# Objective Questions on Analog Electronics | Page – 2

1. The MOSFET switch is on state may be considered equivalent to

The MOSFET switch in its may be considered equivalent to capacitor.

2. A 3-phase full converter has an average output voltage of 200 V for zero degree firing angle & for resistive load for a firing angle of 90°, the output voltage would be

V0 = 3 × Vm / n × cosφ = 0 V ⇒ V0 = 0 V.

3. If a 3-phase, half wave rectifier, if per phase input voltage is 200 V, then the average output voltage is

V0 = 3√2 × Vph / π = 3√2 × 200 / 3.141 = 233.91 V.

4. If C = 8 μF and L = 0.2 mH and V0 = 200 V, the peak electric current through diode is given by

Ipeak = V0 × √( C / L) = 200 × √(8 / 0.2 ×`1000 ) = 40 A.

5. A zener voltage regulator has load requirement of 12 V and 2 A. The zener diode minimum electric current requirement is 0.2 A. The minimum voltage at input is 24 V. What is the series resistance with source required?

Vs = V + R( Iz + IL )
⇒ R = ( 24 - 12 ) / 2.2 = 5.45 Ohm.

6. A uniform plane electromagnetic wave incident normally on a plane surface of dielectric is reflected, with V of 3 what is the percentage of incident power that is reflected?

V = ( 1 + τ) / ( 1 - τ) = 3
τ = 0.5
Pr / Pi = τ2 = (0.5)2 = 0.25 × 100 % = 25 %.

7. In a transistor amplifier the __________ junction is reverse biased.

Because transistor acts as an amplifier only in active region and in active region emitter base junction is forward biased and collector base junction is reversed biased.

8. The average on state electric current for an SCR is 20A for a conduction angle of 120°. The average on state electric current for 60° conduction angle will be

Iav = Im / Form factor [ where form factor for 120° = 1.878, where form factor for 60° = 2.7781 ]
Iav is less than 20 A.

9. For an UJT employed for the triggering of an SCR, stand-off ratio h = 0.64 and dc source voltage and Vbb is 20 V. The UJT would trigger when the emitter voltage is

Ebb = Vt + hbb = 0.3 + 0.64 × 20 = 13.1 V

10. A single phase half wave controlled rectifier has 400sin314t as the input voltage and R as the load. For the firing angle of 60° for the SCR, the average output voltage is

Under construction.

11. A single phase, half bridge inverter has input voltage of 50 V dc, the inverter is feeding a load of 2.4 Ω. The rms output voltage at fundamental frequency is

Fundamental component of the output voltage, V0 = 2Vs / π × sinωt. The rms value of this voltage = 2Vs / √2 × π Volts.

12. Thermocouple work on which of the following effects ?

Thermoelectric effect (Seebeck effect) : The phenomenon of production of an electric current in a thermocouple by keeping its two junctions at different temperatures is called Seebeck effect (thermoelectric effect). The magnitude and direction of thermo emf depends on the metals and the temperature difference between the two junctions.

13. When a loop composed of two dissimilar metals could be made to carry a continuous electric current simply by maintaining the two junctions at different temperatures, the above effect is known as

The Seebeck effect is the conversion of temperature differences directly into electricity and is named after the physicist T.J Seebeck, who, in 1821 discovered that a compass needle would be deflected by a closed loop formed by two metals joined in two places, with a temperature difference between the junctions. This was because the metals responded differently to the temperature difference, creating a electric current loop and a magnetic field.

14. Piezoelectric materials serve as a source of

Piezoelectric materials serve as a source of ultrasonic waves.

15. A 1 cm piezoelectric transducer having a g-coefficient of 58 V / Kg / m2 is subjected to a constant pressure of 10 Kg / m2 for about 15 minutes. The piezo voltage developed by the transducer will be

Voltage developed = g-coefficient × pressure = 58 × 10-3 = 58 mV.

16. Dummy strain gauges are used for

Dummy strain gauges are used for compensation of temperature changes.

17. When an electric current is passed through the junction of two different metals, heat is absorbed for liberated depending on the direction of the electric current the above phenomenon is known as

It is the converse of Seebeck effect. It states that if a electric current is passed through a junction of two different metals heat is either evolved or absorbed at that junction depending on the direction of current.
If the direction of electric current in a thermocouple is reversed the junction at which the heat was being evolved starts absorbing heat and at the other junction where heat was being absorbed starts evolving heat, hence the Peltier effect is reversible.

18. Which of the following pairs is commonly used in thermocouples ?

Type T ( copper – constantan ) thermocouples are suited for measurements in the − 200 to 350°C range. Often used as a differential measurement since only copper wire touches the probes. Since both conductors are non-magnetic, there is no curie point and thus no abrupt change in characteristics. Type T thermocouples have a sensitivity of about 43 μV / °C.

19. If a piece of metal is made to have a temperature gradient between its two ends, an e.m.f. is observed to exist between those ends. The above phenomenon is known as

The absorption or evolution of heat energy if a electric current allowed to flow in a conductor having its different parts at different temperature, is known as Thomson effect.

20. Piezoelectric effect is carried out in

In piezoelectric effect when a mechanical strain is applied to one face of a suitably cut face of a piezoelectric crystal, it causes an emf to be developed in opposite surface of that piece. Reverse is also true.