written 8.2 years ago by |
DC analysis of the circuit:
1.VG=R2(R1+R2)(VDD−VSS)–5V=30(180+30)X10–5=−3.571V
Applying KVL to gate source loop we get,
- VG−VGS−IDQRS+5=0
∴VGSQ=5+VG−IDQRS=5−3.571–1000IDQ=1.429–1000IDQ
But IDQ=K(VGSQ−VT)2
IDQ=1X10(−3)(1.429–1000IDQ−0.8)2
IDQ=1X10(−3)(0.629−1000IDQ)2
IDQ=1X10(−3)(0.395−1258IDQ+106IDQ2)
IDQ=3.95X10(−4)−1.258IDQ+103IDQ2
By solving quadratic equation,
IDQ=2.066X10(−3)A or IDQ=0.1911X10(−3)A
If we select IDQ=2.066X10(−3)thenVDSQ will be negative.
So IDQ = 0.1911 mA
IDQ = 0.1911 mA
VGSQ= 1.429 – 1000X 0.1911 X 10^(-3) = 1.2379 V
gm=2K(VGSQ−VT)
= 2X1X10(−3)(1.2379–0.8)=8.758X10(−4) = 0.8758 mA/V
AC analysis:
Step1: Draw the small signal equivalent circuit The small signal equivalent circuit is shown in fig.
Step2: Calculate the voltage gain
Vo=−gmVgsRD
And Vi=Vgs+gmVgsRS==Vgs(1+gmRS)
AV=VoVi=(−gmVgsRD)(Vgs(1+gmRS))
=(−gmRD)((1+gmRS))=(−0.8758X6.3)(1+0.8758X1) = -2.941V
AV = -2.941V
Step2: Input Resistance:
Ri=(R1||R2)
= 180K || 30K = 25.714 Ω
Ri = 25.714 Ω
Step3: Input Resistance:
Apply KCL at node D of fig.
Io=gmVGS+IRD
Apply KVL to DSG loop of fig to write,
Vo+gmVGS−gmVGSRS=0
∴gmVGS=Vo(RS−1)
IRD=VoRD
Io=Vo(RS−1)+VoRDButRS−1=RS
∴ Io=Vo(1RS+1RD)
Ro=VoIo=R(SXRD)R(S+RD) = R_S ||R_D = 6.3K || 1K = 863.013 Ω
Ro = 863.013Ω