# Question Bank for Valve Electronics (Diode and Triode) ### MCQs on Valve Electronics

Ques: Thermionic emission from a heated filament varies with its temperature T as
(a) T–1
(b) T
(c) T2
(d) T3/2

Ans. (c)

Ques: In diode, when there is saturation current, the plate resistance (rp) is
(a) Zero
(b) Infinite
(c) Some finite quantity
(d) Data is insufficient

Ans. (b)

Ques: The ripple factor in a half wave rectifier is
(a) 1.21
(b) 0.48
(c) 0.6
(d) None of these

Ans. (a)

Ques: The electrical circuits used to get smooth d.c. output from a rectifier circuit is called
(a) Filter
(b) Amplifier
(c) Full wave rectifier
(d) Oscillator

Ans. (b)

Ques: The grid in a triode valve is used
(a) To increases the thermionic emission
(b) To control the plate to cathode current
(c) To reduce the inter-electrode capacity
(d) To keep cathode at constant potential

Ans. (b)

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Ques: Plate voltage of a triode is increased from 200 V to 225 V. To maintain the plate current, change in grid voltage from 5V to 5.75 V is needed. The amplification factor is
(a) 40
(b) 45
(c) 33.3
(d) 25

Ans. (c)

Ques: The amplification factor of a triode is 50. If the grid potential is decreased by 0.20 V, what increase in plate potential will keep the plate current unchanged
(a) 5 V
(b) 10 V
(c) 0.2 V
(d) 50 V

Ans. (b)

Ques: Select the correct statement
(a) In a full wave rectifier, two diodes work alternately
(b) In a full wave rectifier, two diodes work simultaneously
(c) The efficiency of full wave and half wave rectifiers is same
(d) The full wave rectifier is bi-directional

Ans. (a)

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Ques: Diode is used as a/an
(a) Oscillator
(b) Amplifier
(c) Rectifier
(d) Modulator

Ans. (c)

Ques: In a diode valve, the state of saturation can be obtained easily by
(a) High plate voltage and high filament current
(b) Low filament current and high plate voltage
(c) Low plate voltage and high cathode temperature
(d) High filament current and high plate voltage

Ans. (b)

Ques: The introduction of a grid in a triode valve affects plate current by
(a) Making the thermionic emission easier at low temperature
(b) Releasing more electrons from the plate
(c) By increasing plate voltage
(d) By neutralising space charge

Ans. (d)

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Ques: Select the correct statements from the following
(a) A diode can be used as a rectifier
(b) A triode cannot be used as a rectifier
(c) The current in a diode is always proportional to the applied voltage
(d) The linear portion of the I–V characteristic of a triode is used for amplification without distortion

Ans. (a) and (d)

Ques: The unit of mutual conductance of a triode valve is
(a) Siemen
(b) Ohm
(c) Ohm metre
(d) Joule Coulomb–1

Ans. (a)

Ques: In the circuit of a triode valve, there is no change in the plate current, when the plate potential is increased from 200 volt to 220 volt and the grid potential is decreased from – 0.5 volt to –1.3 volt. The amplification factor of this valve is
(a) 15
(b) 20
(c) 25
(d) 35

Ans. (c)

Ques: A triode has a mutual conductance of 2 x 10–3 mho and an amplification factor of 50. The anode is connected through a resistance of 25 x 103 ohms to a 250 volts supply. The voltage gain of this amplifier is
(a) 50
(b) 25
(c) 100
(d) 12.5

Ans. (b)

Ques: The amplification factor of a triode valve is 15. If the grid voltage is changed by 0.3 volt the change in plate voltage in order to keep the plate current constant (in volt) is
(a) 0.02
(b) 0.002
(c) 4.5
(d) 5.0

Ans. (c)

Ques: In a triode valve
(a) If the grid voltage is zero then plate current will be zero
(b) If the temperature of filament is doubled, then the thermionic current will also be doubled
(c) If the temperature of filament is doubled, then the thermionic current will nearly be four times
(d) At a definite grid voltage the plate current varies with plate voltage according to Ohm’s law

Ans. (c)

Ques: The amplification factor of a triode is 20. If the grid potential is reduced by 0.2 volt then to keep the plate current constant its plate voltage is to be increased by
(a) 10 volt
(b) 4 volt
(c) 40 volt
(d) 100 volt

Ans. (b)

Ques: The amplification produced by a triode is due to the action of
(a) Filament
(b) Cathode
(c) Grid
(d) Plate

Ans. (c)

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Ques: Coating of strontium oxide on Tungsten cathode in a valve is good for thermionic emission because
(a) Work function decreases
(b) Work function increases
(c) Conductivity of cathode increases
(d) Cathode can be heated to high temperature

Ans. (a)