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Supply Rejection Improvement Of An Audio Amplifier By Supply Regulation Of Some Internal Stages

IP.com Disclosure Number: IPCOM000007496D
Publication Date: 2002-Apr-01
Document File: 5 page(s) / 41K

Publishing Venue

The IP.com Prior Art Database

Abstract

In audio applications, the required quality level is always increasing. Among the parameters affecting the quality (especially in applications where the same battery is supplying different circuits) the Supply Rejection Ratio is one of the most important (the other ones are noise and distortion). For mono-supply applications, the negative supply is the ground and we are concerned about the rejection of the positive supply only (PSRR). Current PSRR requirements can reach 90 to 100 dB, which are difficult to attain with prior art and require a new approach to provide this high level of rejection.

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Supply Rejection Improvement Of An Audio Amplifier By Supply Regulation Of Some Internal Stages

In audio applications, the required quality level is always increasing.  Among the parameters affecting the quality (especially in applications where the same battery is supplying different circuits) the Supply Rejection Ratio is one of the most important (the other ones are noise and distortion).  For mono-supply applications, the negative supply is the ground and we are concerned about the rejection of the positive supply only (PSRR).  Current  PSRR requirements  can reach 90 to 100 dB,  which are difficult to attain with prior art and require a new approach to provide this high level of rejection.

              Prior art uses a voltage regulator to supply the application rather than to supply it directly with the supply voltage as illustrated in Fig.1.

                                           Figure 1 Prior Art

              In this way the supply rejection of the regulator is added to that of the initial application to provide the PSSR figure.   While improved PSRR performance can be achieved using this approach, this prior art does not solve the problems of:

-  reducing die area and power dissipation

-  reducing the number of pads of the circuit or reducing the external circuitry to make the chip work

-  eliminating additional components to the application (i.e., regulator output capacitor)

-   improving output voltage range as well as output power capability.

The invention uses a new voltage regulator separation scheme, as briefly illustrated in Fig.2.

Figure 2- New Voltage Regulator Separation Scheme

               A regulator is used to supply only the first stage(s) of the audio amplifier.  This enables the load of the regulator to be low (low current, low capacitance) so that the regulator can be designed with classical operational amplifier design techniques and without requiring an external decoupling capacitor. The high current path of the amplifier and its output stage continues to be supplied directly by the battery voltage, resulting in the higher possible output power.

              In one embodiment, the invention is utilized in a classical two-stage CMOS operational amplifier of Fig.3.

       
   
 
   

Ground

 
 

Figure 3 Embodiment of the Invention within a Two-Stage CMOS Amplifier

              In this schematic the output stage M6/M7 continue to be supplied by the battery while the differential stage M8 to M12 is supplied by the new regulated voltage.  The voltage at the interface between the first and second stage (node VD) is referenced to the ground and relatively independent of the supply voltage. In these conditions it has been possible to change the supply of the differential stage without changing anything in the second stage as well as in the interface between both stages.

              One advantage of the invention is that the drain-source voltage of the input transistors M11/M12 don’t vary with the main supply voltag...