0195136047.pdf

9.1 TRANSISTOR SWITCHES 425

RC

C

E

S Vo

+ VCC

+

=

=

−

RC

E

C

S Vo

+ VCC

+

−

(a)

RC

C

S

E

Vo

Vsat

ICEO

+ VCC

+

−

+

−

RC

C

S

E

Vo

+ VCC

+

−

(b)

Figure 9.1.3BJT switch.(a)Off (cutoff) position.

(b)On (saturation) position.

Cutoff

region

Active

region

VT

Saturation

region

Vi

VCC

(VCC − Vsat) + VT

VCC

Vo

Vsat

0.7 V

RB

βRC

Figure 9.1.4Transfer characteristic of BJT

switch.

Figure 9.1.4 shows the transfer characteristic of a BJT switch relatingViandVo, whereas
Figure 9.1.5 depicts a plot of a typical input-voltage waveform and the resulting output-voltage
waveform, illustrating the essential inherent inversion property of the switch, the waveform
distortion, propagation delays, and rise and fall times of the BJT switch. Note the following
time parameters.