow of charge carriers from E to C or C to E  
N
✓
Some transistors are packaged individually but most are found in integrated  
circuits, IC (more than one transistor)  
✓
Base terminal is thin and lightly doped  
Types of Transistors  
❖
❖
Field-effect transistors (FETs)  
Bipolar transistors (BJT)  
Field-Effect Transistor (Unipolar transistor)  
•
•
It is a type of transistor which uses an electric field to control the flow of current  
It requires only one charge carrier to operate (eg N–channel FET or P–channel  
FET are used for conduction)  
•
•
•
The three terminals of FET are source, gate and drain  
Examples of FET’s are MOSFET, JFET etc  
Are used in low noise amplifier, buffer amplifier and analog switch  
N.B: In this section (according to our level) we will only consider bipolar transistors  
Bipolar Junction Transistor (BJT)  
•
•
Is a type of transistor that uses both electrons and holes as charge carriers  
It is a three-terminal device that acts as electrically controlled switch or as  
amplifier controls.  
•
It consists of a pair of p -n junction diodes that are joined back-to-back  
(sandwich form). The leads (regions) are labeled as base (B), collector (C) and  
emitter (E) (Fig below)  
Types of Bipolar Transistors  
•
•
n-p-n transistor  
p-n-p transistor  
NPN Transistor  
•
Is a type of transistor in which one p–type material is doped with two n–type materials  
•
The diagram and symbol of npn transistor is shown in the figure below  
NB:  
•
The outward arrow shows that the direction of current in npn transistor is from the  
collector through the base to the emitter ( NPN  
→
N
ever  
P
oints iN)  
Construction of NPN Transistor  
•
The construction of NPN transistor is shown in the figure below. The emitter–base  
junction is connected in forward biased while the collector–base junction in reverse biased  
Operation of NPN Transistor  
•
The forward biased is applied across the emitter –base junction while the  
reverse biased is applied across the collector–base junction.(See the fig below)  
•
The emitter is heavily doped. When the forward bias is applied across the  
emitter, the majority charge carriers (electrons) move towards the base. This  
causes the emitter current IE .The electrons enter into the P–type material and  
combine with the holes  
•
Since the base is thin and lightly doped. Thus only a few electrons are  
combined with holes and the remaining (most of electrons) are moved towards  
the collector to constitute the base current IB. This base current enters into the  
collector region  
•
The reversed bias potential of the collector region applies the high attractive  
force on the electrons reaching the collector junction. Thus attract or collect the  
electrons at the collector  
•
The whole of the emitter current is entered into the base. Thus, we can say that  
the emitter current is the sum of the collector or the base current (IE = IC + IB)  
N.B  
•
The collector current in any transistor is less than the emitter current because  
of the recombination of holes and electrons occurring in the base area  
In order for a transistor to work, the emitter should always be connected in  
forward biased while Collector is always connected in reverse biased  
The main difference between PNP and NPN transistor is that, the current conduction  
in NPN carried by electrons while the charge carriers in PNP are carried by holes  
•
•
PNP Transistor  
•
•
Is a type of transistor in which one n–type material is doped with two p–type materials  
The diagram and symbol of pnp transistor is shown in the figure below  
•
The inward arrow shows that the direction of current in pnp transistor is from the  
emitter to collector (PNP  
→
Points iN Permanently  
)
Construction of PNP Transistor  
•
The construction of PNP transistor is shown in the figure below. The emitter–base junction  
is connected in forward biased while the collector–base junction in reverse biased  
The emitter in forward biased attracts the electrons towards the battery and hence  
constitutes the current to flow from emitter to collector  
•
•
•
The base of the transistor is always kept positive with respect to the  
collector so that the hole from the collector junction cannot enter into the base  
And the base–emitter is kept in forward due to which the holes from the  
emitter region enter into the base and then into the collector region by  
crossing the depletion layer  
Working of PNP Transistor  
•
The emitter base junction is connected in forward biased due to which the emitter  
pushes the holes in the base region. These holes constitute the emitter current  
When these holes move into the base, they combine with electrons.  
•
•
Since the base of the transistor is thin and very lightly doped .Hence only a few  
holes combine with the electrons and the most (remaining) are moved towards the  
collector space charge layer. Hence develops the base current (See the fig below)  
•
•
The collector base region is connected in reverse biased. The holes which collect  
around the depletion region when coming under the impact of negative polarity  
attracted by the collector. This develops the collector current. The complete emitter  
current flows through the collector current IC  
Thus, we can say that the emitter current is the sum of the collector or the base  
current (IE = IC + IB)  
Difference between PNP and NPN transistors  
NPN Transistor  
PNP Transistor  
It consists of an N–type sandwiched by  
two P–type semiconductor  
It consists of an P–type sandwiched by  
two N–type semiconductor  
It has holes as majority charge carriers  
It has electrons as majority charge carriers  
It is slower than NPN because holes  
are slower than electrons  
Less preferred than NPN  
It is faster than PNP because electrons  
are faster than holes  
Most preferred than PNP (most used)  
Both collector and base are negative  
with respect to emitter  
The collector is positive with respect to  
both emitter and base  
Why always NPN most used rather than PNP?  
•
This is because the mobility of electrons in N type material is more than the  
mobility of holes in P type material  
•
Addition reason: Silicon is used to make N type semiconductors. Since it is  
found abundantly in nature  
Uses of Transistors  
•
They are used in virtually all electronic devices such as calculators  
,
televisions  
,
radios  
,
computers, etc.  
Application of Transistor  
➢
➢
As an amplifier (electronic amplifier)  
As a switch (electronic switch)  
Transistor as Regulator/Amplifier/Active Mode  
Electronic amplifier is the circuit that increases the amplitude (intensity) of a given input  
OR  
is the magnification of input signal  
Types of electronic Amplifiers  
•
•
Single-stage amplifier  
Multi-stage amplifier  
NB:  
o
o
o
Relationship between the input and output of an amplifier is called the transfer function  
The magnitude of the transfer function is referred to as the gain  
Amplifiers commonly used in radio and television transmitters and receivers  
,
stereo equipment  
,
microcomputers and digital musical instruments  
o
o
Transistors are commonly used as amplifying elements  
In this section we will consider Single-Stage Amplifiers Only  
Single-Stage Amplifier  
•
•
Is the type of amplifier which consists of only one amplifying device.  
It consists of transistor (amplification stage) which is connected to a load resistor  
through which a load current flows  
Types of Single-Stage Amplifiers  
➢
➢
➢
Common-collector (CC) amplifiers  
Common-base (CB) amplifiers  
Common- emitter (CE) amplifiers  
Common-Collector Amplifier  
•
The base terminal of the transistor serves as the input, the emitter the output,  
while the collector is common to both. The emitter-base junction is forward-biased by  
the power supply VEE while the collector- base junction is reverse-biased by VCC  
Mechanism  
➢
The input signal is fed to the base-collector circuit while the output signal is  
tapped from the emitter terminal with respect to the ground  
➢
C1 and C2 are coupling capacitors to provide direct current isolation at the input  
and output of the amplifier  
Common-Base Amplifier  
•
The emitter terminal serves as the input, the collector as the output, and the  
base is common to both. The emitter-base junction is forward-biased by the  
power supply VEE while the collector base junction is reverse-biased by VCC  
Mechanism  
o
The input signal is fed to the emitter-base circuit while the output signal is tapped  
from the collector-base circuit  
o
C1 and C2 are coupling capacitors to provide direct current isolation at the input  
and output of the amplifier  
Common-Emitter Amplifier  
•
The base terminal of the transistor serves as the input, the collector as the  
output, and the emitter is common to both. The emitter-base junction is  
forward-biased by power supply VBB while the collector-base junction is  
reverse-biased by power supply VCC  
Mechanism  
•
The input signal is fed to the base-emitter circuit and the amplified signal is  
tapped from the collector terminal with respect to the ground emitter circuit  
•
C1 and C2 are coupling capacitors to provide direct current isolation at the input  
and output of the amplifier  
Light Dependant Resistor (LDR)  
•
•
Is a semiconductor device whose resistance depends on the light falling on it  
Is used to switch on or switch off automatically street lamp  
Thermistor  
➢
➢
Is a semiconductor device whose resistance depends on thermal energy  
It is used to control maximum temperature of hot liquid  
Example Form IV NECTA 2002 QN: 10 SECT. C  
(a) Explain how an extrinsic semiconductor is constructed  
(b) Describe the model of action and application of PN junction diode  
(c) The diagram below shows a circuit in which important components A and B are removed  
Answer the following questions  
(a) What does A and B represents?  
(b) Describe briefly the purpose of component A and B  
(c) Draw a well labeled circuit diagram for the circuit above  
Integrated Circuit (IC)  
•
Is a combination of several resistors, capacitors or transistors which are built  
out of the same crystal  
OR  
•
Is an electronic device made of semiconductor material containing up to billions  
of electronic circuits and components such as transistors in a small space (chip)  
Two or more transistors can be joined together to increase the amplification  
An integrated circuit is sometimes known as a chip or microchip  
•
•
Information Signals  
•
Are current or voltage variations (waveforms) through which information is  
relayed in electronic circuits  
Types of Information Signals  
•
•
Analogue signals  
Digital signals  
Analogue Signals  
•
Are electrical signals that convey or store information by means of variation in  
a continuous wave form  
•
Electrical signals may represent information by changing factors such as their  
voltage, current, frequency or total charge. The information is converted from  
some physical form (such as sound, light, temperature, pressure) to an  
electrical signal by a device known as a transducer  
Transducer:  
OR  
Is a device that converts an input signal of one form into an output  
signal of another form  
Is a device which converts a physical variation such as temperature,  
pressure or sound to an electrical signal  
OR  
Is a device which converts energy from one form to another  
Digital Signals  
•
Are electrical signals that convey or store information by means of variation in  
a non-continuous wave form  
•
Digital signals have only two amplitude levels, usually called nodes. This  
means the values can only be given in one of two ways. The values may be  
specified, for Example, as  
1
or  
0
,
TRUE or FALSE, and HIGH or LOW  
•
•
Digital signals are often derived from analogue signals  
The main advantage of digital signals over analogue signals is that the signal  
level or value need not be precise  
•
•
It can be approximated within a fixed number of digits or bits  
The process of approximating the precise value within a fixed number of digits  
is called quantization  
•
Signal can be distorted during transmission can still read correctly  
Class Activity – 5  
1. Explain why an ordinary junction transistor is called bipolar?  
ANS: Because, the transistor operation is carried out by two types of charges  
carriers (majority and minority carriers)  
2. Why transistor is called current controlled device?  
ANS: Because, the output voltage, current or power is controlled by the input  
current in a transistor  
3. What is the significance of the arrow –head in the transistor symbol?  
ANS  
:
Is to show the conventional direction of current flow.(From emitter – to – base  
in case of p-n-p transistor and from base – to – emitter in case of n – p –n transistor)  
N.B  
:
Arrow head is always marked on the emitter and not for collector, since collector  
always reverses its leakage current opposite to the direction of emitter current  
4. Discuss the need for biasing the transistor.  
ANS: For normal operation, base–emitter junction should be forward biased  
and the collector base Junction reverse biased  
5. What are the differences between a semiconductor and an insulator in terms of  
their conductivity  
6. In case the transistor is not biased properly, what would happen?  
ANS: It would  
➢
Work inefficiently  
➢
Produce distortion in the output signal  
➢
With the change in transistor parameters or temperature rise, the operating  
point may shift and the amplifier output will be unstable  
7. Which of the transistor currents is always the largest? Which is always the  
smallest? Which two currents are relatively close?  
ANS: The emitter current IE is always the largest one. The base current IB is  
always the smallest one .The collector current IC and emitter current IE are  
relatively close in magnitude  
8. Why collector is made larger than emitter and base?  
ANS: Collector is made physically larger than emitter and base because  
collector is to dissipate much power  
9. Why silicon type transistors are more often used than germanium type?  
ANS: This is because:  
(i) At room temperature, Silicon crystal has fewer free electrons than  
Germanium crystal. This implies that Silicon will have much smaller  
collector cut off current than Germanium  
(ii) The variation of collector cut off current with temperature is less in  
Silicon compared to Germanium  
(iii) The structure of Germanium crystals will be destroyed at higher  
temperature while Silicon crystal are not easily damaged by excess heat  
10. Why the width of the base region of a transistor is kept very small compared to other  
region? (ANS: In order to pass most of the injected charge carriers to the collector)  
11. Why emitter is always forward biased?  
ANS: Emitter is always forward biased with respect to base so as to supply the  
majority charge carriers to the base.  
12. Why collector is always reverse biased w.r.t base?  
ANS: In order to remove the charge carriers from the base–collector junction  
13. A diode has a certain characteristic when operating. Explain this characteristic  
.
ANS: The main operating characteristic of a diode is that it allows current in  
one direction and blocks current in the opposite direction.  
14. Distinguish between semiconductors and conductors and give one example for each  
15. What must the conditions be for a LED to emit light?  
ANS: A LED emits light when the diode is forward biased allowing current to flow.  
16. Transistors have two main functions, what are they?  
ANS: Amplification and switching.  
17. Define doping. (ANS: The process of adding impurities to the intrinsic or pure semiconductor )  
18. What are the differences between a conductor, semiconductor and insulator in  
terms of their energy levels?  
19. Your provided with a diode, a resistor R an a.c source of low voltage and  
connecting wires , Sketch the circuit diagram for a half – wave rectifier and  
indicate the terminals where the output voltage vo may be connected  
ANS:  
20. What is meant by Donor impurity in semiconductors?  
(ANS: Is an atom introduced into the semiconductor (doping) to provide an extra  
electron for conduction)  
21. Explain how intrinsic semiconductor can be changed into a transistor  
22. Mention two types of :  
(a) Semiconductors  
(b) Diode  
23. What is the difference between analogue and digital signals?  
24. (a) What is the difference between PNP and NPN transistors?  
(b) Draw their circuit symbols, label them and describe each connection  
25. The semiconductor diode can be used as a rectifier as used in the circuit below  
(a) What does the term rectification means?  
(b) The rectification described by the circuit above is half wave rectification. Sketch  
its waveform which would be seen on a suitably adjusted CRO and explain why  
the output voltage is so rectified  
26. The output power of a signal is 50 W and the input power is 0.1 W .Calculate the power gain  
27. What is an IC?  
28. State the majority carriers for a p – type semi conductors  
29. Explain how doping produces an n – type semiconductor  
30. Study the circuit diagrams in the figure below carefully. Which of the two circuits  
will light a bulb?  
31. Name two semiconductors which are widely used in electronics  
32. Explain why semiconductor solid state diodes are fast replacing the vacuum type  
diodes in many applications  
33. (a) Give an illustration of a p – n junction diode and its symbol  
(b) How does a junction diode work?  
34. How is an intrinsic semiconductor different from an extrinsic semiconductor?  
.Explain how an extrinsic semiconductor is constructed?  
35. Define the following terms (a) Transistor (b) Doping (c) Rectification  
36. Draw the circuit diagram of the (a) half wave rectification (b) Full wave rectifier(use 2 diodes)