近似解法：
`V_B` = `10*(10k)/(10k+40k)` = 2V
`V_E` = `2-0.7` = 1.3V
`I_E` = `(1.3)/(1k)` = 1.3mA
`r_π` = `(26m*100)/(1.3m)` = 2kΩ
電路化成電晶體的交流等效電路(如下圖)
`A_i` = `i_o/i_i` = `(8k)/(8k+2k)*100*(4k)/(4k+4k)` = 40
近似解法：
`V_B` = `10*(10k)/(10k+40k)` = 2V
`V_E` = `2-0.7` = 1.3V
`I_E` = `(1.3)/(1k)` = 1.3mA
`r_π` = `(26m*100)/(1.3m)` = 2kΩ
電路化成電晶體的交流等效電路(如下圖)
`A_v` = `v_o/(v_i)` = `-(100i_b*4k////4k)/(i_b*2k)` = `-(200k*i_b)/(2k*i_b)` = -100
`Z_i` = 8k//2k = 1.6kΩ
`V_i*(1.6k)/(1.6k+1.5k) = Vi`
`A_v` = `(1.6k)/(1.6k+1.5k)*A_v` = `(1.6k)/(3.1k)*-100` = -51.6
將電路圖化成交流等效電路(如下圖)，由交流等效電路可以看出
Ri = RB1 // rπ
將電路圖化成交流等效電路(如下圖)，由交流等效電路可以看出
`V_o` = `-β*i_b*(R_(B2)*R_C)/(R_(B2)+R_C)`
= `-β * i_b * R_o`
`V_i` = `i_b*r_π`
`A_v` = `V_o/V_i`
= `(-β*i_b* R_o ) / (i_b*r_π)`
= `(-β* R_o ) / r_π`
= `(-β* R_o ) / ((β+1)*r_e)`
≒ `(- R_o ) / r_e`
先算 `r_π`
`400k*I_B+0.7+1k*I_E` = 10.7
`400k*I_B+100k*I_B` = 10
`500k*I_B` = 10
`I_B` = `10/(500k)`
`I_B` = 0.02 mA
`r_π` = `(25m)/(0.02m)`
=1250Ω
化成交流等效電路，如下圖
`A_V` = `V_o/V_i` = `(-100*i_b*1k)/(i_b*1.25k)`
= `-100/1.25` = - 80
先算 `r_π`
`400k*I_B+0.7+1k*I_E` = 10.7
`400k*I_B+100k*I_B` = 10
`500k*I_B` = 10
`I_B` = `10/(500k)`
`I_B` = 0.02 mA
`r_π` = `(25m)/(0.02m)`
=1250Ω
化成交流等效電路，如下圖
`R_i` = 400k//`r_π`
= 400k//1.25k ≒ 1.25kΩ