A working transistor with its three legs marked and R is tested using a multimeter. No conduction is found between P and Q . By connecting the common (negative) terminal of the multimeter to R and the other (positive) terminal to P or Q , some resistance is seen on the multimeter. Which of the following is true for the transistor?
- (A)It is an npn transistor with as base✓ Correct
- (B)It is a pnp transistor with R as collector
- (C)It is a pnp transistor with as emitter
- (D)It is an npn transistor with as collector
Answer: (A)
Solution
Only the base of a transistor forms a single conducting p–n junction with each of the other two legs; the collector–emitter path is two junctions back-to-back and never conducts (that is the 'no conduction between P and Q'). Since R conducts to BOTH P and Q, R must be the base — which immediately rules out every 'collector' or 'emitter' option. Reading the ohmmeter in the usual analog convention (current leaves the COMMON/negative lead), placing COMMON on R drives current out of R into P and Q, so the base sits at the higher potential and both junctions are forward biased with the base as the p-region — an npn transistor. Hence npn with R as base.
JEE Main · 2008 · 27 Apr 2008 · Offline · Single correct option