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Determine voltage gain, Input resistance and output resistance for the MOSFET amplifier shown.

The diagram below

enter image description here

1 Answer
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DC analysis of the circuit:

1.VG=R2(R1+R2)(VDDVSS)5V=30(180+30)X105=3.571V

Applying KVL to gate source loop we get,

  1. VGVGSIDQRS+5=0

VGSQ=5+VGIDQRS=53.5711000IDQ=1.4291000IDQ

  1. But IDQ=K(VGSQVT)2

    IDQ=1X10(3)(1.4291000IDQ0.8)2

    IDQ=1X10(3)(0.6291000IDQ)2

    IDQ=1X10(3)(0.3951258IDQ+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

  1. VGSQ= 1.429 – 1000X 0.1911 X 10^(-3) = 1.2379 V

  2. gm=2K(VGSQVT)

= 2X1X10(3)(1.23790.8)=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.

enter image description here

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+gmVGSgmVGSRS=0

gmVGS=Vo(RS1)

IRD=VoRD

Io=Vo(RS1)+VoRDButRS1=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

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