## Sunday, July 31, 2011

### Kerala Engineering Entrance (KEAM -Engineering) 2011 Questions (MCQ) on Transistor Amplifier and Oscillator

Today we will discuss two multiple choice questions involving transistors which appeared in Kerala Engineering Entrance (KEAM -Engineering) 2011 question paper. These questions are simple as is the case with questions in this section at the level expected in the case of 12th grade (Plus Two) students.
(1) In a common emitter transistor amplifier, the output resistance is 500 KΩ and the current gain β = 49. If the power gain of the amplifier is 5×106, the input resistance is
(A) 325 Ω
(B) 165 Ω
(C) 198 Ω
(D) 225 Ω
(E) 240 Ω
Power gain Ap is given by
Ap = βac Av where βac is the current gain and Av is the voltage gain (for small a.c. signals).
Since Av = βacRo/Ri where Ro is the output resistance and Ri is the input resistance, we have
Ap = βac2Ro/Ri
Therefore Ri = βac2Ro/Ap = 492×(500×103) /(5×106) = 240 Ω, very nearly.
(2) A transistor oscillator is (i) an amplifier with positive feedback (ii) an amplifier with reduced gain (iii) the one in which dc supply energy is converted into ac output energy. Then
(A) all (i), (ii) and (iii) are correct
(B) only (i) and (ii) are correct
(C) only (i) and (iii) are correct
(D) only (ii) and (iii) are correct
(E) only (ii) is correct
Statements (i) and (iii) are correct where as statement (ii) is incorect. So the correct option is (c).