Q1 .
Explain why anordinary junction transistor(BJT)is
called bipolar.
Ans.
An ordinary junction transistor is calledbipolarbecauseboth
electrons and holes(i.e., two types of charge carriers)are
involved in its operation.
● In anNPN transistor, themajority carriersin the
emitter areelectrons, and they flow through the baseinto
the collector.
● In aPNP transistor, themajority carriersareholes,
moving from emitter to collector.
● However, theminority carriers(holes in N-type and
electrons in P-type) also play a role, especially in the base
region where recombination occurs.
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So, unlike devices like field-effect transistors (FETs), which
rely on only one type of carrier (either electrons or holes),
bipolar junction transistors (BJTs)depend on themovement
and interaction of both types of charge carriers,hence the
namebipolar.
Q2. Why is a transistor called acurrent-controlleddevice?
Ans.
● In a transistor, especially a BJT (Bipolar Junction
Transistor), theinput is the base currentand theoutput
is the collector current.
● Asmall currentflowing into the base cancontrola much
larger currentflowing from collector to emitter.
● This means the transistoramplifies current— theoutput
current depends directly on the input current.
● Because theoperation of the transistor is based on
controlling current using another current, it is calleda
current-controlled device.
● This is different from FETs (Field Effect Transistors),
which arevoltage-controlled devices.
A transistor is called a current-controlled device because its
output current is controlled by the input (base) current.
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Q3. What is thesignificance of the arrowheadin the
transistor symbol? Why are NPN transistors commonly used
over PNP transistors?
Ans.
● It helps to identify whether the transistor isNPNor PNP.
NPN Transistor:
● Arrow pointsoutwardfrom the emitter.
● Indicates currentflows outof the emitter — meaning
emitter is negativecompared to base.
● Electronsare the majority carriers (negative charge
carriers).
PNP Transistor:
● Arrow pointsinwardtoward the emitter.
● Indicates currentflows intothe emitter — meaning
emitter is positivecompared to base.
● Holesare the majority carriers (positive charge carriers).
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● Why NPN Use Over PNP Transistors.
NPN transistors use electrons as majority charge carriers,
which move faster than holes in PNP transistors.
They are more efficient and suitable for high-speed switching.
Most circuits use a positive power supply, and NPN transistors
work better with it.
It is easier to provide the required biasing in NPN using a small
positive voltage at the base.
NPN transistors usually have better current handling and are
more widely available.
Grounding the emitter in NPN makes circuit design simpler
Q4. What do you mean bybiasing in NPNtransistor?
Ans.
Biasing in an NPN transistor means applying proper voltages
to its terminals to make it work in the desired mode.
The base is given a small positive voltagecompared to the
emitter to forward bias the base-emitter junction.
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The collector is given a higher positive voltagethan the base
to reverse bias the collector-base junction.
This setup allows current to flow from collector to emitter,
controlled by the base current.
Proper biasing is necessaryfor the transistor to work as an
amplifier or a switch.
Q5. Why is thecollector made largerthan the emitter and
base?
Ans.
The collector is made larger because it handles more current
and power than the base and emitter.
Collector current (Ic) is nearly equal to emitter current (Ie)
because base current (Ib) is very small.
Equation:Ie = Ib + Ic
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Since most of the current flows through the collector, it
needs a larger area todissipate heatsafely.
Larger size helps prevent overheatingand increasesthe
reliability of the transistor.
Thebase is very thinto allow easy movement of carriers, and
theemitter is medium-sizedfor efficient injectionof
electrons (in NPN).
Q6. Can a transistor be obtained byconnecting two
semiconductor diodesback to-back?
Ans.
No, a transistor cannot be obtained by connecting two
semiconductor diodes back to back.
Although it looks like two PN junctions,it is notpractically
possiblebecause the middle region (base) in a transistorisvery
thin and lightly doped, unlike in diodes.
In a transistor, the base controls the current, but two diodes
joined togethercannot provide this control, so theywon’t
behave like a transistor.
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Q7. Why isCE configurationthe most popular amplifier
circuit?
Ans.
● CE configuration is widely used because it provideshigh
voltage gainandmoderate current gain, making it
excellent for amplification.
● It is commonly used in practical circuits likeradios, audio
systems, and communication devicesdue to its good
performance.
● Theoutput signal is strong and clearly amplified, which
makes CE the preferred choice among the three
configurations (CB, CC, CE).
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●
● Hence, due to its high gain and wide usability, the CE
configuration is the most popular for amplifier
applications.
Q8. What do you mean byoperating point?
Ans.
● Theoperating point(also calledquiescent point or
Q-point) is theDC current and voltageat a specificpoint
on the transistor's characteristic curve.
● It shows thesteady conditionof the transistor whenno
input signalis applied.
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● It is decided by thebiasing circuit, and it helps in setting
the transistor’s mode of operation.
● A properly set operating point ensures that the transistor
works in theactive regionfor faithful amplification.
Thus, the operating point is crucial for stable and
distortion-free transistor operation.
Q9. Why is afixed-bias circuitnot used commonly?
Ans.
● A fixed-bias circuit is not commonly used because it isnot
stable for long-term use.
● It getseasily affected by temperature, which can
disturb the transistor's working.
● There isno self-adjusting feature, so small changescan
impact the circuit.
● It isless reliablefor practical applications.
Instead, voltage divider bias is preferred as it gives better
stability and reliable performance in real circuits.
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Q10. Write a fewapplications of BJT.
Ans.
● Used as anamplifierin radios, audio systems, and
communication devices.
● Works as aswitchin digital and electronic circuits.
● Used insignal processingto boost weak signals.
● Found inoscillator circuitsto generate AC signals.
● Used involtage regulatorsandmotor drivers.
Q11. What is a Transistor? Showtransistor actionfor PNP
and NPN transistors.
Ans.
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● Atransistoris a semiconductor device used for
amplification and switching.
● Transistors are mainly of two types:BJT (Bipolar
Junction Transistor)andFET (Field Effect Transistor).
● BJTworks usingboth electrons and holes(bipolar) and is
current-controlled.
● FETworks using only one type of charge carrier and is
voltage-controlled.
Transistor Action (For BJT):
● It has three regions – Emitter, Base, and Collector.
● There are two types: NPN and PNP transistors.
● InNPN, when the base-emitter junction is forward-biased
and the collector-base junction is reverse-biased,
electronsflow from emitter to collector.
● InPNP, when the base-emitter junction is forward-biased
and collector-base is reverse-biased,holesflow from
emitter to collector.
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Q12. Define the following termsw.r.t. BJT: α, β, γ , ICO,
ICBO, ICEO.
Ans.
Common Compare Area Larger Area Smaller Area
Terminal of Remaining Terminal → Terminal →
Two Output Input
Base (CB Collector vs Collector (large Emitter
mode) Emitter area)
Emitter Collector vs Collector (large Base
(CE mode) Base area)
Collector Emitter vs Base Emitter Base
(CC mode) (moderate
area)
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Term Definition
α Amplification factor inCommon Base (CB)
(Alpha) configuration. It is the ratio of collector current to
emitter current:
α = IC / IE
β Amplification factor inCommon Emitter (CE)
(Beta) configuration. It is the ratio of collector current to
base current:
= IC / IB
β
γ Amplification factor inCommon Collector (CC)
(Gamm configuration. It is the ratio of emitter current to
a) base current:
= IE / IB = β + 1
γ
ICBO Leakage current betweenCollector and Basewith
Emitter open(reverse saturation current).
ICO Another name forICBO(often used interchangeably
in different textbooks).
ICEO Leakage current betweenCollector and Emitterwith
Base open. It is given by:
CEO = (1 + β) ×
I
ICBO
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Q13. Draw differentcircuit configurations of biasingin
Transistors.
Ans.
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15
Q14. Differentiate betweenCB, CE, and CC configuration.
Ans.
Point CB (Common CE (Common CC (Common
Base) Emitter) Collector)
Common Base is Emitter is Collector is
terminal connected to connected to connected to both
both input and both input and input and output.
output. output.
Input Between Between Base Between Base and
connectio Emitter and and Emitter. Collector.
n Base.
Output Between Between Between Emitter
connectio Collector and Collector and and Collector.
n Base. Emitter.
Current Very low (less High. It Very high.
gain than 1). multiplies the
current.
Voltage High. High. Low.
gain
Power Medium. Highest among Medium.
gain the three.
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Input Low. It takes Medium. High.
resistanc small signals.
e
Output High. Medium to Low.
resistanc High.
e
Used for High-speed Most To connect two
circuits like amplifiers, circuits without
radio audio systems, loss (impedance
frequency. speakers. matching).
Q15. Draw and explainoutput characteristics of n-p-n
silicon transistorsin CB and CE configurations. Indicate
cut-off, saturation, and active regions.
Ans. CE configuration.
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CB configuration.
1. Cut-off Region:
● Transistor is OFF.
● No current flows from collector to emitter.
● Junctions are not forward-biased.
● Used when you want the transistor to act like a switch
(open).
2. Active Region:
● Transistor is ON and amplifying.
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● In CE, base-emitter is forward-biased; collector-base
is reverse-biased.
● In CB, emitter-base is forward-biased; collector-base
is reverse-biased.
● Current is controlled (by base in CE, by emitter in CB).
● Used in amplifiers.
3. Saturation Region:
● Transistor is fully ON.
● Both junctions are forward-biased.
● Maximum current flows.
● Acts like a closed switch.
● Used in digital logic (to represent ON state).
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Q16. DiscussCC configurationwith Input and output
characteristics.
Ans.
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The Common Collector (CC) configuration is a basic transistor
configuration where:
● Collector terminal is common to both input and output.
● Input is applied between base and collector.
● Output is taken between emitter and collector.
Input Characteristics – (Ib vs Vbc)
1.Input is applied between base and collector.
2.Graph is plotted between Ib (Y-axis) and Vbc (X-axis).
3.Curve looks like a forward-biased diode.
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4.Non-linear relationship – small change in Vbc gives large
change in Ib.
5.As Vbc increases, Ib increases rapidly.
Output Characteristics – (Ie vs Vec)
1.Output is taken between emitter and collector.
2.Graph is plotted between Ie (Y-axis) and Vec (X-axis)
for different Ib values.
3.Curves are almost horizontal (flat).
4.Ie remains almost constant for a given Ib, even if Vec
changes.
5.As Ib increases, the Ie also increases (curves shift
upward).
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Q17. DrawCE, CB and CC circuit configurations of NPN and
PNPtransistor.
Ans.
● CE Configuration for NPN And PNP
● CB Configuration for NPN And PNP
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● CC Configuration for NPN And PNP
Q18. Discuss theEmitter bias techniquein brief.
Ans.
Definition:
Emitter bias is a transistor biasing method used tostabilize
the operating point(Q-point), even if temperatureor
transistor β (gain) changes.
Circuit Setup:
It usestwo power supplies– one positive (+Vcc)and one
negative (–Vee). A resistor (Rb) connects the baseto +Vcc, and
another resistor (Re) connects the emitter to –Vee.
Working Principle:
Theemitter resistor (Re)providesnegative feedback.If the
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current increases, the voltage across Re increases, which
reducesVbeand automatically brings the current backdown.
Advantages:
This method givesexcellent thermal stabilityandkeeps the
transistor current steady even if β varies. It’s very reliable for
stable amplifier operation.
Disadvantage:
The main drawback is the need fordual power supplies,which
makes the circuit slightly more complex and may not be ideal
for all applications.
Q19. Discuss theCollector to Base bias techniquein brief.
Ans.
Definition:
This biasing method connects thebase resistor (Rb)directly
to thecollectorinstead of the power supply. It helpsgive
moderate stabilityto the transistor’s operating point.
Circuit Setup:
A resistorRb is connected from base to collector,andRcis
connected from collector to+Vcc. The base-emitterjunction is
forward biased.
Working:
Ifcollector current (Ic)increases,collector voltage(Vc)
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drops. This reducesbase current (Ib), which in turn decreases
Ic — givingnegative feedbackandpartial stability.
Advantage:
It providesbetter stabilitythan fixed bias withoutneeding
extra power supplies. It'ssimpler than emitter bias.
Disadvantage:
Stability isnot as strongas emitter bias. Also,performance
depends onβand may vary with temperature.
Q20 to Q22
Click on the ink:-Solution
Q23.How to identifytransistor leads?
Ans.
Hold the flat side of the transistor facing you.
ForNPN or PNP (e.g., BC547): the leads fromleftto right
areEmitter, Base, Collector.
Use a multimeter indiode modeto confirm:
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● Base to emitter and base to collector will show a voltage
drop (~0.6–0.7V) for NPN.
● Reverse polarity confirms PNP if drop is seen when red
probe is on emitter/collector and black on base.
Q24. Draw theDC load line curve of BJTand show Q Point.
Ans.
It is a straight line drawn on the output characteristics of a
BJT. It represents all possible combinations ofcollector
current (Ic)andcollector-emitter voltage (Vce)fora given
load resistor (Rc) and supply voltage (Vcc). The line is obtained
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using KVL:
Vcc = IcRc + Vce
● When Ic = 0, Vce = Vcc (cut-off point).
● When Vce = 0, Ic = Vcc/Rc (saturation point).
This line helps in finding theQ-point (quiescentpoint)which
indicates the BJT's operating region.
Q25.A BJT has a base current of 200 micro A and an
emitter current of 20 mA. Determine collector current and
beta.
Ans.
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