0195136047.pdf

366 SEMICONDUCTOR DEVICES

EXAMPLE 7.3.3

Considering the circuit shown in Figure E7.3.3(a), find the state of operation and operating point

if the BJT hasβ=80 and other typical values of a silicon BJT at room temperature.

(a)

(b)

E

B

C

+

+

+

−

−

−

RB = 60 kΩ

RE = 4 kΩ

vBE

iB

VCC = 20 V

iE

iC

vCE

E

B

C

+

+

+− −

−

60 kΩ

4 kΩ

vBE = 0.7 V

iB

20 V

iC =

80 iB

iE =

81 iB

vCE

Figure E7.3.3

Solution

Let us check the state of operation through some preliminary calculations. Application of KVL

yields

vBE=vCE−RBiB=VCC−REiE−RBiB

If we assume the saturated state, thenvCE=VsatandiB>0, so that

vBE=Vsat−RBiB< 0. 2

which is in violation of the saturation condition:vBE=Vγ= 0 .7V.

If we assume the cutoff state, theniB=0 andiE=iC=ICEO, so that

vBE=VCC−REICEO∼=20 V

which is in violation of the cutoff condition:vBE<Vγ.

Having thus eliminated saturation and cutoff, the active-state model is substituted, as shown

in Figure E7.3.3(b).

The outer loop equation gives

20 − 60 iB− 0. 7 − 4 × 81 iB= 0

whereiBis the base current in mA. Solving,