A new generation of direct-coupled stereo amplifier ICs can directly drive headphones and speakers , eliminating the need for bulky and expensive output coupling capacitors. Many of these amplifiers also have a charge pump that generates an internal negative voltage rail, which provides a bipolar output swing when powered by a single positive voltage. However, if the application requires switching the amplifier output between two or more headphones or other loads, this cannot be achieved using a simple electronic analog switch. Many analog switches cannot handle signals that exceed the positive supply voltage VDD or are below ground. Depending on the maximum value of VDD, one of the following two solutions can be used.
If VDD is lower than 2.8V (Figure 1), a switch can be selected for IC2, such as Maxim's MAX4762, which can handle negative VSS signals as low as -5V, while VDD can range from 1.8V to 5.5V, with a typical on-resistance of 0.6Ω. If VDD drops to between 2.8V and 5V, IC2 uses a dual-power, low-on-resistance switch, such as Maxim's MAX4679 (Figure 2), and the differential stereo amplifier IC1 used is MAX972 2B to handle higher VDD. For the negative power supply of the switch, the negative voltage generated in the MAX9722B can be used, eliminating the need for an additional charge pump power supply circuit.
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To improve the design of the mobile phone , a stereo headphone jack can be used to accommodate both the headset and microphone of the hands-free kit. The top contact of the stereo headphone plug is for the headset, the ring contact is for the microphone, and the shell contact is a common connection to ground (Figure 3). When the hands-free kit is connected, one channel of the headphone amplifier IC1 must be turned off. Although the MAX4411 amplifier has its own individual channel shutdown function, when it is turned off, the output impedance of the device to ground is 2 kΩ.
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Electret microphones generally have an open-drain JFET output circuit that requires a 2kΩ resistor R1 to be connected to a low-noise, positive supply voltage of about 2V. This resistor provides a DC bias for the JFET, so that the audio output signal of the electret microphone appears at the output terminal. In most applications, the output of the microphone is directly connected to a high-impedance, low-noise amplifier IC3 through an AC coupling capacitor C1.
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The amplifier's 2kΩ off impedance to ground becomes a heavy load for the microphone, shunting the microphone's DC bias away from its optimal operating range, reducing output and SNR. Adding analog switch IC2 between the microphone and headphone amplifier outputs maintains the microphone's bias and resistive load.
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