6. 如圖2-6所示之JFET電晶體電路,已知該電晶體截止電壓 \( V_{GS(off)} = -5V \),直流閘源極電壓 \( V_{GS} = -4V \) 時,\( I_D = 0.5mA \),則 \( R_1/R_2 \) 的值為何?