It is often the case that electronic apparatuses having audio output units generate unpleasant audible popcorn noise due to a sudden change of a bias voltage during a startup and a shutdown of the audio output unit. In order to prevent or suppress generation of such popcorn noise, the audio output unit is designed to have a sufficiently large time constant to allow the bias voltage rise and fall slowly during its startup and shutdown, respectively.
When the apparatus has a multiplicity of output amplifier circuits for a CD player and a MD player for example which are started up or shut down simultaneously, their bias voltages can be raised and dropped simultaneously using a common time constant circuit.
However, if the electronic apparatus has audio output units adapted to individually and independently start up or shut down a multiplicity of output amplifier circuits, the output amplifier circuits cannot be controlled by a single time constant circuit. In this case, each output amplifier circuit requires its own time constant circuit.
Take a cellular phone for example. It has four output amplifier circuits for an earphone, a back loudspeaker, a receiver, and an external output. Since cellular phones are powered by a battery, their power consumption is preferably reduced as much as possible to prolong the life of the battery. In a cellular phone, therefore, the output amplifier circuits are designed to operate in such a way that only one output amplifier circuit currently in use is powered but the rest of the output amplifier circuits are shut down. Thus, a multiplicity of time constant circuits are provided one for each output amplifier circuit to allow the individual output amplifier circuits to be started up and shut down without generating popcorn noise.
These time constant circuits respectively require a condenser and a charge and discharge circuit. These output amplifier circuits occupy a large area on an IC chip, since each of the output amplifier circuits includes a condenser. This hinders miniaturization of the IC chip. If these condensers are externally provided, the IC chip requires additional connection pins. In addition, a space is needed for installing the condensers.