ELECTRONICS

1. A two-input logic gate comprises an OR gate whose output is connected to the input of a NOT
gate. What is the output state of the gate when its inputs are both HIGH?

ANSWER: LOW
NOT(HIGH OR HIGH) = LOW

2. A two-input logic gate comprises an AND gate whose output is connected to the input of a NOT
gate. What input state results in a LOW output state?

ANSWER: Both inputs HIGH

3. A two-input logic gate comprises a two-input AND gate with a NOT gate connected to each
input. What is the output state of the gate when its inputs are both HIGH?

ANSWER: LOW

(4) The common-emitter current gain of a bipolar junction transistor is 10. Find the base current needed for
a collector current of 100 mA when the transistor is biased in the active mode.

Ans: 𝟏𝟎 𝐦𝐀
𝐼𝐵 = 𝐼𝐶 /𝛽 = 100 mA/10 = 10 mA

(5) When a bipolar junction transistor is biased in the forward-active mode, the collector current is 500 mA
when the base current is 5 mA. What is the common-emitter current gain of the transistor?

Ans: 𝟏𝟎𝟎
𝛽 = 𝐼𝐶 /𝐼𝐵 = 500 mA/(5 mA) = 100

(6) The common-emitter current gain of a bipolar junction transistor is 50. Find the base current needed for
an emmiter current of 102 mA when the transistor is biased in the active mode.

Ans: 𝟐 𝐦𝐀
𝐼𝐵 = 𝐼𝐶 /𝛽 = 𝐼𝐸 /(1 + 𝛽) = 102 mA/51 = 2 mA

Contest 20a

(1) What is the minority carrier type in the base region of a bipolar junction transistor with n-type majority
carriers in its emitter?

Ans: N-type

(2) The base of a bipolar junction transistor is p-type. What is the majority carrier type in its collector?

Ans: N-type
(3) If the minority carriers in the collector of a bipolar junction transistor are p-type, what type of bipolar
transistor is the device?

Ans: NPN

1. Name the process by which an intrinsic semiconductor is converted to an extrinsic

semiconductor.

ANSWER: Doping

2. What type of extrinsic semiconductor is produced by introducing phosphorous impurities into

pure silicon?

ANSWER: N type

3. Name the usual dopant used to convert pure silicon into p-type silicon?

ANSWER: Boron
(1) A current of 16 mA flows through a forward-
biased light emitting diode. How many electrons
flow through the diode every second?
Ans: 𝟏.𝟎×𝟏𝟎𝟏𝟕 𝑅=𝐼/𝑒=16×10−3 A/(1.60×10−19
C)=1.0×1017 s−1
(2) Water flows through a pipe with a volume flow
rate of 0.5 m3 s−1. Find the flow speed in a section
of the pipe of cross sectional area 20 cm2.
Ans: 𝟐𝟓𝟎 𝐦/𝐬 𝑣=0.5 m3/s÷(20×10−4 m2)=250
m/s
(3) A constant 500 N force acts on a 20 kg mass
initially moving at 5 m s−1. Find the change in
linear momentum of the object after 4 s.
Ans: 𝟐𝟎𝟎𝟎 𝐍 𝐬 Δ𝑝=𝐹Δ𝑡=500 N×4 s=2000 N s
1. What is an intrinsic semiconductor?
ANSWER: A pure semiconductor with equal electron and hole concentrations.
2. What is a p-type semiconductor?
ANSWER: A semiconductor with excess holes.
3. What is a compensated semiconductor?
ANSWER: One that is doped to have equal hole and electron concentrations.

1. The magnitude of the gain of an inverting amplifier is . What is the output voltage when the
input voltage is ?

ANSWER: 64 mV
 1. If the collector of a bipolar junction transistor is n-type, what is the conductivity type of
the base?

ANSWER: P-type
2. What is the biasing condition for the base-emitter junction of a bipolar junction transistor
in saturation?

ANSWER: Forward bias

3. Name the bipolar junction transistor that has n-type base.

ANSWER: PNP
1. Name the state of a p-n junction when the p-type terminal is connected to the positive terminal of
a battery and the n-type terminal is connected to the negative terminal of the battery.
ANSWER: Forward biased
2. Name the state of a p-n junction when the n-type terminal is connected to the positive terminal of
a battery and the p-type terminal is connected to the negative terminal of the battery.
ANSWER: Reverse biased
3. Name the current that flows across a reverse biased p-n junction.
ANSWER: Reverse saturation current

1. Which of the following pure materials is an electrical insulator at 0 K: Silicon dioxide, silver,
silicon, gold?
ANSWER: Silicon dioxide and silicon
2. Which of the following pure materials is a semiconductor at 300 K: Lead, germanium,
polyethylene, nickel?
ANSWER: Germanium
3. Which of the following pure materials is known to undergo a transition to a superconducting
state: Lead, glass, mercury, polyethylene?
ANSWER: Lead and mercury

1. What logic gate is obtained when a NOT gate is connected to the output of an AND gate?

ANSWER: NAND

2. What logic gate is obtained when a NOT gate is connected to the output of a NOR gate?

ANSWER: OR

3. What logic gate is obtained by joining the inputs of a NAND gate?

ANSWER: NOT

1. What does the acronym LED stand for?

ANSWER: Light emitting diode

2. What does the acronym BJT stand for?

ANSWER: Bipolar junction transistor

3. What does the acronym CMOS stand for?

ANSWER: Complementary metal-oxide semiconductor

1. What is the effect on output of increasing rotor speed of a simple dc motor?
1. Name the high resistance region that separates the p- and n-type regions of a pn junction.
ANSWER: Depletion region
Or depletion layer, or space charge region/layer, depletion zone, junction region
2. What happens to the width of the depletion region of a pn junction when the junction is reverse
biased?
ANSWER: It increases
3. What physical process accounts for the presence of a depletion layer in an un-biased pn junction?
ANSWER: Diffusion of majority charge carriers across the metallurgical junction
ANSWER: Increase in output
(1) A current of 16 mA flows through a forward-biased light emitting diode. How many electrons flow
through the diode every second?

Ans: 𝟏. 𝟎 × 𝟏𝟎𝟏𝟕
𝑅 = 𝐼/𝑒 = 16 × 10−3 A/(1.60 × 10−19 C) = 1.0 × 1017 s−1

1. Distinguish between cold-cathode emission and thermionic emission from a metallic

surface
ANSWER:
Cold-cathode emission occurs when electrons are ejected from the surface of a metal by
the action of an electric field alone; whereas thermionic emission occurs when electrons
are emitted from the surface due to the high temperature of the metal.

2. Mention the three ways in which a bipolar junction transistor is connected in a circuit.

ANSWER: Common base; Common emitter; and Common collector.

 1. Name the current which flows when a p-n junction diode is reverse biased.
ANSWER: Reverse saturation current
2. How is a p-n junction diode reverse biased?
ANSWER: By connecting the n terminal to the positive terminal of a battery and the
p terminal to the negative terminal of the battery
3. How is a p-n junction diode forward biased?
ANSWER: By connecting the p terminal to the positive terminal of a battery and the
n terminal to the negative terminal of the battery
1. A common-emitter amplifier has a base current of 10 μA. What is the collector current if
the current gain of the transistor is 100?
ANSWER: 1 mA
2. A common-emitter amplifier has a collector current of 100 mA. What is the base current
if the current gain of the transistor is 100?
ANSWER: 1 mA
3. A common-emitter amplifier has a base current of 100 μA and a collector current of 1
mA. What is the current gain of the transistor?
ANSWER: 10
1. What is the output pulse sequence from a NOT gate when the input sequence is 010?
ANSWER: 101
2. What is the output pulse sequence from a NOT gate when the input sequence is 100?
ANSWER: 011
3. What input pulse sequence to a NOT gate produces the output sequence 000?
ANSWER: 111
1. What property of a p-n junction allows it to be used as a rectifier?
ANSWER: It has a much higher resistance for current flowing in one direction than
for current flowing in the opposite direction Accept it allows current to flow in only
one direction
2. Where is the depletion layer in a p-n device located?
ANSWER: At the p-n junction, extending partly into each type of material
3. What are the minority carriers in the p-type region of a p-n device?
ANSWER: Electrons
PREAMBLE: A bipolar junction transistor can be operated in several modes. When asked,
give one and only one mode.
1. Give one mode of operation of a bipolar junction transistor.
ANSWER: Active, saturation, or cut-off
2. Give another mode of operation of a bipolar junction transistor.
ANSWER: Active, saturation, or cut-off
3. Give the third mode of operation of a bipolar junction transistor.
ANSWER: Active, saturation, or cut-off
PREAMBLE: Give the bias conditions for operation of a BJT in the given mode.

1. Active.
ANSWER: Emitter-base junction forward biased and collector-base junction reverse biased
2. Saturation.
ANSWER: Emitter-base junction forward biased and collector-base junction forward biased
3. Cut-off.
ANSWER: Emitter-base junction reverse biased and collector-base junction reverse biased
1. What are the majority carriers in the emitter of an npn transistor?
ANSWER: Electrons
2. What are the majority carriers in the collector of a pnp transistor?
ANSWER: Holes
3. What are the minority carriers in the base of an npn transistor?
ANSWER: Electrons
1. A bipolar junction transistor has an n-type base region. What are the majority charge carriers in
the collector region?
ANSWER: Holes
2. What are the minority charge carriers in the collector region of a bipolar junction transistor with
a p-type emitter?
ANSWER: Electrons
3. What are the minority charge carriers in the emitter region of a bipolar junction transistor with a
p-type collector?
ANSWER: Electrons
4. What are the majority carriers in the emitter of an npn transistor?
ANSWER: Electrons
5. What are the majority carriers in the collector of a pnp transistor?
 ANSWER: Holes
6. What are the minority carriers in the base of an npn transistor?
ANSWER: Electrons

PREAMBLE: A BJT has small signal gain 𝛽 = 20.0 and is operated in active mode.

1. Find the emitter current when the base current is 50.0 𝜇A.
ANSWER: 1.05 mA
𝐼𝑒 = (1 + 𝛽)𝐼𝑏 = 21.0 × 50.0 𝜇A = 1050 𝜇A = 1.05 mA
2. Find the base current required for a collector current of 10.0 mA.
ANSWER: 0.500 mA
𝐼𝑏 = 𝐼𝑐 /𝛽 = 10.0 mA ÷ 20.0 = 0.500 mA
3. Find the emitter current when the collector current is 50.0 mA.
ANSWER: 52.5 mA
𝐼𝑒 = (1 + 1/𝛽)𝐼𝑐 = 1.05 × 50.0 mA = 52.5 mA
1. A light-emitting diode (LED) has two terminals, an anode and a cathode. Which terminal is at a
higher potential than the other when the LED emits light?
ANSWER: Anode
2. The anode of a light emitting diode is maintained at 0.5 V above its cathode. Why does the LED
not emit light?
ANSWER: The applied potential difference may be less than the forward voltage drop of
the LED
“0.5 V is too small” may be accepted
3. What name is given to the current that flows when the cathode of a light emitting diode is held at
a higher potential than the anode?
ANSWER: Reverse current
1. How many diodes are used in a full-wave rectifier that employs a centre-tapped transformer?
ANSWER: 2
2. How many diodes are needed in a full-wave rectifier that does not use a centre-tapped
transformer?
ANSWER: 4
3. What is the function of the smoothing capacitor in a half-wave rectifier?
ANSWER: Filter out a.c. signals
1. Name the type of electronic effect the OCH3 group will exert on a benzene ring.
 ANSWER: Electron donating mesomeric/resonance effect and Electron withdrawing,
inductive effect.

2. Name the type of electronic effect the ethyl group will exert on the amino functional group in
ethylamine.
ANSWER: Electron donating inductive effect.

3. Name the type of electronic effect the nitro group on carbon 3 of butanoic acid will exert on the
acid.
ANSWER: Electron withdrawing inductive effect

The elementary charge is approximately 1.6 × 10−19 C.

1. Find the electron flow rate through a copper wire carrying a current of 1.6 A.
ANSWER: 1019 s −1
𝑅 = 𝐼/𝑞 = 1019 s−1
2. Find the magnitude of current in a copper wire through which the electron flow rate is 5 × 1015 s−1.
ANSWER: 0.8 mA
𝐼 = 𝑅𝑞 = 8 × 10−4 A = 0.8 mA
3. Find the electron flow rate through a copper wire carrying a current of 8 A.
ANSWER: 5 × 1019 s −1

The current gain 𝛽of a Bipolar Junction transistor (BJT) is 20.

1. Find the collector current when the base current is 5 mA.

ANSWER: 100 mA
𝐼𝐶 = 𝛽𝐼𝐵
2. Find the base current when the collector current is 80 mA?
ANSWER: 4 mA
𝐼𝐵 = 𝐼𝐶 /𝛽
3. Find the emitter current when the base current is 10 mA?
ANSWER: 210 mA
𝐼𝐸 = 𝐼𝐶 + 𝐼𝐵 = (𝛽 + 1)𝐼𝐵 = 210 mA

1. What is the output state of a two-input AND gate when each input is held at logic level 1?
 ANSWER: 1
2. What is the output state of a two-input OR gate when each input is held at logic level 1?
ANSWER: 1
3. What is the output state of a two-input NAND gate when each input is held at logic level 1?
ANSWER: 0

1. ratio of collector current to base current for a certain BJT is 4. Find the ratio of collector current to
emitter current.
ANSWER: 0.8 𝛼 = 𝛽/(𝛽 + 1) = 4 ÷ (4 + 1) = 0.8
2. The ratio of collector current to base current for a certain BJT is 19. Find the ratio of collector current to
emitter current.
ANSWER: 0.95 𝛼 = 19/20 = 0.95
3. The ratio of collector current to emitter current for a certain BJT is 0.99. Find the ratio of collector
current to base current.
ANSWER: 99 𝛽 = 𝛼/(1 − 𝛼) = 0.99/0.01 = 99
1. Find the reactance of a 5 𝜇F capacitor at an angular frequency of 400 rad/s.
ANSWER: 500 Ω
𝑋𝐶 = 1/𝜔𝐶 = 1/(400 rad/s × 5 × 10−6 F) = 500 Ω
2. Find the reactance of a 5 𝜇H inductor at an angular frequency of 400 rad/s.
ANSWER: 0.002 Ω = 2 mΩ
𝑋𝐿 = 𝜔𝐿 = 400 rad/s × 5 × 10−6 H = 0.002 Ω
3. Find the reactance of a 20 𝜇F capacitor at an angular frequency of 500 rad/s.
ANSWER: 100 Ω
𝑋𝐶 = 1/𝜔𝐶 = 1/(500 rad/s × 20 × 10−6 F) = 100 Ω
: A light emitting diode operates with a forward current of 200 mA and a forward voltage drop of 2
V. The LED is connected in series with a resistor.

1. Find the resistance of the resistor for normal operation of the LED using a 3 V battery.
ANSWER: 5Ω
𝑅 = (3 V − 2 V)/(200 mA) = 5 Ω
2. Find the resistance of the resistor for normal operation of the LED using a 9 V battery.
ANSWER: 35 Ω
𝑅 = (9 V − 2 V)/(200 mA) = 35 Ω
3. Find the resistance of the resistor for normal operation of the LED using a 12 V battery.
ANSWER: 50 Ω
𝑅 = (12 V − 2 V)/(200 mA) = 50 Ω