45. 如右圖所示之並聯-串聯式(Shunt-Series)負回授放大電路，電晶體參數 gm1 = gm2 = 6 mA/V，若忽略爾利效應(Early Effect)及基體效應(Body Effect)，電阻 RS = RD = 10 KΩ 及 RF = 80 KΩ，請問電流放大倍數 Af = Io / Is為何？