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A new generation of directly 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 that provides a bipolar output swing when powered from a single positive voltage. However, if an application needs to switch the output of an amplifier between two or more headphones or other loads, a simple electronic analog switch alone is certainly not possible. Many analog switches cannot handle signals that are above the positive supply voltage V DD or below ground. According to the maximum value of V DD , one of the following two schemes can be used.If V DD is lower than 2.8V (Figure 1), select a switch for IC 2 , such as Maxim's MAX4762, which can handle V SS negative signals as low as -5V, while V DD can range from 1.8V to 5.5 V, the typical on-resistance is 0.6Ω. If V DD drops between 2.8V and 5V, IC 2 uses a dual-supply, low-on-resistance switch, such as Maxim's MAX4679 (Figure 2), with a differential stereo amplifier IC 1 for the MAX9722B to handle Higher V DD . For the negative supply of the switch, the negative voltage generated in the MAX9722B can be used, eliminating the need for an additional charge pump supply circuit.
To improve the design of the phone, a stereo headphone jack can be used to accommodate both the headset and the microphone of the hands-free kit. The top contact of the stereo headphone plug is for the headphone, the ring contacts are for the microphone, and the case contacts are connected to the ground (Figure 3). When connecting the handsfree kit, one channel of the headphone amplifier IC 1 must be turned off. Although the MAX4411 amplifier has its own separate channel shutdown function, the output impedance of the device to ground is 2 kΩ when turned off.
An electret microphone typically has an open-drain JFET output circuit that requires a 2kΩ resistor R 1 to be connected to a low noise, approximately 2V positive supply voltage. The resistor provides a DC bias to the JFET, allowing the audio output signal of the electret microphone to appear on the output terminal. In most applications, the output of the microphone is connected directly to a high-impedance, low-noise amplifier IC 3 via an AC-coupling capacitor C 1 .
The 2kΩ on-off impedance of the amplifier becomes the heavy load of the microphone, shunting the DC bias of the microphone, leaving it to its optimal operating range, reducing output and SNR. Adding an analog switch IC 2 between the microphone and the headphone amplifier output maintains the microphone's bias and resistive load.
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