Amplifier circuit and methods of operation thereof

Abstract

A signal amplifying circuit and associated methods and apparatuses, the circuit comprising: a signal path extending from an input terminal to an output terminal, a gain controller arranged to control the gain applied along the signal path in response to a control signal; an output stage within the signal path for generating the output signal, the output stage having a gain that is substantially independent of its supply voltage, and a variable voltage power supply comprising a charge pump for providing positive and negative output voltages, the charge pump comprising a network of switches that is operable in a number of different states and a controller for operating the switches in a sequence of the states so as to generate positive and negative output voltages together spanning a voltage approximately equal to the input voltage.

1 - 40 . (canceled) 
   
   
       41 . An audio amplifier circuit for providing an output signal to an audio transducer, said audio amplifier circuit comprising:
 a power amplifier having an audio input for receiving an audio input signal, an audio output for providing said output signal, and a pair of power supply rail connections substantially symmetric about a reference potential, forming a bi-polar power supply input; and   a charge pump power supply for providing a bi-polar power supply voltage across said pair of power supply rail connections of said power amplifier, wherein said charge pump power supply has a select input for selecting an operating mode of said power supply, wherein in a first operating mode, said bi-polar power supply voltage is equal to a first voltage, and wherein in a second operating mode said bi-polar power supply voltage is substantially equal to a rational fraction of said first voltage−, wherein in both of the first operating mode and the second operating mode, the bipolar power supply voltage is substantially symmetric about the reference potential.   
   
   
       42 . The audio amplifier circuit of  claim 41 , wherein in said second operating mode said power supply voltage is substantially equal to half of said first voltage. 
   
   
       43 . The audio amplifier circuit of  claim 41 , wherein said charge pump comprises a plurality of capacitors and a switching circuit for switching said capacitors alternatively from a series to a parallel combination, and wherein in response to said select input, a pattern of switching of said switching circuit is altered, whereby a rational relationship between said power supply voltage in said first operating mode and said power supply voltage in second operating mode is provided. 
   
   
       44 . The audio amplifier circuit of  claim 41 , wherein said charge pump power supply includes a switched-capacitor voltage divider that uses a series-coupling of capacitors in a first switching phase of said second operating mode to produce said rational fraction of said first voltage and couples at least one of said capacitors to at least one of said power supply rail connections in a second switching phase of said second operating mode to apply said rational fraction of said first voltage to said at least one power supply rail connection. 
   
   
       45 . The audio amplifier circuit if  claim 41 , further comprising a first amplifier stage circuit having an output coupled to said audio input of said power amplifier, wherein said first amplifier stage has a pair of first stage power supply rail connections connected to a fixed voltage power supply, wherein a fixed voltage of said fixed voltage power supply does not change substantially with selection of said operating mode of said power supply. 
   
   
       46 . The audio amplifier circuit of  claim 41 , further comprising a control circuit for selecting said operating mode in conformity with a volume setting of said audio amplifier circuit. 
   
   
       47 . The audio amplifier circuit of  claim 46 , wherein said audio amplifier circuit further comprises a potentiometer coupled to said audio input for controlling a magnitude of said output signal, and wherein said control circuit selects said magnitude in conformity with a control level of said potentiometer. 
   
   
       48 . The audio amplifier circuit of  claim 47 , wherein said potentiometer is a digitally controlled potentiometer, and wherein said control level is determined from a digital control value supplied to said potentiometer. 
   
   
       49 . The audio amplifier circuit of  claim 41 , further comprising a control circuit for providing a selection signal to said select input in conformity with an amplitude of said audio input, whereby clipping of said power amplifier is prevented when said second operating mode of said charge pump power supply is selected by instead selecting said first operating mode. 
   
   
       50 . The audio amplifier circuit of  claim 49 , wherein said control circuit generates an indication of an amplitude of said audio input, and wherein said control circuit provides said selection signal to said select input in conformity with said indication of said amplitude of said audio input, whereby said first operating mode is selected when said indication of amplitude indicates that a higher power level is required from said audio output and said second operating mode is selected when said indication of amplitude indicates that a higher power level is not required from said audio output. 
   
   
       51 . The audio amplifier circuit of  claim 41 , wherein said charge pump power supply comprises:
 a first flyback capacitor; and   a first switching circuit for alternatively coupling said first flyback capacitor in said first operating mode between a positive one of said power supply input terminals and a positive one of said pair of power supply rail connections in first phase and between said pair of power supply input terminals in a second phase, and wherein when said charge pump power supply is in said second operating mode, said first switching circuit couples said positive one of said power supply input terminals to said positive one of said pair of power supply rail connections.   
   
   
       52 . A method of improving efficiency of an amplifier for providing a power output to an audio transducer, said method comprising:
 receiving an indication of a magnitude of a signal to be amplified by said amplifier;   selecting an operating mode of a charge pump power supply that provides a bi-polar power supply output symmetric about a reference potential; and   supplying a bi-polar power supply voltage to said amplifier from said charge pump in conformity with said selected operating mode, wherein in a first operating mode of said charge pump power supply, said power supply voltage is equal to a first voltage and in a second operating mode of said charge pump power supply, said power supply voltage is substantially equal to a rational fraction of said first voltage, wherein in both of the first operating mode and the second operating mode, the bipolar power supply voltage is substantially symmetric about the reference potential.   
   
   
       53 . The method of  claim 52 , wherein said receiving receives a volume control level corresponding to a gain of a signal path of said signal and wherein said selecting is performed in conformity with a magnitude of said volume control level.