0195136047.pdf

418 TRANSISTOR AMPLIFIERS

8.3.11Starting from Equations (8.3.9), (8.3.11), and

(8.3.12), show that equations (8.3.13) through

(8.3.15) hold forro→∞.

8.3.12Consider the CC BJT amplifier circuit shown in

Figure 8.3.2(a) withR 1 =150 k,R 2 =150 k,

RE=2k, RL=3k, andVCC=10 V. The

BJT hasβ= 100 ,ICQ= 1 .2 mA, andVA= 75

V. DetermineRi,Rin,Av 1 , andAi.

8.3.13Develop a formula for the power gainAPfor a

common-collector stage if the power gain of an

amplifier stage is defined as the ratio of the load

powervLiLto the power delivered by the stagev 1 is.

(See Figure 8.3.2 for notation.)

8.3.14In the CC amplifier stage of Figure 8.3.2(a), letR 1

=32 k,R 2 =22 k,RE= 400 ,RL= 250 ,

andVCC=9 V. Given thatVA=70 V,β=50, and

ICQ=4 mA for the BJT, findRi,Rin,Av 1 , andAi.

*8.3.15Obtain an expression for the power gainAPfor
a common-base stage if the power gain of an
amplifier stage is defined as the ratio of the load
powervLiLto the power delivered by the source
v 1 is. (See Figure 8.3.3 for notation.)
8.3.16Show that Equation (8.3.17) holds for the circuit
of Figure 8.3.3(b).
8.3.17Consider the CB BJT amplifier circuit shown in
Figure 8.3.3(a), withRC=1k, RL=6k,
andRin= 20 . The transistor parameters are
given byβ= 60 ,VA=70 V, andgm= 0 .03 S.
FindRi,RE,Av 1 , andAi.
8.3.18Consider the CE BJT amplifier circuit shown in
Figure P8.3.18. In order to make it into a common-
base amplifier, terminalsaandbare connected

together, and capacitorCEis disconnected from

ground and used to couple a signal from a source

to the emitter. DetermineRi,Rin,Av 1 , andAi.

8.4.1For the CS JFET amplifier circuit of Figure

8.4.1(a),RD=2kandRL=3k. The JFET

withro=15 khas a voltage gainAv 1 =− 4. 5

when the entire source resistance is bypassed. Find

gm.

8.4.2Considering the CS JFET amplifier circuit of Fig-

ure 8.4.1(a), a portion of the source resistance

RSSin the JFET CS stage is sometimes left unby-

passed. LetRSS=RSS 1 +RSS 2 in whichRSS 1 is

that portion ofRSSthat is not bypassed andRSS 2 is

that part ofRSSthat is bypassed. Find expressions

for the amplifier voltage gainAv 1 and the current

gainAi. Comment on the results, particularly with

reference to Equations (8.4.3) and (8.4.4).

8.4.3The common-source amplifier of Figure 8.4.1(a)

withRL = 300 , RD = 150 , andRSS=

100 (fully bypassed) has a JFET withVA= 80

V,ro=2k, andVP=4V.

(a) ComputegmandIDSSif the voltage gain is

− 2 .8.

(b) If the voltage gain is to be reduced to− 1. 4

by leaving part ofRSSunbypassed, findRSS 1 ,

which is that portion ofRSSthat is not by-

passed.

*8.4.4Consider Figure 8.4.1 (a) of a CS JFET ampli-

fier withR 1 =330 k, R 2 =110 k, RD =

1k, RSS=1k, RL=1k, IDQ=6 mA,

VA=90 V,VP =4 V, andIDSS =20 mA.

ComputeRin,Av 1 , andAi.

C

E

Input 9000 Ω

30,000 Ω

CB

CE

B

250 Ω

750 Ω C

C

1000 Ω

+

−

9 V

ICQ = 5 mA

+

−

vL

β = 150

vA = 50 V

a

b

Figure P8.3.18