Stereo-amplifier IC's outputs drive multiple loads
Choose the analog switch based on it’s supply voltage range.
Jean-Jacques Avenel, Maxim Integrated Products, Lesigny, France; Edited by Brad Thompson and Fran Granville -- EDN, November 21, 2005
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, VDD, or below ground. Depending on VDD's maximum value, you can apply one of two approaches.
If VDD falls below 2.8V (Figure 1), choose a switch for IC2 such as Maxim's MAX4762, which handles negative signals down to VSS of –5.5V, where VDD can range from 1.8 to 5.5V with a typical on-resistance of 0.6Ω. If VDD 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 VDD. 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.
