240ln05 - FET AMPLIFIER CIRCUITS FET REGIONS OF OPERATION...

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1 FET AMPLIFIER CIRCUITS LN #5 SATURATION TRIODE Linear I D V DS FET REGIONS OF OPERATION For applications in amplifiers FETS are biased in the saturation region where the drain current for an ideal device is independent of V DS and depends only upon V GS . We already know that in saturation, FETs can be described by ( ) 2 2 ' T GS eff D V V L W k I = with k’ = μ n C ox , L eff = L- x d This implies that the TRANSCONDUCTANCE ) ( ' | T GS eff V DS D m V V L W k V I g GS = = and the OUTPUT CONDUCTANCE |0 D o GS o I g V if r VDS = = −−−−−> −−> = The above analysis suggests the following small signal equivalent circuit g m v gs D S G + - v gs BECAUSE OF THE INSULATOR (OXIDE) UNDER THE GATE, THE IDEAL MOSFET HAS ZERO GATE CURRENT THE DEPENDENCE OF I D ON V DS (IN SATURATION) IN REAL DEVICES IS MODELED AS ( ) ) 1 ( 2 ' 2 DS T GS eff D V V V L W k I λ + = In this case D T GS V DS D o o DS T GS eff V GS D m I V V L W k V I r g V V V L W k V I g GS DS = = = + = = 2 ) ( 2 ' | 1 ) 1 )( ( ' |
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2 g m v gs + _ v gs r o D S G THERE IS AN ADDITIONAL VDS DEPENDENCE THROUGH L eff THE MODIFIED CIRCUIT NOW IS MOSFET CIRCUIT PERFORMANCE AND AMPLIFER CONFIGURATIONS Common Source Amplifier v i + V DD V o V GG R D V D I D Triode Cutoff Saturation V i V o Cutoff Saturation Triode & Linear Physical Circuit Device Characteristic Transfer Curve + _ v gs r o + _ v i g m v gs v o + R D Note: We will ignore g mb
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240ln05 - FET AMPLIFIER CIRCUITS FET REGIONS OF OPERATION...

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