0195136047.pdf

7.3 BIPOLAR JUNCTION TRANSISTORS 365

10

Load line

8

iC, mA

iB=μ100 A

80 Aμ

60 Aμ

40 Aμ

20 Aμ

iB= 0

vCE,V

(b)

6

4

2

0

0246810121416

Figure E7.3.1Continued

Corresponding to 65-μA interpolated static curve and load line [see Fig. E7.3.1(b)], we get

vCE 2 =1 V andiC 2 = 5 .5 mA. Hence,

Av=

vCE

vS

=

1 − 9. 4

2 − 1

=− 8. 4

and

Ai=

iC

iB

=

( 5. 5 − 1. 3 ) 10 −^3

( 65 − 15 ) 10 −^6

=

4. 2

50

× 103 = 84

EXAMPLE 7.3.2

Given that a BJT hasβ=60, an operating point defined byICQ= 2 .5 mA, and an Early voltage
VA=50 V. Find the small-signal equivalent circuit parametersgm,ro,andrπ.

Solution

gm=

ICQ

VT

=

2. 5 × 10 −^3

25. 681 × 10 −^3

= 97. 35 × 10 −^3 S

ro∼=

VA

ICQ

=

50

2. 5 × 10 −^3

=20 k

rπ∼=

β

gm

=

60

97. 35 × 10 −^3

= 616