0195136047.pdf

420 TRANSISTOR AMPLIFIERS

denotes a reference to the midband frequency

range.)

(g) The linear model of a common-emitter BJT

amplifier shown in Figure P8.5.6(e) is appli-

cable at low frequencies. Obtain an expression

for the current gainAIl=I 2 /ISin terms of

AImandωl, whereωlcorresponds to 0.707AIm

on the frequency response curve, andωliden-

tifies the lower end of the useful bandwidth of

the amplifier.

(h) The hybrid-πequivalent circuit for comput-

ing the high-frequency performance of a BJT

RC-coupled amplifier is shown in Figure

P8.5.6(f). Analyze the circuit (by representing

in terms of isolated input and output sections)

to findAIh= I 2 /IS in terms ofAImand

ωh, whereωhcorresponds to 0.707AImon the

frequency response curve, and denotes high-

band frequencies.

(i) Sketch the frequency response curve of anRC-

coupled amplifier by plotting the current gain

as a function of frequency on semilogarithmic

paper and showing the bandwidth.

RB VB VC

RS

hie hfeIB hoe

IS IB IC

IL

RL

BC

E
(a)

VS

Internal

resistance

of source

Source

signal

+

−

Base bias

equivalent

resistance

Collector

load

resistance

Figure P8.5.6