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 12^th 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 A_p is given by

A_p = β_ac A_v where β_ac is the current gain and A_v is the voltage gain (for small a.c. signals).

Since A_v = β_acR_o/R_i where R_o is the output resistance and R_i is the input resistance, we have

A_p = β_ac²R_o/R_i

Therefore R_i = β_ac²R_o/A_p = 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).