Vacuum Tubes - Electronics Engineering test questions

Vacuum Tubes - Electronics Engineering test questions

(1) Vacuum tubes are still being used in

(A) Electroplating plants
(B) Radio transmitters
(C) Radio receivers
(D) Public address systems

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(2) Work function of thoriated tungsten = _________ and work function of pure tungsten = __________

(A) 2.63 eV, 4.5 eV
(B) 1.1 eV, 1.5 eV
(C) 4.52 eV, 2.5 eV
(D) 3.98 eV, 8.5 eV

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(3) An oxide coated cathode cannot be used if the voltage at the anode is

(A) 6 V
(B) 100 V
(C) 1 V
(D) 500 V

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(4) The ratio (Small change in plate current / small change in plate voltage) with constant plate voltage is known as

(A) Transconductance (gm)
(B) AC plate resistance (rp)
(C) Amplification factor (μ)
(D) None of the above

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(5) In case of a cathode, the emitter efficiency is

(A) Emission current / electron density
(B) Emission current / W
(C) Emission current / volt
(D) Emission current /°C

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(6) In a vacuum tube diode, the rate of electrons collection at plate = the rate of electrons emission by cathode only when the tube is working in

(A) Temperature limited conditions
(B) Space charge limited conditions
(C) Low temperature region
(D) Both (A) and (B) above

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(7) Thoriated tungsten is used as emitter material in tubes upto

(A) 30W
(B) 300W
(C) 10W
(D) 1000W

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(8) Usually, an anode surface is blacked and roughened to

(A) Arrest escaping electrons
(B) Retard fast electrons that are emitted by cathode
(C) Dissipate heat produced at the anode
(D) Eliminate outside interference

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(9) If anode of vacuum tube is not connected to any external circuit and cathode is heated then

(A) Glass tube gets charged
(B) Electrons form a space charge
(C) Electrons escape through glass tube
(D) Protons are emitted from anode

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(10) In a CRT, __________ has the highest positive voltage

(A) Anode
(B) Heater
(C) Control grid
(D) Cathode

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(11) In a pentode tube, a control grid is provided to

(A) Restrict the secondary emission from the plate
(B) Accelerate the electrons emitted from the plate
(C) Collect electrons from the space change
(D) Control the number of electrons moving from cathode to plate

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(12) A triode can also be used as an amplifier because

(A) It has different values of plate resistance for ac and dc
(B) Any small change in grid voltage is capable of causing a larger change in plate voltage
(C) Plate is a high power source
(D) It has three elements like a transistor

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(13) The plate current in a triode, tetrode or pentode increases when the

(A) Plate voltage is made less positive
(B) Screen grid voltage is made less positive
(C) Control grid voltage is made more negative
(D) Control grid voltage is made less negative

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(14) The vacuum tube that cannot be used as an amplifier is

(A) Pentode
(B) Diode
(C) Tetrode
(D) Triode

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(15) Function of suppressor grid is

(A) To suppress secondary emission effects
(B) To dissipate heat
(C) To reduce space charge
(D) All of the above

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(16) The control grid in a pentode is provided between

(A) Screen grid and plate
(B) Screen and suppressor grid
(C) Cathode and screen grid
(D) Plate and suppressor grid

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(17) In a triode, the value of parasitic capacitance increases as

(A) Current decreases
(B) Current increases
(C) Signal frequency decreases
(D) Signal frequency increases

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