Nt1310 Unit 6 Midpoint Biases

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Midpoint Bias It is desirable to bias a JFET near the midpoint of its transfer characteristic curve where IDSS =I_DSS/2 Under ac signal condition, it allows the maximum amount of drain current swing between IDSS and 0. IDSS =IDSS/2 then Schokley’s equation can be written as, I_DSS/2=〖I_DSS (1-V_GS/V_(GS(off)) )〗^2 1/2=(1-V_GS/V_(GS(off)) )^2 V_GS=0.29V_(GS(off)) V_GS=V_(GS(off))/3.4 From this V_GS value required R_S can be determined. To set the drain voltage at the midpoint, choose R_D to get the desired voltage drop.

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