Audio microphones are commonly used in a variety of consumer applications such as cellular telephones, digital audio recorders, personal computers and teleconferencing systems. In particular, lower-cost electret condenser microphones (ECM) are used in mass produced cost sensitive applications. An ECM microphone typically includes a film of electret material that is mounted in a small package having a sound port and electrical output terminals. The electret material is adhered to a diaphragm or makes up the diaphragm itself.
Another type of microphone is a microelectromechanical systems (MEMS) microphone, in which a pressure sensitive diaphragm is etched directly onto an integrated circuit. As such, the microphone is contained on a single integrated circuit rather than being fabricated from individual discrete parts.
Most ECM and MEMS microphones also include a preamplifier that can be interfaced to an audio front-end amplifier via a cord and plug for a target application such as a cell phone or a hearing aid. In many cases, the interface between the preamplifier and front-end amplifier is a three-wire interface coupled to a power terminal, signal terminal and ground terminal. In some systems, however, a two-wire interface is used in which two of the terminals are combined into a signal, thereby reducing the cost of the system by using two wires instead of three wires.
For all types of transducers, and microphones in particular, designing electronic interfaces can be challenging. Combining a power and signal interface into a single interface poses a number of particular design challenges with respect to voltage swing and low supply voltage use, among others. Increasing voltage swing in a transducer generally increases the range of the transducer. Lowering the supply voltage is usually associated with decreasing the power consumption and is often relevant in mobile applications. In some instances, lowering the supply voltage, however, may have an adverse impact on the voltage swing.