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+// Example 7.5: Operating point

+clc, clear

+VP=-5; // in volts

+IDSS=12e-3; // in amperes

+// From Fig. 7.34(a)

+VDD=18; // in volts

+R1=400; // in kilo-ohms

+R2=90; // in kilo-ohms

+RD=2e3; // in ohms

+RS=2e3; // in ohms

+// Applying Thevnin's theorem to obtain simplified circuit in Fig. 7.34(b)

+VGG=VDD*R2/(R1+R2); // in volts

+// Plotting transfer characteristics

+VGS=[VGG:-0.01:VP]; // Gate source voltage in volts

+// Using Shockley's equation

+ID=IDSS*(1-VGS/VP)^2; // Drain current in amperes

+ID=ID*1e3; // Drain current in mili-amperes

+plot2d(VGS,ID,rect=[-5,0,3,12]);

+xtitle("Transfer Characteristics","VGS (V)","ID (mA)");

+// Plotting bias line

+// From the KVL for the gate-loop

+ID=(-VGS+VGG)/RS; // Source current in amperes

+ID=ID*1e3; // Source current in mili-amperes

+plot(VGS,ID,"RED");

+// Intersection of transfer curve with the bias curve

+// Putting VGS = VGG-ID*RS in Shockley's equation and solving, we get

+// ID^2*RS^2 + (2*RS*VP - 2*VGG*RS - VP^2/IDSS)*ID + (VGG-VP)^2

+// Solving the equation

+p_eq = poly([(VGG-VP)^2 (2*RS*VP-2*VGG*RS-VP^2/IDSS) RS^2],"x","coeff");

+p_roots= roots(p_eq);

+IDQ=p_roots(1); // in amperes

+// Writing the KVL for the drain source loop

+VDSQ=VDD-IDQ*(RD+RS); // in volts

+IDQ=IDQ*1e3; // in mili-amperes

+disp(VDSQ,"VDSQ (V) =");

+disp(IDQ,"IDQ (mA) =");
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