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What is thermal runaway of a transistor?

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Description : Describe thermal runaway of transistor and explain how it can be avoided.

Answer : Concept of thermal runaway:  1.We know that  IC = β IB + (1+ β) ICO  Where ICO is the leakage current and ICO is strongly dependent on temperature. 2. Leakage current approximately doubles for ...   Thermal runaway can be avoided by : 1) Using stabilization circuitry 2) Heat sink 

Description : Define thermal runaway.

Answer : Thermal runaway  The problem with increasing temperature causing increasing collector current is that more current increase the power dissipated by the transistor which, in turn, increases its temperature ... self-reinforcing cycle is known as thermal runaway, which may destroy the transistor. 

Description : Compare between BJT and FET on the basis of (i) Biolar/Unipolar (ii) Tharmal Runaway (iii) Noise (iv) Applications

Answer : Parameter BJT FET Bipolar/Unipolar It is bipolar device i.e. current in this device is carried by electrons and holes. It is unipolar device i.e. current ... in oscilloscopes, electronic voltmeters and other measuring and testing equipment because of their high input impedance.

Description : What is meant by stabilization in transistor?

Description : What is the main use of a transistor?

Answer : Main use of a transistor as amplifier and as switch.

Description : What is self biasing of transistor?

Description : Why biasing is required for a transistor?

Description : What is role of HFE in transistor ?

Description : What do you mean by biasing of a transistor?

Description : Explain the thermal triggering method of SCR. Enlist different triggering methods of SCR.

Answer : Types of Triggering: 1) Forward voltage triggering 2) Thermal triggering (Temperature triggering) 3) Radiation triggering (Light triggering) 4) dv/dt triggering 5) Gate triggering ... generated leakage current increases. This current gets multiplied internally and thyristor is turned on. 

Description : Classify condensers used in thermal power station. Explain each type in brief. 

Answer : Types of condenser: Classification: 1. Jet Condenser (Mixing Type) 2. Surface condenser (non Mixing type)  Explanation:-   1. Jet Condenser (Mixing Type) In Jet condenser the steam ... there in no direct contact between the steam and cooling water. This condensate is reused in boiler. 

Description : "Efficiency of thermal power station is low". — Justify. 

Answer : Justification: Efficiency of thermal power station is low":- Overall efficiency of T.P.P depends upon efficiency of boiler, turbine and alternator. The heat produced due to combustion of coal ... losses in thermal power plant is 71%, So efficiency of thermal power plant is less about 29% 

Description : State advantages of Pulverized coal w.r.t. thermal power plant. 

Answer : Advantages of Pulverized coal w.r.t. thermal power plant.  1. Due to pulvarization amount of fuel required to produce same amount of heat reduces. 2. Time required for combution reduces ... of air and thorough mixing of air and fuel, very high-combustion temperatures can be attained.

Description : State the function of super heater of a thermal power plant. 

Answer : Function of Super heater: -  Its function is to increase the temperature of steam by absorbing heat from exhausted hot flue gases.

Description : State the function of cooling tower and condenser in thermal power station.

Answer : 1. Condenser:   Function of condenser is to convert exhaust steam again into water by reducing its temperature with the help of cold water. Also it reduces back pressure of steam turbine 2 ... tower : The function of cooling tower is to reduce the temperature of water coming from condenser.

Description : Write any four advantages and four disadvantages of thermal power plant.

Answer : .embed-container { position: relative; padding-bottom: 56.25%; height: 0; overflow: hidden; max-width: 100%; } .embed-container iframe, .embed-container object, .embed-container embed { position: absolute; top: 0; left: 0; width: 100%; height: 100%; }

Description : Explain ash disposal and dust collection in a thermal power plant.

Answer : Ash disposal:-  Various methods of disposal of ash are:  Mechanical handling system.  Pneumatic system. Hydraulic system  Steam jet system  Commonly used ash handling ... be done either manually or automatically.  This removed fly ash is collected in separate container

Description : State any four factors governing Selection of site for thermal power station. 

Answer : Following Factors are to be considered:- 1. It should locate near coal mine. 2. Sufficient quantity of water should be available. 3. Sufficient large space should be available. 4. It should be located ... laws of the land and the town planning. 14. The interests of national defense must be served.

Description : The overall efficiency of Thermal Power Plant is equal to _______________ .  (A) Regenerative Cycle Efficiency (B) Renkine Cycle Efficiency (C) Carnot cycle efficiency (D) None of the above

Answer : The overall efficiency of Thermal Power Plant is equal to Regenerative Cycle Efficiency. 

Description : In a thermal power plant, ash is collected in

Answer : In a thermal power plant, ash is collected in hoppers or bunkers

Description : The unit heat rate characteristic of a thermal power unit shows :  (A) Heat input per kWh of output versus the megawatt output of unit  (B) Heat input per kW of output versus the megawatt output ... hour output of unit  (D) Heat input per kW of output versus the megawatt hour output of unit

Answer : The unit heat rate characteristic of a thermal power unit shows : Heat input per kWh of output versus the megawatt output of unit 

Answer : Thermal overload relays are used to protect the motor against overcurrent due to heavy loads.

Answer : Thermal circuit breaker has delayed trip action.

Answer : A thermal protection switch can protect against overload.

Description : Compare P.N.P and N.P.N transistor.

Answer : Parameter PNP transistor NPN transistor Definition Two blocks of p- types semiconductors are separated by one thin block of n-type semiconductor. Transistor in which two ntype layer ... Positive Voltage  Emitter Terminal Collector Terminal Ground Signal High Low

Description : Draw construction of SCR using two transistor model. Explain its operation.

Answer : Two Transistor Model of SCR or Thyristor

Description : Draw the V-I characteristics of power transistor. 

Answer : V-I characteristics of power transistor:

Description : Define α and β of the transistor. 

Answer : α: The ratio of collector current IC to emitter current IE for a constant collector to base voltage VCB in the CB configuration is called current gain alpha (α).   β : The ratio of collector ... a constant collector to emitter voltage VCE in the CE configuration is called current gain beta (β) .

Description : List various transistor biasing methods.

Answer : Types of biasing methods: i. Base bias (or fixed bias) ii. Base bias with emitter feedback. iii. Base bias with collector feedback iv. Voltage divider bias (or self bias) v. Emitter bias. 

Description : Transistor operating configurations

Answer : Transistor operating configurations

Description : Define junction field effect transistor (JFET) and give an example.

Answer : Junction Field-Effect Transistor: It is a semiconductor device having three terminals, namely Gate, Drain and Source, in which the current flow is controlled by an electric field set up by an external voltage applied ... : i) n-channel JFET and ii) p-channel JFET Examples of JFET: BFW 10, BFW 11 

Description : Explain working of n-p-n transistor in unbiased condition.

Answer : Construction of NPN transistor: For an unbiased transistor no external power supplies are connected to it. Base is sandwiched between collector & emitter terminal. It is thin & lightly doped ... providing external power supply to transistor & this process is known as Biasing.

Description : Define α & β and derive the relation between (α) &) (β) of transistor.

Answer : α : The ratio of output collector current IC to input emitter current IE in the CB configuration is called current gain alpha (α).  β : The ratio of output collector current IC to the input base current IB in the CE configuration is ... ( 1/β) + 1  hence 1/ α = (1+β )/ β  Therefore, α = β /(1+β)

Description : Define biasing of transistor

Answer : Biasing of transistor is the process that makes the base emitter junction of transistor always forward biased and collector junction reverse biased for achieving the faithful amplification .

Description : With neat diagram explain working of NPN transistor.

Answer : N-p-n transistor is made by sandwiching thin layer of p-type semiconductor between two layers of n-type semiconductor. It has three terminals - Emitter, Base and collector. The npn transistor has two ... up in the collector as it is more positive. In this way, current flows in the transistor.

Description : Draw and explain self-basing of a transistor.

Answer : Transistor Biasing is the process of setting a transistors DC operating voltage or current conditions to the correct level so that any AC input signal can be amplified correctly by the transistor. ExplanationIt ... Applying KVL to o/p loop , Collector-emitter voltage, VCE = VCC − IC RC

Description : Compare between CB , CE & CC transistor configuration.

Answer : Parameter CB CE CC I/P impedance Low Or 50Ω Medium Or 600Ωto 4kΩ High Or 1 MΩ Current Gain Less than or equal to 1 High Highest Voltage Gain Medium Medium Less ... k Ω to 50 k Ω Low Or 50 Ω Phase shift between input and output 0° 360° 0° 

Description : Why transistor is called as bipolar device.

Answer : BJT is called bipolar device because in BJT current conduction takes place due to majority as well as minority charge carriers that is because of electrons as well as holes.

Description : Define the following terms: (i) Stabilization (ii) Thermal runaway

Answer : Stabilization: The process of making operating point independent of temperature changes or variation in transistor parameter is known as stabilization. The maintenance of the operating point stable is called the ... happens, the device gets burnt out. This process is known as Thermal Runaway .

Answer : The thermal noise is due to random Motion of free electrons.

Description : In the common–emitter transistor circuit, if the current gain is 100 and the collector current is 10 mA, the base current is: (1) 10 µA (2) 100 µA (3) 1 A (4) 10 A 

Answer : In the common–emitter transistor circuit, if the current gain is 100 and the collector current is 10 mA, the base current is: 100 µA

Description : The set of transistor characteristics which enable α to be determined directly from the slope is (a) CB transfer characteristics (b) CE transfer characteristics (c) CB input characteristics (d) CE output characteristics

Answer : The common base transfer characteristics

Description : In a transistor Colpitts oscillator, C1 = 0.001 µF, C2 = 0.01 µF and L = 5 µH. The frequency of oscillation in MHz will be (a) 2.37 MHz (b) 10.00 MHz (c) 11.54 MHz (d) 9.09 MHz 

Answer : c

Description : A transistor has a typical value of β = 200 , If the collector current is 15 mA, then base current will be

Answer : A transistor has a typical value of β = 200 , If the collector current is 15 mA, then base current will be 0.075 mA

Description : Beta cut-off frequency of a transistor is the frequency at which the Beta is • 0.707 of low frequency value • Zero • Equal to low frequency value • None of the above.

Answer : Beta cut-off frequency of a transistor is the frequency at which the Beta is 0.707 of low frequency value

Description : If a transistor were operated with emitter and collector interchanged, then  1 .emitter current will increase  2. Collector current will increase  3. Base current will decrease  4. No current flow will take place 

Answer : If a transistor were operated with emitter and collector interchanged, then Collector current will increase 

Description : In a transistor  1. Emitter is heavily doped while the collector is moderately doped 2. Collector base junction is forward biased 3. Emitter is made wider than collector 4. The input resistance is much higher than output resistance 

Answer : In a transistor Emitter is heavily doped while the collector is moderately doped

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