電路能力測試-4：聲音信號放大電路

Circuit Knowledge Test-4: Audio Buffer

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1. What will be the input and output impedance of QA4 stage with C2412 Hfe=100?

計算QA4的輸入及輸出阻抗。（假設電晶體的Hfe值為100）

2. What will be the 6 db lower cut-off frequency and higher cut-off frequency?

計算本電路6 dB的低頻及高頻截止頻率。

3. What will be the audio buffer gain and maximum undistorted audio output RMS value with 5 volt DC supply?

計算本電路的基本放大倍數，及5V電源時的最大不失真電壓值。

4. Why audio buffer needs only one stage while video buffer needs two stages for amplification?

為何音頻電路僅需要單級放大電路，而影像電路必須兩級反向。

5. Calculate the channel cross-talk for this circuit at 100 Hz frequency.

計算本電路在100 Hz時的左右聲道串音比。

試題解答

Circuit Knowledge Test-4: Audio Buffer

ANS-1:

Input impedance of QA4: 4K7 and 6K8 and 560*100 in parallel.? Actually 2K7.

Output impedance of QA4: 330 Ohm.

ANS-2:

Fl = 1/2ΠRC = 1/ (6.28 * 10u * 2K7) = 6 Hz while R = Input impedance, C = CA24.

Fh = 1/2ΠRC = 1/ (6.28 * 0.033u * 330) = 15 KHz while R = RA43, C = CA26.

ANS-3:

Audio buffer gain: RA43/RA49 = 330/560 = 0.6

Maximum undistorted output: 5.0/2.828 =1.75 Vrms.

ANS-4:

Audio signal is symmetric over dc balance line, while video signal is asymmetric with sync and peak white/black level. One transistor amplifier stage will give out reverse phase signal, then video signal has to reverse back to its original phase. However, audio signal also has to keep L/R channels the same phase.

ANS-5:

Channel cross-talk is caused by amplified signal feeding through dc supply line into the amplifier stage of the other channel.

Impedance of CA45 at 100 Hz: Zc = 1/2ΠFC = 1/(6.28 * 100 * 47u) = 33

Cross-talk rati Zc/RA43 = 0.1 (that is -20 db)