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The 750mW bridged load audio amplifier circuit with DC volume control composed of LM 4865 power amplifier IC is shown in the figure below. LM4865 adopts 8-pin SO package and 8-pin mini SMD package (the numbers in parentheses are the SMD package pin number), the power supply voltage VDD = 2.7 ~ 5.5V, the static operating current IDD = 4mA (typical value).
LM4865 contains two amplifiers A1 and A2, where the output of A1 is connected to the input of A2, the two produce output signals with the same magnitude but a phase difference of 180 °, which can drive the output connected between the two output terminals of V01 and V02 Bridge speakers. Compared with a single-ended amplifier, the output power (P0 = 4VDD2 / 2л2RL) produced by the differential bridge output under the same conditions is four times that of the single-ended amplifier, and no coupling capacitor is required.
Connect the headset through the capacitor C0 between the V01 pin of the LM 4865 and ground. As long as the voltage applied to the IC pin HP-Sense exceeds the threshold of 4V, the amplifier A2 is turned off and the speaker is silent. When the earphone is not plugged into the earphone jack, the voltage on the pin HP-Sense is set by the resistor divider composed of R1 and R2, so that the LM4865 operates in the bridge load mode. When the headphone plug is inserted into the socket, A2 is turned off. Therefore, the headset acts as a switch.
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