CHOICE OF INPUT AND OUTPUT 3 parameters---VGS, VDS, ID, (Vsb= for advanced course) ID α (VGS, VDS) ID---captures variation----output either VGS / VDS can be input but if VDS is input, no other terminal is available for output so only VGS can be the input Now what should be Vds?
Where to bias ? Max gain-------MAX IDRD------------MAX ID Min distortion ID= f (VGS)------SATURATION REGION Max current, ID captures variations of VGS faithfully ID= f (VGS, VDS)-----------LINEAR REGION Min current, ID varies with VGS , VDS ---(extra variation) Bits, pilani
Using single DC supply POTENTIAL DIVIDER BIAS Bits, pilani VG = VGS + I D RS
Q point stability Case-1------Vg increases due to power supply fluctuation Vg↑ Vgs↑ Id↑ (Id Rs) ↑ Vgs↓ Case-2----- VT decreases due to temperature fluctuation VT ↓ ( Vgs – VT )↑ Id↑ (Id Rs) ↑ Vgs↓ ( Vgs – VT ) ↓ Bits, pilani
Setting DC BIAS DRAIN TO GATE FEEDBACK BIAS V DD = VGS + I D RD Bits, pilani
Converting to T modelFigure 4.39 Development of the T equivalent-circuit model for the MOSFET. For simplicity, ro has been omitted but canbe added between D and S in the T model of (d). Bits, pilani
How to draw AC model of amplifier? For amplification, only AC behaviour needs to be considered Replace MOS by its model in the circuit Bits, pilani