PWM AMPLIFIER

Abstract: A PWM amplifier comprising a comparator and an integrator is known to amplify, for example, audio signals. Known PWM amplifiers have as one of the drawbacks that the maximally achievable loop gain is low. To overcome the disadvantages of the prior art PWM amplifiers, the invention provides an PWM amplifier (A2) with a second integrator (OPAMP23, 22) in the feedback loop.

Description: PWM amplifier. Description The invention relates to a PWM (pulse width modulation) amplifier. A PWM amplifier, also referred to as class-D amplifier, may be used, for example, for amplifying audio signals. A PWM amplifier of this type is known from United States patent US-A- 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. These and other aspects of the invention are 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. Fig. la shows a known circuit diagram for a class-D amplifier A. The first operational amplifier OPAMPI is arranged as an integrator with capacitor C and an adder with resistors RI and R2, and the second operational amplifier OPAMP2 is arranged as a comparator. A triangular voltage is generated at the output of the integrator OPAMP, by means of the square-wave generator SWG at the input via a resistor RI . The input signal Si is superimposed on this triangular voltage via a resistor R2, and this composite signal is compared with 0. A PWM signal then appears at the output of a comparator OPAMP2. 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 is coupled to the input with which the gain of the assembly can be fixed. The output is coupled via a filter comprising inductances LI and L2 and capacitances C01 and C02 to a loudspeaker L5. Fig. lb also shows a known self-oscillating version of a class-D amplifier A'. In this set-up, the comparator OPAMP2' and resistors R3 and R4 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. la, 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 A2 according to the invention, in which the standard configuration of Fig. la is shown, but with an extension of the loop filter by an extra integrator in the form of an operational amplifier OPAMP23 and capacitance C22. 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. This can be achieved by adding the outputs of operational amplifier OPAMP 1 and integrator 2 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 SWG2 is coupled to the last integrator OPAMP22 so that the noise at the input I of the circuit A2, originating from the generator SWG2, 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. lb 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.

Claims: CLAIMS: 1. A PWM amplifier comprising a comparator and an integrator, which PWM amplifier comprises a feedback loop, characterized in that the PWM amplifier comprises a second integrator which is incorporated in the feedback loop. 2. A PWM amplifier as claimed in claim 1, characterized in that the output of the second integrator is coupled to an input of the comparator, and the output of the first integrator is coupled to the other input of the comparator. 3. An audio signal receiver including a PWM amplifier as claimed in claim 1. 4. A multimedia apparatus including a PWM amplifier as claimed in claim 1. 5. An information reading apparatus including a PWM amplifier as claimed in claim 1.

Inventors: DIJKMANS EISE C, VAN DEN HOMBERG JOHANNES A T M, DIJKMANS, EISE, C., VAN DEN HOMBERG, JOHANNES, A., T., M.
Applicants: KONINKL PHILIPS ELECTRONICS NV, PHILIPS SVENSKA AB, KONINKLIJKE PHILIPS ELECTRONICS N.V., PHILIPS AB
Application year: 1999
Main classification: H03F3
IPC codes: H03F1, H03F3