PROBLEMS 4198.4.5For the CD JFET amplifier of Figure 8.4.2(a),
show that Equations (8.4.8) and (8.4.9) can be
rearranged as follows:
Av 1 =
gm(ro‖RSS‖RL)
1 +gm(ro‖RSS‖RL)Ai=gm(ro‖RSS‖RL)(R 1 ‖R 2 )
RL[1+gm(ro‖RSS‖RL)]
8.4.6In the source-follower circuit of Figure 8.4.2(a)
the voltage gain is to be 0.7 whenRSS= 300
and a JFET withro=5kandgm= 0 .025Sis
used. Find the required load resistance.
8.4.7Obtain an expression for the output resistance seen
by the load when looking back into the ampli-
fier between source and ground for the CD JFET
source-follower circuit of Figure 8.4.2(a).
8.4.8If the circuit of Example 8.4.1 is converted to a
CG amplifier having the same component values,
compute the amplifier voltage gainAv 1 , current
gainAi, andRin.*8.4.9Obtain an expression forRifor the CG JFET am-
plifier circuit of Figure 8.4.3(a).
8.4.10Consider the CG JFET amplifier circuit of Figure
8.4.3(a) withRL=15k, RD= 7 .5k, RSS=
5k, ro = 100 k, andgm = 5 × 10 −^3 S.
EvaluateAv 1 ,Ai, andRin.
8.5.1If the voltage gain of an amplifier is given by
Av=
20 jω
( 120 π+jω)[1+jω/( 5 π× 10 −^4 )]
findωH,ωL, and the midband gain.
8.5.2Consider a CS JFET amplifier (Figure 8.5.2) with
R 1 =520 k, R 2 = 140 , RD=1k, and
RSS= 1 .4k. The FET parameters aregm=
5 mS,ro =20k, andCgs =Cgd =2pF.
The source resistanceRSis 500and the load
resistanceRLis2k. For the low-frequency 3-
dB angular frequency to be 40πrad/s, compute
CS,CD,CG,Av 0 , andωH.
8.5.3DetermineRL,Cgs,andAv 0 of a CS JFET amplifier
(Figure 8.5.2), given thatRS=1kwhich can be
considered to be much less than(R 1 ‖R 2 ), RL=
RD,ωL << ωH,ω3dB= 25 × 106 rad/s, and
ωZ/ωH=75. The transistor parameters aregm=
5 mS,ro>> (RD‖RL), andCgs=Cgd.
8.5.4(a) Let the nonlinear input–output relationship of
a transistor amplifying unit be given byvout=
50 (vd+ 3 v^2 d). Plot the transfer characteristic
and check for what range ofvdthe undistorted
output results.
(b) Let a negative feedback loop be added such
thatvd=vin−Bvout. For values ofB= 0. 03
and 0.08, investigate how the negative feed-
back reduces nonlinear distortion, but reduces
gain.
*8.5.5(a) Consider the voltage amplifier with negative
feedback shown in Figure 8.5.3. ForA =
−100, find the feedback factorBin order to
getAf=−20.
(b) With the value ofBfound in part (a), deter-
mine the resulting range ofAfif the transistor
parameter variations causeAto vary between
−50 and−200.
8.5.6(a) Consider a BJT in the common-emitter con-
figuration. The equations that describe the be-
havior of the transistor in terms of the hybrid
hparameters are given by
vB=hieiB+hrevC
iC=hfeiB+hoevC
Develop theh-parameter equivalent circuit of
the transistor in the common-emitter mode.
(b) Settinghreandhoeequal to zero, obtain theh-
parameter approximate equivalent circuit of
the common-emitter transistor.
(c) Figure P8.5.6(a) shows the small-signal
equivalent circuit of a transistor amplifier in
the common-emitter mode. Find expressions
for gainsAI 1 =IL/IB,AI 2 =IL/IS,AV 1 =
VC/VB, andAV 2 =VC/VS.
(d) Now consider the single-stage transistor am-
plifier shown in Figure P8.5.6(b). The pa-
rameters of the 2N104 transistor arehie=
1 .67 k, hfe=44, and 1/hoe=150 k.
Evaluate the performance of the amplifier by
computing current gain, voltage gain, power
gain, input resistance as seen by the signal
source, and output resistance appearing at the
output terminals.
(e) Next consider the two-stage transistor am-
plifier depicted in Figure P8.5.6(c). Develop
theh-parameter equivalent circuit of the two-
stage amplifier and evaluate its performance.
(f) Then consider the linear model of the
common-emitter BJT amplifier shown in Fig-
ure P8.5.6(d) that is applicable to the midband
frequency range. Find an expression for the
current gainAIm=I 2 /ISthat remains in-
variant with frequency. (Note the subscriptm