Re: [理工] [電子] BJT的輸入阻抗
※ 引述《PuNkChild (脆歐德)》之銘言：
: 用小信號模型算
: -------- --------------
: | | + | | |
: | rπ Vπ gmVπ ro R1
: | | - | | |
: ==== |____________|_______| |
: GND | |
: |↑ix |
: | ==== GND
: Vx
: |
: |
: ==== GND
好久沒碰電子學，憑著一點記憶在算
將 gmvpi and ro 轉成戴維寧等效
-------- --------------
| | + | |
| rπ Vπ gmroVπ R1+ro
| | - | |
==== |____________| |
GND | |
|↑ix |
| ==== GND
Vx
|
|
==== GND
再轉回諾頓
-------- -------------------------
| | + | | |
| rπ Vπ | gmroVπ/(ro+R1) ro+R1
| | - | | |
==== |______ | | |
GND | | |
|↑ix | |
| ==== GND ==== GND
Vx
|
|
==== GND
此時跨在相依電流源的電壓為Vπ
根據歐姆定律可得知等效阻抗為 (ro+R1)/gmro
故Ri= rπ // ((ro+R1)/gmro) // ro+Rl
= rπ // [(ro+Rl)/(1+gmro)]
ps: 相依電壓源與電流源的方向要注意
--
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※ 編輯: ohya74921 來自: 140.115.72.67 (09/02 14:26)
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