Abstract:
A PWM amplifier comprising a comparator and an integrator is known to amplify, for example, audio signals. One drawback of the known PWM amplifiers is the maximally achievable loop gain is low. To overcome the disadvantages of the prior art PWM amplifiers, a PWM amplifier is provided with a second integrator in the feedback loop.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to a PWM (pulse width modulation) amplifier. 
     A PWM amplifier, also referred to as a class-D amplifier, may be used, for example, for amplifying audio signals. 
     A PWM amplifier of this type is known from U.S. Pat. No. 4,504,793. 
     This patent describes such an amplifier which comprises an integrator and a comparator. 
     A drawback of this circuit is that, due to stability requirements, the maximally achievable loop gain is rather low, for example, 40 dB at 1 kHz input signal and 330 kHz switching frequency. Consequently, the distortion numbers and the PSRR (power supply reaction ratio) are not very satisfactory. Moreover, the oscillator is directly coupled to the input of the circuit and the noise in the audio band produced by this oscillator will be added to the input signal. To achieve a large dynamic range, an oscillator producing minimal noise must be used. This configuration is used in, for example, the Sony TA-N88 audio power amplifier. 
     A further drawback of this circuit is that the switching frequency depends on the input signal. 
     It is, inter alia, an object of the invention to provide a PWM amplifier which does not have the above-mentioned drawbacks. A first aspect of the invention provides a PWM amplifier as defined in claim  1 . 
     A PWM amplifier in which the distortion of the output stage as well as the noise contribution of the oscillator are reduced is realized by applying a higher-order integration in the feedback loop. 
     In accordance with a second aspect of the invention, a PWM amplifier has the characteristic features defined in claim  2 . 
     A third, fourth and fifth aspect of the invention provides a receiver, a multimedia apparatus and an information reading apparatus as defined in claims  3 ,  4  and  5 , respectively. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     These and other aspects of the invention will become apparent from and will be elucidated with reference to the embodiments described hereinafter. 
     In the drawing: 
     FIG. 1 shows two embodiments of a PWM amplifier in accordance with the prior art, and 
     FIG. 2 shows an embodiment of a PWM amplifier in accordance with the invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 a  shows a known circuit diagram for a class-D amplifier A. The first operational amplifier OPAMP 1  is arranged as an integrator with a capacitor C and an adder with resistors R 1  and R 2 , and the second operational amplifier OPAMP 2  is arranged as a comparator. A triangular voltage is generated at the output of the integrator OPAMP 1  by means of the square-wave generator SWG at the input via a resistor R 1 . The input signal Si is superimposed on this triangular voltage via a resistor R 2 , and this composite signal is compared with ground. A PWM signal then appears at the output of the comparator OPAMP 2 . This signal drives a set of power switches SW which must supply the power at the output O. From these switches, a feedback resistor R 1  is coupled to the input with which the gain of the assembly can be fixed. The output is coupled via a filter comprising inductances L 1  and L 2  and capacitances C 01  and C 02  to a loudspeaker LS. 
     FIG. 1 b  also shows a known self-oscillating version of a class-D amplifier A′. In this set-up, the comparator OPAMP 2 ′ and resistors R 3  and R 4  have a hysteresis and the square-wave generator at the input may be dispensed with. A large dynamic range can thereby be achieved in a simpler manner.(FM modulation occurs.) The FM swing is rather large, and to keep the switching frequency within a given range, the maximum modulation depth must be limited. 
     The maximally achievable loop gain is equal to that of the standard configuration of FIG. 1 a , so that, also in this case, the distortion numbers and the PSSR are not very satisfactory. Corresponding elements have corresponding reference numerals. 
     FIG. 2 shows an embodiment of a PWM amplifier A 2  according to the invention, in which the standard configuration of FIG. 1 a  is shown, but with an extension of the loop filter by an extra integrator in the form of an operational amplifier OPAMP 23  and capacitance C 22 . In this way, more loop gain can be realized in the low part of the frequency spectrum. For example, at 1 kHz input signal and 330 kHz switching frequency, a loop gain of 76 dB can be achieved. This results in satisfactory distortion numbers and a satisfactory PSRR. The feedback loop includes only a passive circuit element, in this case a resistor R f2 . 
     The advantages of the invention can be achieved by adding the outputs of the first and second integrators and suitably choosing the integration time constants. 
     The order of the filter may of course be further increased but the gain per order then decreases. When using filters having an order which is higher than the first-order, instability should be prevented from occurring in the case of overdrive. This is realized by providing the extra integrators with an amplitude limiter circuit. 
     The square-wave generator SWG 2  is coupled to the last integrator OPAMP 22  so that the noise at the input I of the circuit A 2 , originating from the generator SWG 2 , is compensated by the gain of the previous integrators. The requirements of noise imposed on the generator will thereby be much less stringent. 
     Even when the standard configuration of FIG. 1 b  is extended by extra integrators, this leads to an increased loop gain in the audio band, with the associated better THD (total harmonic distortion) and PSRR (power supply reaction ratio), and creates the possibility of reducing the FM swing. A higher-order loop filter for both standard configurations is therefore a sensible extension. 
     It will be evident to those skilled in the art that a PWM amplifier according to the invention can be adapted at all kinds of subordinate points without departing from the scope of the invention. 
     A PWM amplifier according to the invention may also be used in, for example, an audio signal receiver, a multimedia apparatus and/or an information reading apparatus.