In recent years, with the significant development of information and communication technology and a semiconductor technology, supply and use of all types of portable terminals have rapidly increased. In particular, recent portable terminals have developed to a mobile convergence stage including traditional unique field and other terminal fields. As a representative example of the portable terminals, a mobile communication terminal provides various functions such as a TV watching function (e.g., mobile broadcasting such as Digital Multimedia Broadcasting (DMB) or Digital Video Broadcasting (DVB)), a music playing function (e.g., MPEG Audio Layer-3 (MP3)), a photographing function, and an Internet access function as well as a general communication function such as speech call or message transmission/reception.
Meanwhile, when an earphone is mounted in the portable terminal, a ground terminal of the earphone is connected to a real ground of the portable terminal. As described above, if the ground of the earphone is connected to a real ground of the portable terminal, TDMA noise may be induced in the earphone through the real ground. That is, a portable terminal according to the related art has a problem in that TDMA noise occurs during an earphone call to deteriorate calling quality.
The TDMA noise is simply described. A portable transceiving a wireless in a TDMA scheme transmits the wireless signal for predetermined period. That is, the portable terminal activates a power amplifier for each transmission period to transmit the wireless signal. The power amplifier consumes great power. Due to this, voltage drop of a battery occurs for each transmission period in the portable terminal. As described above, noise generated in an audio output terminal due to voltage drop occurring for each transmission period is generally called TDMA noise.
When the portable terminal provides a radio receiving function, a portable terminal using an earphone as an antenna having no separate antenna for radio reception has been increased. As described, in a case of a portable terminal using an earphone as an antenna for receiving a radio signal, TDMA noise and a radio receiving function must be simultaneously considered. Accordingly, the portable terminal according to the related art has a difficulty in designing a circuit for reducing TDMA noise induced in an earphone.