Newer generation, directly coupled stereo-amplifier ICs can directly drive headphones and speakers and thus eliminate bulky and expensive output-coupling capacitors. Many of these amplifiers also include a charge pump for generating an internal negative-supply-voltage rail to produce a bipolar-output swing while operating from one positive-supply voltage. However, if your applications require switching the amplifier's output between two or more headphones or other loads, you cannot necessarily use a simple electronic analog switch. Many analog switches can't handle a signal that makes excursions above the positive-power-supply voltage, V_DD, or below ground. Depending on V_DD's maximum value, you can apply one of two approaches.

If V_DD falls below 2.8V (Figure 1), choose a switch for IC₂ such as Maxim's MAX4762, which handles negative signals down to V_SS of –5.5V, where V_DD can range from 1.8 to 5.5V with a typical on-resistance of 0.6Ω. If V_DD falls between 2.8 and 5V, use a dual-supply, low-on-resistance switch, such as Maxim's MAX4679 at IC₂ (Figure 2), along with a different stereo amplifier, such as the MAX9722B, IC₁, to handle the higher V_DD. For the switch's negative supply, you can use the negative voltage generated within the MAX9722B to eliminate the need for an additional charge-pump-power-supply circuit.

To enhance a mobile-telephone design, you can use a stereo-headphone jack to accommodate a hands-free combination earphone and microphone. You use the stereo jack's tip connection as the headphone contact, the ring contact for the microphone, and the shell contact as the common connection to ground (Figure 3). When you connect the hands-free combination, you must also turn off one channel of headphone amplifier IC₁. Although the MAX4411 amplifier offers an individual-channel-shutdown feature, the device's outputs present an impedance of 2 kΩ to ground when you switch it off.

An electret microphone capsule typically includes an open-drain JFET-output circuit that typically requires a 2-kΩ resistor, R₁, which connects to a low-noise, positive-supply voltage of approximately 2V. The resistor provides dc bias to the JFET and allows the microphone capsule's audio-output signal to appear on the output terminal. In most applications, the microphone's output connects directly to a high-impedance, low-noise amplifier, IC₃, by ac coupling of capacitor C₁.

The amplifier's 2-kΩ-to-ground off- impedance would heavily load the microphone and halve its dc bias, moving it out of its optimum operating range and reducing its output and SNR. Adding analog switch IC₂ between the microphone and the headphone amplifier's output maintains the microphone's bias and resistive load.