SE538866C2

SE538866C2 - Power amplifier

Info

Publication number: SE538866C2
Application number: SE1550677A
Authority: SE
Inventors: Boström Patrik
Original assignee: Bolecano Holding Ab
Priority date: 2015-05-26
Filing date: 2015-05-26
Publication date: 2017-01-10
Also published as: SE1550677A1; US10090813B2; US20160352293A1

Description

25 30 538 866 long as the supply voltages are constant and the state shift is infinitely fast plus of course, the triangle wave has to be ideal.

The reference signal technique does not have any feedback resulting in an absence of control over the demodulation ﬁlter. There are actually two signal paths, one from the input to the output and one from the supply rails to the output. The channel separation and power supply rejection become very poor. An advantage is that the switching frequency is constant. Supply feed forward can be used to lower the supply voltage dependence.

Using a digital device as a digital signal processor (DSP) for calculating the pulse width instead of comparing the set value to a tri wave signal basically gives the same result. However there are more possibilities to compensate for non-ideal features of components. A DSP solution also is more complicated and expensive.

A way of providing the pulse width modulation is by self-oscillation. A comparator is used to compare the average of the pulse train to the set value giving an error as a result. If the error is negative the positive switch is turned on and vice versa if it is positive. The frequency of operation is close to the frequency where the loop has minus l80° phase shift. Some solutions use the demodulation filter to do the averaging and this gives the amplifier the possibility to adjust errors that occur in and aﬂer the demodulation ﬁlter.

Without additional circuitry the switching frequency would be equal to the filter resonance frequency which usually is around 50kHz and this normally is not acceptable. A zero and zero cancelling resistor is added in the sensing circuit to push the frequency up about a decade. The open loop gain of this topology can be approximated to the ratio between the supply rail voltage and the modulation signal amplitude, usually around 200 times. This open loop gain is not high enough to provide an ampliﬁer With low non-linear distortion but since this loop gain works around the output filter it provides low linear distortion, i.e. gain and phase errors, compared to other topologies where a maximum of about 10dB loop gain can be added around the output ﬁlter, at 20kHz, without causing instability. The result of this post-filter self-oscillating topology is therefore that the linear distortion, i.e. distortion which does not add new frequency content like amplitude and phase, is low for being a class D ampliﬁer but the non-linear distortion is still relatively high compared to a high quality amplifier. The small signal frequency response of this ampliﬁer is close to a desirable first order since it is basically determined by the zero in the feedback path and hence the phase in the audible band 10 15 20 25 30 35 538 866 shiﬂs linearly as the frequency is increased. For example, US 4,041,41l discloses a globally self-oscillating class D ampliﬁer.

WO03/090343 discloses a power ampliﬁer for amplifying an electric signal in an operational frequency range comprising switching means for generating a square wave signal by alternately switching the square wave signal to a ﬁrst supply voltage or a second supply voltage, ﬁlter means for generating a power output signal by low pass ﬁltering the square wave signal, input means for receiving the electric signal and driving the switching means, and a control circuit coupled to the output power signal and the input means for controlling the power ampliﬁer.

A further option is to add a second order active integrator which demodulates the pulse train and gives significant loop gain at low frequencies. The switching frequency is then controlled by this integrator and not by the Q value of the output filter.

The operation of this pre-filter self-oscillating topology is therefore robust and simple and it gives low-non linear distortion, especially at low frequencies.

However, in a pre-ﬁlter self-oscillating topology the linear distortion becomes high, i.e. the frequency response and phase is totally load dependant and a desired response curve can only be obtained with specific load impedance. The switching frequency of all self-oscillating topologies varies with the modulation depth, i.e. pulse width. This is an advantage for the efficiency as the switching losses becomes lower as the conduction losses increase, but if another loop is added to increase the loop gain there is a risk that this loop picks up the switching frequency and then the ampliﬁer would go into a destructive sub-harmonic oscillation condition. It is therefore a desire to provide an alternative power ampliﬁer.

Summary According to a first aspect, a power ampliﬁer is provided comprising an input ampliﬁer, a comparator, a switching circuit, an output ﬁlter and a feedback filter. The input ampliﬁer is arranged to receive an input signal of the power ampliﬁer at a positive input of the input ampliﬁer and an output signal of the feedback ﬁlter at a negative input of the input ampliﬁer, and the input amplifier is further arranged to have a capacitive feedback from an output of the input ampliﬁer to the negative input of the input ampliﬁer. The comparator is arranged to receive the output signal of the input ampliﬁer at a positive input of the comparator and the output signal of the feedback filter at a negative input of the comparator wherein the comparator provides a control signal on an output of the comparator based on a comparison between signal levels of the signals at 10 15 20 25 30 538 866 the inputs of the cornparator. The switching Circuit comprises power switches and is arranged to receive the control signal and control the power switches that connect an output of the switching circuit to either a positive supply voltage or a negative supply voltage based on the control signal. The output ﬁlter is arranged to receive an output signal provided at the output of the switching circuit and provide a ﬁltered signal thereof to an output of the power ampliﬁer. The feedback filter is arranged to receive the output of the power ampliﬁer and provide a ﬁltered feedback signal to the negative input of the comparator and towards the negative input of the input ampliﬁer.

The power ampliﬁer may comprise a clamp circuit arranged to limit the signal at the negative input of the input ampliﬁer at high output signals from the input ampliﬁer, wherein the clamp circuit is connected between the output of the input ampliﬁer and the negative input of the input ampliﬁer. The clamp circuit may comprise an inverting ampliﬁer having its input connected to the output of the input ampliﬁer and its output connected to the negative input of the input ampliﬁer. The inverting ainpliﬁer may comprise a first ampliﬁer stage having its input connected to the output of the input ampliﬁer and its output connected to a second ampliﬁer stage of the inverting ampliﬁer via a voltage divider, and the second ampliﬁer stage has its output connected to the negative input of the input ampliﬁer, wherein the voltage divider is arranged to set a limit for the limiting of the signal at the negative input of the input ampliﬁer.

Alternatively, the clamp circuit may comprise a first zener diode and a second zener diode connected in series with opposite polarities between the output of the input ampliﬁer and the negative input of the input ampliﬁer such that the input ampliﬁer is arranged to operate as a follower when voltage between the output of the input ampliﬁer and the negative input of the input ampliﬁer exceeds a sum of a zener voltage and a forward voltage of the respective zener diodes.

The capacitive feedback from the output of the input ampliﬁer to the negative input of the input ampliﬁer may comprise a capacitor element connected between the output of the input ampliﬁer and the negative input of the input ampliﬁer. The capacitor element may comprise a first capacitor and a second capacitor connected in series between the output of the input ampliﬁer and the negative input of the input ampliﬁer, and wherein a buffer circuit is arranged to provide a replica of the input signal of the power ampliﬁer via a resistor to a junction between the ﬁrst and the second capacitors.

Alternatively, the capacitor element may comprise a first capacitor and a second capacitor connected in series between the output of the input ampliﬁer and the negative l0 15 20 25 30 35 538 866 input of the input ampliﬁer, and wherein a resistor is connected between the positive input of the input ampliﬁer and a junction between the ﬁrst and the second capacitors.

The filtered feedback signal may be provided via a ﬁrst feedback resistor and a second feedback resistor, connected in series, to the negative input of the input ampliﬁer, and a capacitor coupling may be provided from a junction between the ﬁrst feedback resistor and the second feedback resistor to the positive input of the input amplifier.

The power ampliﬁer may be arranged to operate at audio frequencies and provide its output to a loudspeaker.

According to a second aspect, an audio ampliﬁer arrangement is provided comprising a power ampliﬁer according to the first aspect, and a power supply arranged to provide electric power to the power ampliﬁer.

According to a third aspect, an audio amplifier arrangement is arranged to provide multiple amplified audio Channels. The audio ampliﬁer arrangement comprises a power ampliﬁer according to the ﬁrst aspect for each audio channel and a power supply arrangement arranged to provide electric power to the power amplifiers.

The power supply arrangement may comprise a power supply unit for each power ampliﬁer.

The power supply arrangement may comprise a power supply unit arranged to provide electric power to a plurality of the power ampliﬁers.

Other objectives, features and advantages of the present invention will appear from the following detailed disclosure, from the attached dependent claims as well as from the drawings. Generally, all terms used in the claims are to be interpreted according to their ordinary meaning in the technical ﬁeld, unless explicitly defined otherwise herein. All references to "a/an/the [element, device, component, means, step, etc]" are to be interpreted openly as referring to at least one instance of said element, device, component, means, step, etc., unless explicitly stated otherwise. The steps of any method disclosed herein do not have to be performed in the exact order disclosed, unless explicitly stated.

Brief description of the drawings The above, as well as additional objects, features and advantages of the present invention, will be better understood through the following illustrative and non-limiting detailed description of preferred embodiments of the present invention, with reference to the appended drawings. 10 15 20 25 30 538 866 Fig. 1 schematically illustrates a power ampliﬁer according to an embodiment.

Fig. 2 schematically illustrates a power ampliﬁer according to an embodiment.

Fig. 3 schematically illustrates a power ampliﬁer according to an embodiment.

Fig. 4 schematically illustrates an inverting ampliﬁer suitable for a clamp circuit according to an embodiment.

Fig. 5 schematically illustrates a power ampliﬁer according to an embodiment.

Fig. 6 illustrates a filtering approach for an input ampliﬁer according to an embodiment.

Fig. 7 illustrates a ﬁltering approach for an input ampliﬁer according to an embodiment.

Fig. 8 illustrates a filtering approach for an input ampliﬁer according to an embodiment.

Fig. 9 schematically illustrates a power ampliﬁer operating as an audio power ampliﬁer.

Fig. 10 illustrates a conventional approach for feedback for a class D ampliﬁer.

Fig. 11 schematically illustrates a feedback filter according to an embodiment.

Figs 12 to 17 illustrate impedance structures according to embodiments with relation to the feedback ﬁlter.

Detailed description The disclosure below relates to a power ampliﬁer for amplifying an electric input signal in an operational frequency range, e. g. audio frequency range, and providing an output signal. The principle of the power ampliﬁer comprises switching means for generating a square wave signal by alternately switching the square wave signal to a first supply voltage or a second supply voltage, and ﬁlter means for generating a power output signal by low pass ﬁltering the square wave signal. The power ampliﬁer ﬁirther comprises input means for receiving the electric signal and driving the switching means, and a local feedback circuit connecting the output signal towards an input of the switching means, as will be further elucidated below.

Fig. 1 schematically illustrates a power ampliﬁer 100 according to an embodiment. The power ampliﬁer 100 comprises an input ampliﬁer 102, a comparator 104, a switching circuit 106, an output filter 108 and a feedback filter 110. The respective elements 102-110 may be conventional for a class D ampliﬁer except for the particular features set forth below, and for the sake of brevity, the conventional 10 15 20 25 30 35 538 866 elements and their features are not described here not to obscure the inventive contribution.

The input ampliﬁer 102 receives the input signal of the power ampliﬁer 100 at a positive input of the input ampliﬁer 102. On a negative input of the input ampliﬁer 102 a feedback signal from the feedback filter 110 is received through resistor 114, which provides for avoiding that the input ampliﬁer 110 loads the feedback to the class D ampliﬁer, which in turn provides for self-oscillating properties of the class D ampliﬁer. The input ampliﬁer further has a capacitive feedback 112 connected between an output of the input ampliﬁer 102 and the negative input of the input ampliﬁer 102.

Thus, the input ampliﬁer 102 will Work as an integrating regulator for the power ampliﬁer 100 with the input signal as a reference signal and the feedback signal as a present value signal. The output of the input ampliﬁer 102, which may be considered as a control signal to make the output of the power ampliﬁer 100 correspond to the input signal, is provided to a positive input of the comparator 104. A negative input of the comparator 104 receives an output of the feedback ﬁlter 110. An output of the comparator is provided to the switching circuit 106 to control power switches thereof to connect either a positive supply voltage to an output of the switching circuit 106 or a negative supply voltage to the output of the switching circuit 106. The feedback to the comparator 104 will provide for self-oscillating properties of the class D ampliﬁer. The output filter 108 receives the output signal from the switching circuit 106 and provides a filtered output which is the output signal of the power ampliﬁer 100. The ﬁltering filters such that an amplified version of the input signal of the power ampliﬁer 100 is provided at the output while the self-oscillating frequency of the self-oscillating class D ampliﬁer is attenuated. The output signal of the power ampliﬁer 100 is also provided to the feedback ﬁlter 110 which provides loop ﬁltering for the feedback loop back to the comparator 104 and input ampliﬁer 102. A resistor 114 may be provided between the output of the feedback ﬁlter 110 and the negative input of the input ampliﬁer 102.

Fig. 10 illustrates a conventional approach for a class D ampliﬁer 1000 with an inner feedback loop and an outer feedback loop. The ampliﬁer 1000 comprises an integrating ampliﬁer 1010, a switching means 1012, a low pass ﬁlter comprising an inductor 1014 and a capacitor 1016, an inner feedback ﬁlter 1018, and an outer feedback ﬁlter 1020. The inner loop connects the output of the ampliﬁer 1000 via the inner feedback ﬁlter 1018 to an inverting input of the switching means 1012. The outer loop connects the output of the ampliﬁer 1000 via the outer feedback ﬁlter 1020 to an inverting input of the integrating ampliﬁer 1010. The input of the ampliﬁer 1000 is 10 15 20 25 30 538 866 connected to a positive input of the integrating ampliﬁer 1010. The output of the integrating ampliﬁer 1010 is connected to a positive input of the switching means 1012.

While the conventional ampliﬁer as illustrated in Fig. 10 ernploys an approach where the outer feedback ﬁlter 1020 affects the transfer function of the amplifier 1000, the approach demonstrated with reference to embodiments above employs only a loop for the switching circuit 106 with feedback ﬁlter 110 and conditions on that loop is observed, i.e. measured, via the resistor 114 such that the loop for the switching circuit 106 is not affected by the measurement. The observed condition on the loop comprises the ﬁltered output signal which comprises a ﬁrst component being an amplified replica of the input signal and a second component being a residual of the self-oscillating switching. Since the second component is above a desired frequency band of the ampliﬁer 100 and is filtered out by the integrating input ampliﬁer 102, the integrating input ampliﬁer will provide an output, i.e. the control signal discussed above, to the comparator such that the replica of the input signal becomes equal to the input signal.

The above demonstrated approach provides for low distortion and the amplifiefs frequency response is not prone to varying properties of the input ampliﬁer 102, e. g. affected by Variations in properties of a signal source connected to the input of the power ampliﬁer 100, since the frequency properties of the input ampliﬁer 102 does not affect the transfer function of the ampliﬁer; the frequency properties of the input ampliﬁer 102 may only affect the bandwidth of the control signal.

Fig. 2 schematically illustrates a power ampliﬁer 100 according to an embodiment. Here, the same reference numerals will be used as for the embodiment of Fig. 1 where no significant differences in corresponding elements are present and affect the gist of the invention. As the embodiment presented with reference to Fig. 1, the embodiment of the power ampliﬁer 100 of Fi g. 2 comprises an input ampliﬁer 102, a comparator 104, a switching circuit 106, an output ﬁlter 108, a feedback filter 110, a capacitive feedback 112 and a resistor 114. The power amplifier 100 of F ig. 2 further comprises a clamp circuit 116 arranged to limit the signal at a negative input of the input ampliﬁer 102 when the output from the input ampliﬁer 102 is high, e. g. at start-up when there is a large difference between the input signal, i.e. the reference signal when considering the ampliﬁer as a controller, and the output signal of the power ampliﬁer 100. This limiting feature will improve stability under such conditions. The clamp circuit 116 may be implemented in various ways, which will be demonstrated with reference to F igs 3 to 5. 10 15 20 25 30 538 866 Fig. 3 schematically illustrates a power ampliﬁer 100 according to an emhodiment. Here, the same reference numerals will be used as for the emhodiment of Fig. 1 where no significant differences in corresponding elements are present and affect the gist of the invention. As the emhodiment presented With reference to Fig. 2, the emhodiment of the power ampliﬁer 100 of F ig. 3 comprises an input ampliﬁer 102, a comparator 104, a switching circuit 106, an output ﬁlter 108, a feedback ﬁlter 110, a capacitive feedback 112, a resistor 114 and a clarnp circuit 116. Here, the clamp circuit 116 comprises an inverting ampliﬁer 118 having its input connected to the output of the input ampliﬁer 102 and its output connected to the negative input of the input ampliﬁer 102. When the output of the input ampliﬁer 102 becomes very high, i.e. the difference between the feedback signal and the input signal present at respective inputs of the input ampliﬁer 102 is high, the inverting ampliﬁer 118 will limit this by outputtíng a counteracting signal, i.e. a part of the difference Will instead be present across the resistor 114 and not be ampliﬁed by the input ampliﬁer 102. The properties of the inverting ampliﬁer 118 may be set to achieve the desired stability. An example of an implementation of the inverting ampliﬁer 118 will be demonstrated With reference to Fig. 4. Other implementations providing the similar properties are of course equally feasible.

Fig. 4 schematically illustrates an inverting ampliﬁer 118 suitable for the clamp circuit 116 demonstrated with reference to F ig. 3. Since the clamp circuit 116 is to be operating on both half-cycles of a signal, there is provided circuitry to take care of respective half-cycle each connected to a positive supply voltage and a negative supply voltage, respectively. An input of the inverting ampliﬁer 118, Which is to be connected to the output of the input ampliﬁer 102 of Fi g. 3, is connected to an input of a ﬁrst ampliﬁer stage comprising a transistor 120, 122 (for respective half-cycle). The ﬁrst ampliﬁer stage provides its output to a second ampliﬁer stage comprising a transistor 124, 126 (for respective half-cycle) via a voltage divider 128, 130. The voltage divider 128, 130 may comprise a first resistor 132, 134 and a second resistor 136, 138 wherein the first resistor 132, 134 forms a feedback to the input signal of the inverting ampliﬁer 118 and the second resistor 136, 138 is connected to the input of the second ampliﬁer stage. By conﬁguring the features of the voltage divider 128, 130, the limiting properties of the inverting ampliﬁer 118 as desired can be achieved. The output of the second stage is provided as output of the inverting ampliﬁer 118 and is thus connected to the negative input of the input ampliﬁer 102 of F ig. 3. 10 15 20 25 30 35 538 866 Fig. 5 schematically illustrates a power ampliﬁer 100 according to an embodiment. Here, the same reference numerals will be used as for the embodiment of Fig. 1 where no significant differences in corresponding elements are present and affect the gist of the invention. As the embodiment presented with reference to Fig. 2, the embodiment of the power ampliﬁer 100 of Fig. 3 comprises an input ampliﬁer 102, a comparator 104, a switching circuit 106, an output filter 108, a feedback ﬁlter 110, a capacitive feedback 112, a resistor 114 and a clarnp circuit 116. Here, the clamp circuit 116 comprises a first zener diode 140 and a second zener diode 142 connected in series.

The zener diodes 140, 142 are connected with opposite polarities such that a voltage across the series connection will not exceed the zener voltage plus a diode forward voltage for either polarity applied across the series connection. The series connection is connected between the output of the input amplifier 102 and the negative input of the input amplifier 102. When the above discussed limited voltage is reached between the output of the input amplifier 102 and the negative input of the input amplifier 102, the input ampliﬁer will have properties of a follower, and stability is enhanced.

The ﬁltering properties of the input amplifier 102 are of low-pass character due to the capacitive feedback 112. By only applying a single capacitor element, e. g. an integrated capacitance or a discrete component capacitor, as schematically illustrated in Figs 1 to 3 and 5 provides a first order ﬁltering. However, other ﬁltering approaches are also feasible. Figs 6 to 8 illustrate examples of ﬁltering approaches where a second order ﬁltering component is provided. Here, the capacitor element comprises a first capacitor 144 connected in series with a second capacitor 146 for providing the capacitive feedback 112. In the embodiment illustrated in Fig. 6, a first resistor 148 is connected between an input sigiial of the input ampliﬁer 102 and a junction between the ﬁrst capacitor 144 and the second capacitor 146. A second resistor 150 is connected in parallel with the series connection of capacitors, i.e. between the output of the input amplifier 102 and the negative input of the input amplifier 102. Furthermore, the signal from the feedback ﬁlter 110, as illustrated in Fi g. 5, is here provided to a ﬁrst feedback resistor 113 in series with a second feedback resistor 114 towards the negative input of the input amplifier 102. From a junction between the first feedback resistor 113 and the second feedback resistor 114, where is a capacitor 115 connected towards the positive input of the input amplifier 102. This arrangement is arranged to provide for attenuation of frequencies outside the desired frequency band to be ainpliﬁed by the power ampliﬁer 100, i.e. at high frequencies emanating e. g. from residuals of switching operations. This enables improved gain within the desired frequency band to be 10 10 15 20 25 30 35 538 866 ampliﬁed by the power ampliﬁer 100 compared with gain outside the desired frequency band. It is to be noted that this arrangement is optional, and an embodiment without this option will be demonstrated with reference to Fig. 7.

In the embodiment illustrated in Fig. 7, a ﬁrst resistor 152 is connected between an input signal of the input ampliﬁer 102 and the junction between the ﬁrst capacitor 144 and the second capacitor 146. A second resistor 150 is connected in parallel with the series connection of capacitors, i.e. between the output of the input ampliﬁer 102 and the negative input of the input ampliﬁer 102. In the embodiment illustrated in F ig. 8, which may be considered as a variant of the embodiment of Fi g. 7, the first resistor 152 is connected between an output of a buffer circuit 154 and the junction between the first capacitor 144 and the second capacitor 146. The second resistor 150 is also here connected in parallel with the series connection of capacitors, i.e. between the output of the input ampliﬁer 102 and the negative input of the input ampliﬁer 102. The buffer circuit 154 comprises an ampliﬁer configured as a follower with its positive input connected to the input of the input ampliﬁer 102, i.e. the input of the power ampliﬁer 100, and its negative input connected to the output of the ampliﬁer.

Compared with the embodiment demonstrated with reference to Fig. 7, the embodiment of Fig. 8 will cause less load on a signal source connected to the input of the power ampliﬁer, which is particularly advantageous when the output impedance of the signal source is not very low. The arrangement demonstrated with reference to Fig. 5 for improved gain within the desired frequency band to be ampliﬁed by the power ampliﬁer 100 compared with gain outside the desired frequency band may also be used for the embodiment demonstrated with reference to Fig. 8.

Any of the embodiments demonstrated with reference to Figs 6 to 8 may be applied to any of the embodiments demonstrated with reference to Figs 1 to 5 wherein the capacitive feedback 112 as illustrated in any of F igs 1 to 3 and 5 is substituted by the elements as demonstrated with any of Figs 6 to 8.

In the embodiments illustrated with reference to Figs 6 to 8, the application of the first and second capacitors and the application of the ﬁrst and second resistors provides for improved filtering characteristics of the integrating input ampliﬁer with a ﬂat gain in the desired band, a slight increase of gain close to a cut-off frequency and a significant decrease of gain above the cut-off frequency.

Fig. 9 schematically illustrates a power ampliﬁer 100 according to any of the above demonstrated embodiments and their variants Operating as an audio power ampliﬁer and loaded by a loudspeaker 200. The input of the power ampliﬁer is provided 11 10 15 20 25 30 538 866 with an audio signal, e. g. having signals within the frequency range 20 Hz to 20 kHz, which is amplified as demonstrated above and provided to the loudspeaker 200. The power amplifier 100 is powered by a power supply 300. The power amplifier 100 and the power supply 300 may be implemented as a module.

An arrangement for providing a plurality of arnplified audio channels may be provided, wherein each channel comprises a power amplifier 100. A common power supply 300 may be provided for more than one power amplifier 100, or each power ampliﬁer 100 may be powered by a separate power supply 300.

F i g. 11 illustrates a feedback ﬁlter 1110 according to an embodiment. The feedback ﬁlter 1110 is suitable for implementing the feedback ﬁlter 110 demonstrated with reference to Fig. 5. The feedback ﬁlter 1110 comprises a first impedance 1120 connected between an input of the feedback filter 1110 and an output of the feedback ﬁlter 1110, and a second impedance 1130 connected between the output of the feedback filter 1110 and ground or similar signal reference point. Considering the discussion above regarding the feedback loop and the observing of conditions on the feedback, the skilled reader here realizes that, when implementing the feedback ﬁlter 110 by the feedback ﬁlter 1110, the input to the negative input of the input amplifier 102 in some sense corresponds to the signal appearing across the second impedance 1130, and the resistor 114 provides for appropriate measuring without affecting the feedback loop.

With reference to Figs 12 to 16, different suggestions on feasible impedances and impedance structures for the impedances 1120, 1130 of the feedback filter 1110 are demonstrated.

Fig. 12 illustrates an impedance structure 1200 suitable for implementing the first impedance 1120 according to an embodiment. The impedance structure 1200 comprises a first resistor 1202 connected in parallel with a series connection of a capacitor 1204 and a second resistor 1206.

Fig. 13 illustrates an impedance structure 1300 suitable for implementing the first impedance 1120 according to an embodiment. The impedance structure 1300 comprises a first resistor 1302 connected in parallel with a capacitor 1304 and the parallel coupling is connected in series with a second resistor 1306.

F ig. 14i11ustrates an impedance structure 1400 suitable for implementing the second impedance 1130 according to an embodiment. The impedance structure 1400 comprises a first resistor 1402 connected in parallel with a series connection of an inductance 1404 and a second resistor 1406. 12 10 15 20 25 30 35 538 866 Fig. 15 illustrates an impedance structure 1500 suitable for implementing the second impedance 1130 according to an embodiment. The impedance structure 1500 comprises a ﬁrst resistor 1502 connected in parallel with an inductance 1504 and the parallel coupling is connected in series with a second resistor 1506.

Fig. 16 illustrates an impedance structure 1600 suitable for implementing the second impedance 1130 according to an embodiment. The impedance structure 1600 comprises a ﬁrst resistor 1602 connected in series With a capacitor 1604.

The second impedance 1130 may in its most basic embodiment comprise only a resistor 1700, as illustrated in F ig. 17.

Any of the embodiment for the first impedance 1120 may be combined with any of the embodiments for the second impedance 1130 for achieving a feedback filter 110 with desired properties.

With the above demonstrated embodiments and their variants in mind, a general discussion will here be provided, which may apply to one or more of the above demonstrated embodiments, or only provides information for the better understanding of the presented embodiments.

The need to increase the loop gain in order to lower the non-linear distortion of self-oscillating class D ampliﬁers is desired to be achieved without affecting the gain and phase response of the system. This is done by using an error ampliﬁer which has as much gain as possible in e.g. the audio band, i.e. 20 Hz to 20 kHz, but as much attenuation as possible of the switching residual. The correction signal appearing on the output of this error ampliﬁer ideally only contains compensation signals for the audio band content and no remains of the switching frequency. The phase shift of the error ampliﬁer is large at high frequencies and hence the amplitude of the switching frequency residual in the compensation signal is added out of phase on the positive input thereby lowering the effect of the added loop gain due to a lower ratio between supply rails and the modulation signal.

By providing a non-inverting globally self-oscillating class D ampliﬁer, lower non-linear distortion is provided without affecting the frequency response of the same.

This non-inverting globally modulated class D ampliﬁer comprises a comparator which has a positive input and a negative input. The output of this comparator controls two power switches that connect a demodulation ﬁlter to either the positive supply voltage or the negative supply voltage. This demodulation filter provides a power output which is connected to a load, e.g. a loudspeaker. The positive input of the comparator receives an input signal and the negative input receives a feedback signal from the power output. 13 10 15 20 25 30 538 866 The signal appearing on the negative input, minus the signal on the positive input, is proportional to the error on the output of the ampliﬁer.

The input signal to the positive input of the comparator is provided by an integrating, inverting error ampliﬁer which has its positive input connected to the audio input signal hence the audio input signal is the reference for this error ampliﬁer. The error ampliﬁer measures the error on the negative input of the comparator, compares it to the audio input signal on its positive input, amplifies the difference as much as possible in the audio band and attenuates it as much as possible outside of the audio band. What this error ampliﬁer will do is work to make sure that the same signal as it receives on its positive input will appear on the negative input of the comparator. If it succeeds with this entirely, the class D ampliﬁer will have no non-linear distortion in the audio band.

This type of ampliﬁer will have a low linear distortion compared to other class D ampliﬁers. A ﬁlrther advantage is that the frequency and gain response of the class D stage is not affected by the error ampliﬁer.

The frequency response shows large bandwidth and the output impedance is low. By adding an integrating error ampliﬁer the non-linear distortion is substantially reduced.

In order to make the ampliﬁer behave well during clipping and during start-up the integrating error ampliﬁer may be provided with a clamp, so as to limit the error correction signal. The clamp may be connected across an integrating capacitor included in the integrator. ln various embodiments the clamp comprises two zener diodes that will turn the integrator into a follower if the voltage across the integrating capacitor exceeds the zener voltage plus 0.7 V in either direction. Other ways are available to realize this clamp, as demonstrated above. For example, an inverting ampliﬁer may be provided with its input connected to the output of the error ampliﬁer and its output connected to the negative input of the error ampliﬁer. When the error signal becomes too high, the inverting ampliﬁer will limit the error signal to the negative input of the error ampliﬁer.

An embodiment of the power ampliﬁer compared with a conventional globally modulated class D stage with traditional negative feedback around it is likely to provide the following result: The ampliﬁer with the traditional negative feedback provides -3dB at 73 kHz and the absolute phase shift at 20 kHz is -27 degrees. The embodiment of the power ampliﬁer with the integrating error ampliﬁer shows a -3dB point at 120 kHz and 14 538 866 -18 degrees phase shift at 20 kHz. Thus, the frequency and phase response is almost unchanged as compared With the globally self-oscillating class D stage by itself.

The invention has mainly been described above with reference to a few embodiments. However, as is readily appreciated by a person skilled in the art, other embodiments than the ones disclosed above are equally possible within the scope of the invention, as deﬁned by the appended patent claims. 15

Claims

10 15 20 25 30 35 538 866 CLAIMS

1. A power ampliﬁer (100) comprising an input ampliﬁer (102); a comparator (104); a switching Circuit (106); an output filter (108); and a feedback filter (110), wherein the input ampliﬁer (102) is arranged to receive an input signal of the power ampliﬁer (100) at a positive input of the input ampliﬁer (102) and an output signal of the feedback filter (110) at a negative input of the input ampliﬁer (102), and the input ampliﬁer (102) is further arranged to have a capacitive feedback (112) from an output of the input ampliﬁer (102) to the negative input of the input ampliﬁer (102); the comparator (104) is arranged to receive the output signal of the input ampliﬁer (102) at a positive input of the comparator (104) and the output signal of the feedback filter (110) at a negative input of the comparator ( 104) wherein the comparator (104) provides a control signal on an output of the comparator (104) based on a comparison between signal levels of the signals at the inputs of the comparator (104); the switching circuit (106) comprises power switches and is arranged to receive the control signal and control the power switches that connect an output of the switching circuit to either a positive supply voltage or a negative supply voltage based on the control signal; the output ﬁlter (108) is arranged to receive an output signal provided at the output of the switching circuit (106) and provide a ﬁltered signal thereof to an output of the power ampliﬁer (100); and the feedback ﬁlter (110) is arranged to receive the output of the power ampliﬁer (100) and provide a filtered feedback signal to the negative input of the comparator (104) and towards the negative input of the input ampliﬁer (102).

2. The power ampliﬁer (100) of claim 1, comprising a clarnp circuit (116) arranged to limit the signal at the negative input of the input ampliﬁer (102) at high output signals from the input ampliﬁer (102), wherein the clamp circuit (116) is connected between the output of the input ampliﬁer (102) and the negative input of the input ampliﬁer (102). 16 10 15 20 25 30 35 538 866

3. The power ampliﬁer (100) of claim 2, wherein the clamp circuit (116) comprises an inverting ampliﬁer (118) having its input connected to the output of the input ampliﬁer (102) and its output connected to the negative input of the input ampliﬁer (102).

4. The power ampliﬁer (100) of claim 3, wherein the inverting ampliﬁer (118) comprises a ﬁrst ampliﬁer stage having its input connected to the output of the input ampliﬁer (102) and its output connected to a second ampliﬁer stage of the inverting ampliﬁer (102) via a voltage divider (128, 130), and the second ampliﬁer stage has its output connected to the negative input of the input ampliﬁer (102), wherein the voltage divider (128, 130) is arranged to set a limit for the limiting of the signal at the negative input of the input ampliﬁer (102).

5. The power ampliﬁer (100) of claim 2, wherein the clamp circuit (116) comprises a ﬁrst zener diode (140) and a second zener diode (142) connected in series With opposite polarities between the output of the input ampliﬁer (102) and the negative input of the input ampliﬁer (102) such that the input ampliﬁer (102) is arranged to operate as a follower when voltage between the output of the input ampliﬁer (102) and the negative input of the input ampliﬁer ( 102) exceeds a sum of a zener voltage and a forward voltage of the respective zener diodes (140, 142).

6. The power amp1if1er(100) of any one of claims 1 to 5, wherein the capacitive feedback (112) from the output of the input ampliﬁer (102) to the negative input of the input ampliﬁer (102) comprises a capacitor element connected between the output of the input ampliﬁer (102) and the negative input of the input ampliﬁer (102).

7. The power amp1iﬁer(100) of claim 6, wherein the capacitor element comprises a ﬁrst capacitor (144) and a second capacitor (146) connected in series between the output of the input ampliﬁer (102) and the negative input of the input ampliﬁer (102), and wherein a buffer circuit (154) is arranged to provide a replica of the input signal of the power ampliﬁer (100) via a resistor (152) to a junction between the ﬁrst and the second capacitors (144, 146).

8. The power amp1if1er(100) of claim 6, wherein the capacitor element comprises a ﬁrst capacitor (144) and a second capacitor (146) connected in series 17 10 15 20 25 30 538 866 between the output of the input ampliﬁer (102) and the negative input of the input ampliﬁer (102), and wherein a resistor (152) is connected between the positive input of the input ampliﬁer (102) and a junction between the first and the second capacitors (144, 146).

9. The power ampliﬁer of any one of claims 1 to 8, wherein the filtered feedback signal is provided via a first feedback resistor(113) and a second feedback resistor (114), connected in series, to the negative input of the input ampliﬁer (102), and a capacitor coupling (115) is provided from a junction between the ﬁrst feedback resistor (113) and the second feedback resistor (114) to the positive input of the input ampliﬁer (102).

10. The power ampliﬁer (100) of any one of claims 1 to 9, arranged to operate at audio frequencies and provide its output to a loudspeaker (200).

11. An audio ampliﬁer arrangement comprising a power ampliﬁer of any one of claims 1 to 10; and a power supply arranged to provide electric power to the power ampliﬁer.

12. An audio ampliﬁer arrangement arranged to provide multiple amplified audio channels, the audio ampliﬁer arrangement comprising A power ampliﬁer of any one of claims 1 to 10 for each audio channel; and A power supply arrangement arranged to provide electric power to the power ampliﬁers.

13. The audio ampliﬁer arrangement of claim 12, wherein the power supply arrangement comprises a power supply unit for each power ampliﬁer.

14. The audio ampliﬁer arrangement of claims 12, wherein the power supply arrangement comprises a power supply unit arranged to provide electric power to a plurality of the power ampliﬁers. 18