LA4550 Continued from preceding page. C7 (C8) : Output capacitor. The low cutoff frequency is determined by the following formula. 1 fL : Low cutoff frequency fL= 2πC7RL RL : Load resistance To make the low frequency response in the bridge amplifier mode identical with that in the stereo mode, the capacitor value must be doubled. C9 : Decoupling capacitor. Used for the ripple filter. Since the rejection effect is saturated the capacitor value more than needed. This capacitor, being also used for the time constant of the muting circuit, affects the starting time. C10 : Power source capacitor. Application Circuits Voltage gain adjust · Stereo mode The voltage gain is determined by on-chip resistor R1 (R2) and external feedback resistor Rf as follows : R1 VG=20 log [dB] Rf1+R2 Any voltage gain can be obtained by external resistor Rf. · [BTL 1] Bridge amplifier 1 mode R1 Vo1≈ Vi R2+R4+Rf1+Rf2 R3 Vo2≈ – Vi R2+R4+Rf1+Rf2 Vo = Vo1 – Vo2 R1+R3 = Vi R2+R4+Rf1+Rf2 ∴ Vo R1+R3 VG=20 log =20log [dB] Vi R2+R4+Rf1+Rf2 Assuming R2=R4=50Ω, R1=R3=20kΩ and Rf1=Rf2, the voltage gain is obtained by : R1 VG=20 log [dB] Rf1+R2 [BTL 2] Bridge amplifier 2 mode The CH1 is a noninverting amplifer and the CH2 is an inverting amplifier. The total voltage gain, being apparently higher than that of the CH1 by 6dB, is approximately calculated by the following formula. VG=20log R2/R1+6 (dB) To reduce the voltage gain, Rf is connected and the following formula is used. VG=20log R2/Rf+R1+6 (dB) No.1718–4/9
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