Question

Fig. shows V_CE vs V_i characterstics of a transistor, known as transfer characterstics. The curve is divided into three regions as shown in Fig. The transistor is to be used as (a) an amplifier (b) a switch. The part of transfer–characterstics used is

(1) (a) and (c) any of I, II or II

(2) (a) – region I, (b) – region II

(3) (a) – region II; (b) region I and III

(4) (b) region I and II (b) region II

Answer 1

(3) (a) – region II; (b) region I and III

In a transistor if V_i is low so that t emitter–base junction is NOT FORWARD biased V₀ is high. If V_i is high the transistor is driven into saturation region and V₀ is low. One of these two positions can be used as “off” and other as “on” state of transistor. The transistor works as a switch. 

Region II is the “active region” of a transistor. In this region transistor is used as an amplifier. A small variation in Vi produces a small change in input current I_B . This produces a large change in output current I_C and therefore a large change in output voltage V₀ . This is amplification.