A silicon transistor with beta 50 is used in the circuit be

A silicon transistor with beta = 50 is used in the circuit below. R_B = 5 kOhm (a) If the switch is in position A, what is the voltage V_out? (b) If the switch is moved to position B. what is the new V_out?

Solution

(a) When switch is in position A Base current Ib=-Vbe/Rb <= 0 A so transistor is in cuttoff Ic=0,

KVL gives Vout=Vcc-Ic*Rc as Ic= 0;

Vout= Vcc=100 V

(b)When switch is in position B,KVL to base emitter loop gives

100-Ib*Rb-Vbe=0

Ib=100-Vbe/Rb=100-0.7/5k=99.3/5k=19.86 mA

Ib=19.86 mA

Beta= 50 given

Assuming Transistor is in active region let\'s find voltages

Ic=Beta*Ib=50*19.86=993 mA

Vout=Vcc-Ic*Rc=100-993m*5k=-4865 (negative value means our assumption of transistor in active region is wrong)

Transistor is in saturation region

Vout=Vce=0.2 V