tailieunhanh - Introduction to Electronics - Part 7

The Source Follower Đi theo này sử dụng thiên vị cố định: IG = 0 ⇒ VGSQ = 0 ⇒ ID = IDS rất lớn Rin thu được bằng cách hy sinh sự ổn định thiên vị, mà không phải là rất quan trọng trong mạch này anyway, như chúng ta sẽ thấy. Tương đương với tín hiệu nhỏ được xây dựng theo cách thông thường: | The Source Follower Introduction to Electronics 159 The Source Follower Small-Signal Equivalent Circuit This follower uses fixed bias IG 0 VGSQ 0 ID IDSS Tremendously large Rn is obtained by sacrificing bias stability which isn t very important in this circuit anyway as we shall see. The small-signal equivalent is constructed in the usual manner Fig. 229. Source follower small-signal equivalent circuit. The Source Follower Introduction to Electronics 160 Fig. 230. Source follower small-signal equivalent circuit Fig. 229 repeated . Voltage Gain This one requires a little more algebra. Beginning with and v. V V V v v in gs o gs in o gmVgs in RL gmvgs V Rg 7 Rl vgs gm V 1 Ì Rg rl 233 234 We replace vgs in eq. 234 with eq. 233 and solve for vo vn V v o in vo gm V d r . Rg 7 Rl 235 1Ì 1 1Ì 1 gm V Rg 7 Rl vo v in gm p V Rg 7 Rl 236 Av v vin igm 1 1 V m RL Rl to typically Rl 237 The Source Follower Introduction to Electronics 161 Fig. 231. Source follower small-signal equivalent circuit Fig. 229 repeated . Input Resistance Replacing vo in eq. 233 with eq. 234 v. v v v_ in gs o gs gs d R. rg J 238 gm V R But vgs iin Rg v in i inRG iR gm V r . 1 - RgJ Rl 239 Solving for vn in Rin - Rg 1 gmRG RL 240 Because IG 0 RG can be several MQ. With the additional multiplying factor of RL Rin can become extremely large