Saturday, April 14, 2012

Electronics & comm. Ques and Ans-1



Drift current in the semiconductors depends upon  [GATE 2011]
(a) only the electric field
(b) only the carrier concentration gradient
(c) both the electric field and the carrier concentration
(d) both the electric field and the carrier concentration gradient


Which of the following correctly depicts the capacitance of cylindrical capacitor



  



Which of the following can be ignored in a transmission line at radio wave frequency
(a) R and G           (b) G and L
(c) R and C           (d) L and C

Which of the following is correct
(a) In pre-emphasis magnitude of low frequencies are increased
(b) In de-emphasis magnitude of high frequencies are decreased
(c) In pre-emphasis magnitude of high frequencies are decreased
(d) In de-emphasis magnitude of low frequencies are increased

Which of the following logic family is used when the noise is very high
(a) DTL                        (b) TTL
(c) RTL                        (d) HTL

Which of the following statement is correct for a transistor
(a) Collector region is physically larger than emitter region
(b) Collector is heavily doped and emitter is lightly doped
(c) Emitter, Base is heavily doped and collector is lightly doped
(d) In a PNP transistor the collector collects the electrons from the base

In a PNP transistor under normal biased condition which of the following flows from emitter to base in forward biased junction
(a) Minority carrier hole
(b) Majority carrier holes
(c) Minority carrier electrons
(d) Majority carrier electrons

For a common base PNP transistor which of the following statement is correct
(a) Input resistance is greater than output resistance
(b) Output resistance is greater than input resistance
(c) Both input and output resistance are equal
(d) None of these

Which of the following correctly depicts the relation between ICBO and ICEO






In a NPN transistor when both the collector-base and emitter-base junction is forward-biased then the transistor operates in
(a) Active region           (b) Cut off region
(c) Saturation region      (d) Inverted region

Which of the following is correct
(a) Both UJT and FET are unipolar
(b) UJT is unipolar and FET is bipolar
(c) UJT is bipolar and FET is unipolar
(d) Both UJT and FET are bipolar

Answers
(1)-c Explanation: When a semiconductor is subjected to external electric field the charge carriers inside it attains certain velocity called drift velocity(Vd). The current due to this motion of charge carriers is called drift current. Drift current= qE(nμn+pμp). Where μn and μp are the mobilities of electrons and holes respectively. q is the charge of electron. E is the external electric field. p and n are concentration of charge carriers.
(2)-c Explanation: r2 is the outer radius and r1 is the inner radius of the capacitor
(3)-a Explanation: At radio wave frequency inductive reactance(L) becomes much greater than resistance(R) and capacitative susceptance(C) much grater than conductance(G). So R and G can be ignored. In this case the transmission line will be said as a losless transmission line.
(4)-b Explanation: In FM system at higher frequencies the noise becomes more. Hence pre-emphasis is used to increase the amplitude of higher frequencies before modulation. At the receiver end the amplitude of those higher frequencies is decreased to get back the original signal. This process at the receiver end is called de-emphasis.
(5)-d Explanation: High Threshold Logic(HTL) is used when the noise is very high. Another advantage of HTL is its increased noise margin. The disadvantage of HTL is its slow speed (HTL propagation delay-110ns, TTL propagation delay-10ns).
(6)-a Explanation: In a Bipolar Junction Transistor the collector region is physically larger than the emitter region. The doping concentration of Emitter > Collector > Base. In a PNP transistor the emitter is P type so it emits holes which is collected by the collector from the base.
(7)-b Explanation: Normal biased condition means emitter-base junction is forward biased and collector-base junction is reverse biased. So in a PNP transistor under normal biased condition holes flows from p-type emitter towards base.
(8)-b Explanation: From the input and output characteristic of the transistor it is confirmed that the output resistance is greater than the input resistance. You can know this from the slope of the curve. The more horizontal the curve the higher is the resistance.
(9)-c Explanation: We know the equation IC=αIE+ICBO. Which implies that IC=α(IC+IB)+ICBO Now putting IB=0 and solving for IC we will get the answer.
(10)-c Explanation: In a transistor in the saturation region the base-emitter and collector-base region are forward biased.
(11)-a Explanation: see the following classification


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