Improvement of PSRR for amplifiers for hand-held / hands-free applications
Original Publication Date: 2003-Jul-25
Included in the Prior Art Database: 2003-Jul-25
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In future Smart Phone devices it is intended to use the same speaker for hand-held mode (conventional conversation with the device at the ear), hands-free mode (freehand conversation) and for ringing mode (phone-ringing). For this requirement it's necessary to have a sufficient sound pressure level (SLP) by dint of a very sensitive speaker with a low impedance (e.g. 8 Ohm / 0.5W). The usage of such a speaker will decrease the voltage swing across a conventional speaker at hand-held mode. The problem is that the signal to noise ratio will be decreased and thereby the noise will be stronger, because the Power Supply Rejection Ratio (PSRR) of an amplifier is independent of the output voltage swing. The new approach is a amplifier system with two different pairs of amplifiers in one circuit (Fig. 1) connected to the speaker. For every mode (hand-held or hands-free/ringing mode) one pair is at each case enabled and the other tri-stated. The configuration regulates the supply voltage level of the speaker and can vary the noise level that way. Figure 1 shows an example of such a circuit.