Question

From the given circuit, using a silicon BJT, what is the value of VBC?

(a) 6.13 V

(b) -6.13 V

(c) 7 V

(d) -7 V

This question was addressed to me in an internship interview.

My question comes from BJT DC biasing topic in section BJT Biasing of Analog Circuits

Answer 1

Right answer is (b) -6.13 V

For explanation I would say: Consider the BJT to be in saturation. Then IC=12-0.2/2.2k=5.36 mA

And IB=12-0.8/240k=0.047 mA

IBMIN=ICSAT/β=5.09/50=0.1072mA which is greater than above IB.

Hence transistor is in the active region.

Thus IC=βIB.

VBE=0.7V

IB=12-0.7/240=47.08μA

IC=50×47.08=2.354 mA

VCE=VCC-ICRC=12-2.354*2.2=12-5.178=6.83V

Hence VBC = 0.7-6.83 = -6.13V.