Common Emitter Amplifier with Base Biasing & Emitter Negative Feedback

Assumption:

1. ZOUT = 1.2KΩ
2. VR4 = 0.5V
3. VCC = 5V
4. VBE = 0.6V
5. hFE = 100
6. f3dB = 40HZ

Calculation:

1. R3 = ZOUT = 1.2KΩ
2. ICQ1(sat) = (VCC - VR4) / R3 = (5 - 0.5) / 1200 = 3.75mA
3. ICQ1(q) = ((VCC - VR4) / 2) / R3 = ((5 - 0.5) / 2) / 1200 = 1.9mA
4. IBQ1 = ICQ1(q) / hFE = 1.9m / 100 = 19uA
5. IEQ1 = IBQ1 + ICQ1 = 19u + 1.9m = 1.919mA
6. R4 = VR4 / IEQ1 = 260Ω
7. R2 = (VBE + VR4) / (10 * IBQ1) = (0.6 + 0.5) / (10 * 19u) = 5.8KΩ
8. R1 = (VCC - VBE - VR4) / (11 * IBQ1) = (5 - 0.6 - 0.5) / (11 * 19u) = 18.7KΩ
9. ZC2 = R4 / 10 = 27Ω
10. C2 = 1 / (2 * π *  f3dB * ZC2) = 1 / (2 * 3.14 * 40 * 27) = 147uF
11. C3 = 1 / (2 * π *  f3dB * R3) = 1 / (2 * 3.14 * 40 * 1.2K) = 3uF 
12. Re = VT / IEQ1 = 25m / 1.919m = 13Ω
13. ZIN = R1 || R2 || (hFE * Re) = 1 / (1 / 18.7K + 1 / 5.8K + 1 / (100 * 13)) = 1KΩ
14. C1 = 1 / (2 * π *  f3dB * ZIN) = 1 / (2 * 3.14 * 40 * 1K) = 4uF

 Part selection:

1. R1 = 18KΩ
2. R2 = 5.1KΩ
3. R3 = 1.2KΩ
4. R4 = 270Ω
5. C1 = 4.7uF
6. C2 = 100uF
7. C3 = 1mF (Pick larger capacitor to improve frequency response)

Note:

1. This circuit is the primary stage of another circuit. If load RL is connected to  this circuit, the ZOUT becomes R3 || RL.
2.  For removing annoying high frequency noise, a small bypass capacitor can be placed between the circuit and next stage.

小電容麥克風的使用方式

1. 上圖是在大部份電料行買的到的小電容麥克風, 正式名稱叫做駐極體式電容麥克風(Electret condenser microphone)。
2. 這種麥克風利用駐極體來提供聲音訊號。駐極體部份不需額外供電即可提供永久電荷, 但因為訊號電壓太小, 所以還會內建一顆JFET把訊號放大到容易使用的地步。圖中的麥克風就是兩腳式的, 它的Source和Ground由一條引線接在一起;若為三腳式的, 則Source和Ground就不會接在一起。
3. 下圖是我自己的接線方法, 記得在輸出端要加上一顆電容以免下一級的直流訊號逆流而入。
   
4. 這裡是其它網站提供的介紹和結構圖和接線圖。