Electronics Engineering Multiple Choice Questions and Answers (MCQs) Part 4:
150. In phase modulation, phase deviation is proportional to
(a) carrier amplitude.
(b) carrier phase.
(c) message signal.
(d) message signal frequencies.
Answer : (c) message signal.
151. The waveform A cos (ωt + k cos ω2t) is
(a) amplitude modulated.
(b) frequency modulated.
(c) phase modulated.
(d) frequency as well as phase modulated.
Answer : (d) frequency as well as phase modulated.
152. In a superheterodyne receiver arrange the following components sequentially from input to the output:
1. Antenna. 2. Mixer. 3. IF amplifier. 4. Audio amplifier.
(a) 1, 2, 3 and 4.
(b) 4, 2, 3 and 1.
(c) 1, 3, 2 and 4.
(d) 4, 3, 2 and 1.
Answer : (a) 1, 2, 3 and 4.
153. An AM superheterodyne receiver with IF of 455 kHz is tuned to the carrier frequency of 1,000 kHz. The image frequency is
(a) 545 kHz
(b) 1 MHz
(c) 1,455 kHz
(d) 1,910 kHz
Answer : (d) 1,910 kHz
154. The main advantage of pre-emphasis circuit in FM transmitter is
(a) to increase the carrier power.
(b) to improve the signal to noise ratio at low audio frequencies.
(c) to increase the bandwidth of side band.
(d) to improve the signal to noise ratio at high audio frequencies.
Answer : (d) to improve the signal to noise ratio at high audio frequencies.
155. The types of modulation used generally in TV transmission for video and audio signals, respectively, are
(a) FM and AM.
(b) FM and FM.
(c) AM and AM.
(d) AM and FM.
Answer : (d) AM and FM.
156. Which of the following pulse modulation is analog?
(a) PCM.
(b) Differential PCM.
(c) PWM.
(d) Delta.
Answer : (c) PWM.
157. Which one of the following pulse communication systems is digital?
(a) PAM
(b) PCM
(c) PPM
(d) PWM
Answer : (b) PCM
158. What are the three steps in generating PCM in the correct sequence?
(a) Sampling, quantizing and encoding.
(b) Encoding, sampling and quantizing.
(c) Sampling, encoding and quantizing.
(d) Quantizing, sampling and encoding.
Answer : (a) Sampling, quantizing and encoding.
159. Pulse code modulation is commonly used in telemetry because
(a) It ensures immunity from noise during transmission.
(b) The bandwidth requirement of the channel is reduced.
(c) It removes quantization error.
(d) It permits lower rate of sampling than what is normally required under Shanon’s theorem.
Answer : (a) It ensures immunity from noise during transmission.
160. In delta modulation, the slope overload distortion can be reduced by
(a) decreasing the step size.
(b) decreasing the granular noise.
(c) decreasing the sampling.
(d) increasing the step size.
Answer : (d) increasing the step size.
161. Time division multiplexing needs
(a) transmission of data samples.
(b) transmission of data at random.
(c) constant data transmission.
(d) transmission of data of only one measurand.
Answer : (a) transmission of data samples.
162. Time division multiplexing requires
(a) constant data transmission.
(b) transmission of data samples.
(c) transmission of data at random.
(d) transmission of data of only one measurand.
Answer : (b) transmission of data samples.
163. Which one of the following is capable of giving the highest data speed?
(a) Coaxial cable link.
(b) Microwave LOS link.
(c) Microwave satellite system.
(d) Optical fibre system.
Answer : (d) Optical fibre system.
164. Consider the following statements:
In optical communications, the losses in optical fibres can be caused by
1. impurities. 2. microbending. 3. attenuation in glass. 4. stepped index operation.
Which of these statements are correct?
(a) 1, 2 and 3.
(b) 1, 3 and 4.
(c) 1, 2 and 4.
(d) 2, 3 and 4.
Answer : (a) 1, 2 and 3.
165. Which one of the following photodetector does not provide gain?
(a) Phototransistor.
(b) Photoconductor.
(c) Avalanche photodiode.
(d) p-i-n photodiode.
Answer : (d) p-i-n photodiode.
166. In a forward biased photodiode an increase in incident light causes the diode current to
(a) increase.
(b) remain constant.
(c) decrease.
(d) remain constant while the voltage drop across the diode increases.
Answer : (d) remain constant while the voltage drop across the diode increases.
167. Dark current in a semiconductor photodiode is
(a) the forward bias current.
(b) the forward saturation current.
(c) the reverse saturation current.
(d) the transient current.
Answer : (c) the reverse saturation current.
168. A solar cell is actually a device which utilises
(a) photoconductive effect.
(b) photovoltaic effect.
(c) photoemissive effect.
(d) photoresistive effect.
Answer : (b) photovoltaic effect.
169. The light emitting diode (LED) emits light of a particular colour because
(a) it is fabricated from a fluorescent material.
(b) transition between energy levels of the carriers takes place while crossing the p-n junction.
(c) heat generated in the diode is converted into light.
(d) the band gap of the semiconductor material used in the fabrication of the diode is equal to the energy hV of the light photon.
Answer : (d) the band gap of the semiconductor material used in the fabrication of the diode is equal to the energy hV of the light photon.
170. An LED made using GaAs emits radiation in:
(a) Visible region.
(b) Ultraviolet region.
(c) Infrared region.
(d) Microwave frequency region.
Answer : (c) Infrared region.
171. In which one of the following, two optical polarisers, one in front and other in back are needed?
(a) LED.
(b) LCD.
(c) LDR.
(d) LSI.
Answer : (b) LCD.
172. The efficiency of a LED for generating light is directly proportional to the
(a) applied voltage.
(b) current injected.
(c) temperature.
(d) level of doping.
Answer : (a) applied voltage.
173. As compared to an LED, an LCD has the distinct advantage of
(a) extremely low power requirement.
(b) providing a silver display.
(c) being extremely thin.
(d) giving two types of displays.
Answer : (a) extremely low power requirement.
174. Consider the following statements in connection with the biasing of semiconductor diodes.
1. LEDs are used under forward bias condition.
2. Photodiodes are used under forward-bias condition.
3. Zener diodes are used under reverse-bias condition.
4. Variable capacitance diodes are used under reverse-bias condition.
Which of these statements are correct ?
(a) 1, 2 and 3.
(b) 1, 2 and 4
(c) 2, 3 and 4.
(d) 1, 3 and 4.
Answer : (d) 1, 3 and 4.
175. Optocoupler is used to
(a) reduce SCR turn-off time.
(b) protect IGBTs against dv/dt.
(c) regulate gate signal.
(d) isolate gating circuitry from power lines.
Answer : (d) isolate gating circuitry from power lines.
176. Consider the following devices:
1. SCR 2. GTO 3. BJT 4. MOSFET 5. IGBT
Which of these devices do not belong to the family of transistors?
(a) 1 and 2 only.
(b) 1, 2 and 3 only.
(c) 2, 3 and 5 only.
(d) 1, 2, 3, 4 and 5.
Answer : (a) 1 and 2 only.
177. Which one of the following statements is not correct ?
(a) Power MOSFETs are so constructed as to avoid punch through.
(b) In a power MOSFET, the channel length is relatively large and channel width is relatively small.
(c) Power MOSFETs do not experience any minority carrier storage.
(d) Power MOSFETs can be put in parallel to handle large currents.
Answer : (a) Power MOSFETs are so constructed as to avoid punch through.
178. Consider the following statements :
1. BJT has lower power losses than MOSFETs.
2. MOSFETs have lower power losses than IGBTs.
3. SCRs have lower power losses than MOSFETs.
Which of these statements are correct ?
(a) 1, 2 and 3.
(b) 1 and 2.
(c) 2 and 3.
(d) 1 and 3.
Answer : (c) 2 and 3.
179. Which semiconductor power device, out of the following, is not a current triggered device ?
(a) Thyristor.
(b) GTO.
(c) TRIAC.
(d) MOSFET.
Answer : (d) MOSFET.
180. Thyristor is a semiconductor switch which is
(a) unilateral and astable.
(b) bilateral and astable.
(c) unilateral and bistable.
(d) bilateral and bistable.
Answer : (c) unilateral and bistable.
181. In the forward blocking region of a silicon controlled rectifier, the SCR is
(a) in the off-state.
(b) in the on-state.
(c) reverse biased.
(d) at the point of breakdown.
Answer : (a) in the off-state.
182. Once an SCR is turned on, it remains so until the anode current goes below
(a) Trigger current.
(b) Breakover current.
(c) Threshold current.
(d) Holding current.
Answer : (d) Holding current.
183. After firing an SCR, the gate pulse is removed. The current in the SCR will
(a) remain the same.
(b) immediately fall to zero.
(c) rise up.
(d) rise a little and then fall to zero.
Answer : (a) remain the same.
184. The anode current through a conducting SCR is 10 A. If its gate current is made one-fourth, then what will be the anode current?
(a) 0 A
(b) 5 A
(c) 10 A
(d) 20 A
Answer : (c) 10 A
185. In a thyristor, the holding current IH is
(a) more than latching current IL.
(b) less than IL.
(c) equal to IL.
(d) equal to zero.
Answer : (b) less than IL.
186. An SCR can be turned off
(a) by passing a negative pulse to its gate.
(b) by removing the gate supply.
(c) by reverse biasing it.
(d) by forcing the current through gate to become zero.
Answer : (c) by reverse biasing it.
187. Circuit turn-off time of an SCR is defined as the time
(a) taken by the SCR to turn off.
(b) required for the SCR current to become zero.
(c) for which the SCR is reverse biased by the commutation circuit.
(d) for which the SCR is reverse biased to reduce its current below the holding current.
Answer : (c) for which the SCR is reverse biased by the commutation circuit.
188. When compared to those of a symmetrical thyristor, the turnoff time and reverse blocking voltage of an asymmetrical thyristor are respectively
(a) large and large.
(b) large and small.
(c) small and large.
(d) small and small.
Answer : (d) small and small.
189. The sharing of the voltage between thyristors operating in series is influenced by their
(a) di/dt capabilities.
(b) dv/dt capabilities.
(c) junction temperature.
(d) static v-i characteristics and leakage currents.
Answer : (d) static v-i characteristics and leakage currents.
190. The di/dt protection for an SCR is achieved through the use of
(a) R in series with SCR.
(b) R-L in series with SCR.
(c) RL across SCR.
(d) L in series with SCR.
Answer : (d) L in series with SCR.
191. A triac is a
(a) 2 terminal switch.
(b) 2 terminal bilateral witch.
(c) 3 terminal unilateral switch.
(d) 3 terminal bidirectional switch.
Answer : (d) 3 terminal bidirectional switch.
192. Triac cannot be used in
(a) ac voltage regulators.
(b) cycloconverters.
(c) solid state type of switch.
(d) inverter.
Answer : (c) solid state type of switch.
193. Consider the following statements:
A four-layer PNPN device having two gate leads can be turned on by applying a
1. positive current pulse to the cathode gate.
2. positive current pulse to the anode gate.
3. negative current pulse to the anode gate.
4. negative current pulse to the cathode gate.
Which of these statements is/are correct?
(a) 1 alone.
(b) 1 and 3.
(c) 2 alone.
(d) 2 and 4.
Answer : (d) 2 and 4.
194. An LASCR
(a) can conduct in both directions.
(b) has a built-in antiparallel diode.
(c) is preferred in high voltage and high current applications where optical isolation is used between trigger source and switching device.
(d) can be turned off by negative gate signal.
Answer : (c) is preferred in high voltage and high current applications where optical isolation is used between trigger source and switching device.
195. In a GTO, anode current begins to fall when the gate current
(a) is negative peak at time t = 0
(b) is negative peak at t = storage period ts
(c) just begins to become negative at t = 0
(d) just begins to become positive at t = 0
Answer : (b) is negative peak at t = storage period ts
196. UJT is known as
(a) voltage controlled device.
(b) current controlled device.
(c) relaxation oscillator.
(d) none of the above.
Answer : (a) voltage controlled device.
197. For an UJT employed for triggering an SCR, stand-off ratio η = 0.64 and dc source voltage VBB is 20 V. The UJT would trigger when the emitter voltage is
(a) 6.4 V.
(b) 12.8 V.
(c) 6.55 V.
(d) 13.1 V.
Answer : (d) 13.1 V.