AMPLIFIER CIRCUIT AND METHOD FOR AMPLIFYING AN AUDIO SIGNAL

Abstract

An amplifier circuit (1) for amplifying an audio signal, includes: an input stage (I) for receiving a first input signal representing the audio signal; a transformer (T) including a primary winding (L.sub.1) and a secondary winding (L.sub.2), defining a first terminal, a second terminal and a middle terminal; a differential amplifier (U), having a first input (U.sub.+), connected to the first terminal of the secondary winding (L.sub.2); a second input (U.sub.), connected to the middle terminal of the secondary winding (L.sub.2), and an output (U.sub.out), connected to the second terminal of the secondary winding (L.sub.2); an additional transformer (T) including an additional primary winding (L.sub.1) and secondary winding (L.sub.2), defining a first terminal, a second terminal and a middle terminal; an additional differential amplifier (U), having a first input (U.sub.+), connected to the first terminal of the additional secondary winding (L.sub.2), a second input (U.sub.+) connected to the middle terminal of the additional secondary winding (L.sub.2) and an output (U.sub.out) connected to the second terminal of the additional secondary winding (L.sub.2); an output circuit (O) for receiving as input the output signals from the differential amplifier (U) and from the additional differential amplifier (U) to generate a differential output signal (V.sub.out).

Claims

1. An amplifier circuit for amplifying an audio signal, comprising: an input stage having a first pin for receiving a first input signal representing the audio signal, and a second pin for receiving a second input signal representing the audio signal and 180 out of phase with respect to the first input signal; a transformer including a primary winding and a secondary winding, the secondary winding defining a first terminal, a second terminal and a middle terminal; a differential amplifier, having a first input, electrically connected to the first terminal of the secondary winding by a first connecting branch; a second input, electrically connected to the middle terminal of the secondary winding by a second connecting branch; and an output connected to the second terminal of the secondary winding, an additional transformer including an additional primary winding and an additional secondary winding, the additional secondary winding defining a first terminal, a second terminal and a middle terminal, wherein the primary winding and the additional primary winding are electrically connected in series between the first pin and the second pin; an additional differential amplifier, having a first input, electrically connected to the first terminal of the additional secondary winding by a first connecting branch; a second input, electrically connected to the middle terminal of the additional secondary winding by a second connecting branch; and an output connected to the second terminal of the additional secondary winding; an output circuit for receiving as input the output signals from the differential amplifier and from the additional differential amplifier to generate a differential output signal.

2. The amplifier circuit according to claim 1, wherein the primary winding of the transformer and the additional primary winding of the additional transformer are configured to generate a magnetic flux and an opposite, additional magnetic flux.

3. The amplifier circuit according to claim 1, wherein the transformer and the additional transformer are identical, placed side by side and oriented in the same way, so that an external magnetic flux generates the same current in the transformer and in the additional transformer.

4. The amplifier circuit according to claim 1, wherein, for each of said differential amplifier and additional differential amplifier, the output is connected to the second input by a first impedance.

5. The amplifier circuit according to claim 4, wherein, for each of said differential amplifier and additional differential amplifier: the second connecting branch (r.sub.2) includes a second impedance, so that the second input is connected to the middle terminal of the respective secondary winding by the second impedance; the output is connected to the second terminal of the respective secondary winding by a third impedance.

6. The amplifier circuit according to claim 5, comprising an earth connection between the second impedance and the middle terminal.

7. The amplifier circuit according to claim 5, wherein said first impedance, second impedance and third impedance each comprise a resistive component and a capacitive component.

8. The amplifier circuit according to claim 1, wherein the transformer and the additional transformer are identical.

9. The amplifier circuit according to claim 1, wherein the transformer comprises a core and the additional transformer comprises an additional core which is distinct from the core of the transformer.

10. The amplifier circuit according to claim 1, wherein the output circuit comprises a differential output amplifier to generate the differential output signal.

11. An audio amplifier, comprising: an amplifier circuit according to claim 1; a power amplifier.

12. The audio amplifier according to claim 11, comprising a first stage including the amplifier circuit and a second stage including the power amplifier, wherein the second stage is configured to receive the differential output signal balanced by the first stage.

13. The audio amplifier according to claim 12, comprising a third stage including an additional amplifier circuit according to claim 1, the third stage being configured to receive an amplified output signal from the second stage.

14. A method for amplifying an audio signal, comprising the following steps: receiving a first input signal, representing the audio signal, at a first pin of an input stage; at a second pin of the input stage, receiving a second input signal, representing the audio signal and 180 out of phase with respect to the first input signal; providing a transformer including a primary winding and a secondary winding, the secondary winding defining a first terminal, a second terminal and a middle terminal; providing a differential amplifier, having a first input, electrically connected to the first terminal of the secondary winding by a first connecting branch; a second input, electrically connected to the middle terminal of the secondary winding by a second connecting branch; and an output connected to the second terminal of the secondary winding, providing an additional transformer including an additional primary winding and an additional secondary winding defining a first terminal, a second terminal and a middle terminal, wherein the primary winding and the additional primary winding are electrically connected in series between the first pin and the second pin; providing an additional differential amplifier, having a first input, electrically connected to the first terminal of the additional secondary winding by a first connecting branch; a second input, electrically connected to the middle terminal of the additional secondary winding by a second connecting branch; and an output connected to the second terminal of the additional secondary winding of the additional transformer; generating a differential output signal from the output signals from the differential amplifier and from the additional differential amplifier to make the differential output signal available to an output stage.

15. The method according to claim 14, comprising the following steps: via the primary winding of the transformer, generating a magnetic flux; via the additional primary winding of the additional transformer, generating a magnetic flux, opposite of the magnetic flux generated by the primary winding.

16. The method according to claim 14, wherein the transformer and the additional transformer are identical, placed side by side and oriented in the same way, so that an external magnetic flux generates the same current in the transformer and in the additional transformer.

17. The method according to claim 16, wherein, for each of said differential amplifier and additional differential amplifier: the output is connected to the second input of the respective differential amplifier by a first impedance; the second connecting branch includes a second impedance, so that the second input is connected to the middle terminal of the respective secondary winding by the second impedance and the second impedance is connected to the middle terminal by an earth connection; the output is connected to the second end of the respective secondary winding by a third impedance.

18. The method according to claim 14, comprising a step of providing a differential output amplifier having an output circuit to generate the differential output signal.

19. The audio amplifier according to claim 12, comprising a pre-processing stage, located between the first stage and the second stage and configured for receiving the differential output signal from the first stage and for feeding the processed differential output signal to the second stage, the pre-processing stage including an analog-to-digital converter, a digital signal processor and a digital-to-analog converter.

20. A loudspeaker comprising an audio amplifier according to claim 14, wherein: the audio amplifier is configured for receiving an input power signal from a network through an input connector, the amplifier circuit is configured for outputting a balanced differential signal and for feeding the balanced differential output signal to the pre-processing stage, and the power amplifier is configured for receiving the pre-processed balanced differential signal from the processing stage and for feeding an amplified signal to the loudspeaker.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0073] This and other features will become more apparent from the following description of a preferred embodiment, illustrated by way of non-limiting example in the accompanying drawings, in which:

[0074] FIG. 1 illustrates an amplifier circuit according to one or more aspects of this disclosure;

[0075] FIGS. 2A-2C illustrates an audio amplifier according to one or more aspects of this disclosure;

[0076] FIGS. 3A and 3B illustrate an audio amplifier according to one or more aspects of this disclosure.

DESCRIPTION OF EXAMPLE EMBODIMENTS

[0077] The numeral 1 in the drawings denotes an amplifier circuit. The amplifier circuit 1 comprises an input stage I for receiving as input an audio signal V.sub.in, representing the audio signal to be amplified. The input stage I comprises a first input node IN.sub.+ responsible for receiving a first input signal V.sub.in+. The input stage I comprises a second input node IN.sub. responsible for receiving a second input signal V.sub.in. Therefore, the input stage I is an input stage for balancing the input audio signal V.sub.in, where the second input signal V.sub.in is 180 out of phase with the first input signal V.sub.in+.

[0078] The amplifier circuit 1 comprises an isolating stage TI, connected to the input stage I.

[0079] The isolating stage TI comprises a transformer T and an additional transformer T. The transformer T includes a primary winding L.sub.1, having a first terminal m.sub.1 (that is, a first end) and a second terminal m.sub.2, or second end. The first terminal m.sub.1 of the primary winding L.sub.1 is connected to the first input node IN.sub.+ to receive the first input signal V.sub.in+. The additional transformer T includes an additional primary winding L.sub.1, having a first terminal m.sub.1 (that is, a first end) and a second terminal m.sub.2, (that is, a second end). The first terminal m.sub.1 of the additional primary winding L.sub.1 is connected to the second input node IN.sub.. The second terminal m.sub.2 of the primary winding L.sub.1 and the second terminal m.sub.2 of the additional primary winding L.sub.1 are electrically connected in series so that the transformer T and the additional transformer T are electrically connected in series between the first input node IN.sub.+ and IN.sub..

[0080] Preferably, the second terminal m.sub.2 of the primary winding L.sub.1 is connected to the first input node IN.sub.+ through a first input resistor R.sub.in+ and the second terminal m.sub.2 of the additional primary winding L.sub.1 is connected to the second input node IN.sub.. In particular, the primary winding L.sub.1 and the additional primary winding L.sub.1 are connected to each other so as to have discordant polarity. In particular, the magnetic flux generated by the primary winding L.sub.1 is the opposite of the magnetic flux generated by the additional primary winding L.sub.1.

[0081] The transformer T comprises a secondary winding L.sub.2, including a first terminal m.sub.1 (that is, a first end) and a second terminal m.sub.2, (that is, a second end). The secondary winding L.sub.2 comprises a first secondary winding L.sub.21 including the first terminal m.sub.1 of the secondary winding L.sub.2, and a second secondary winding L.sub.22 including the second terminal m.sub.2 of the secondary winding L.sub.2. The first secondary winding L.sub.21 and the second secondary winding L.sub.22 are connected to each other by a middle terminal m.sub.3 of the secondary winding L.sub.2. The first secondary winding L.sub.21 and the second secondary winding L.sub.22 are connected to each other so as to have concordant polarity. The primary winding L.sub.1 and the secondary winding L.sub.2 of the transformer T have concordant polarity, that is, additive polarity.

[0082] The additional transformer T comprises a secondary winding L.sub.2, including a first terminal m.sub.1 (that is, a first end) and a second terminal m.sub.2, (that is, a second end). The secondary winding L.sub.2 comprises an additional first secondary winding L.sub.21 including the first terminal m.sub.1 of the additional secondary winding L.sub.2, and a second secondary winding L.sub.22 including the second terminal m.sub.2 of the additional secondary winding L.sub.2. The additional first secondary winding L.sub.21 and the additional second secondary winding L.sub.22 are connected to each other by a middle terminal m.sub.3 of the additional secondary winding L.sub.2. The additional first secondary winding L.sub.21 and the additional second secondary winding L.sub.22 are connected to each other so as to have concordant polarity. The additional primary winding L.sub.1 and the additional secondary winding L.sub.2 of the additional transformer T have concordant polarity, that is, additive polarity.

[0083] The transformer T and the additional transformer T define a transformer pair. In an example, the amplifier circuit 1 may comprise a plurality of transformer pairs.

[0084] The primary winding L.sub.1 of the transformer T and the additional primary winding L.sub.1 of the additional transformer T are connected in such a way as to have discordant polarity.

[0085] Shown by way of example in FIG. 1 is an amplifier circuit 1 where the primary winding L.sub.1 of the transformer T and the additional primary winding L.sub.1 of the additional transformer T are connected in such a way as to have discordant polarity and the polarity of both of the first primary windings L.sub.21 and L.sub.21 is concordant with the polarity of the respective second secondary windings L.sub.22, L.sub.22. The polarity of both the primary winding L.sub.1 of the transformer T and the additional primary winding L.sub.1 of the additional transformer T may be inverted with respect to those shown in FIG. 1. The polarities of all the windings may also be inverted with respect to those shown in FIG. 1.

[0086] In particular, it is noted that the secondary winding L.sub.2 and the additional secondary winding L.sub.2 are in separate magnetic circuits.

[0087] In the drawings, the stray capacitance produced between the primary winding L.sub.1 and the first secondary winding L.sub.21 of the transformer T and between the primary winding L.sub.1 and the second secondary winding L.sub.22 of the transformer T are labelled C1 and C2 respectively; the stray capacitance produced between the additional primary winding L.sub.1 and the additional first secondary winding L.sub.21 of the additional transformer T and between the additional primary winding L.sub.1 and the additional second secondary winding L.sub.22 of the additional transformer T are labelled C3 and C4 respectively.

[0088] The amplifier circuit 1 comprises an amplifying stage A, connected to the isolating stage TI.

[0089] The amplifying stage A comprises a differential amplifier U, including a first input U.sub.+, defining a non-inverting input for the differential amplifier U, a second input U.sub., defining an inverting input for the differential amplifier U, and an output U.sub.out for generating a differential signal at a first output node OUT.sub.+. The first input U.sub.+ is connected to the first terminal m.sub.1 of the secondary winding L.sub.2 by a first connecting branch r.sub.1, and the second input U.sub. is connected to the middle terminal m.sub.3 of the secondary winding L.sub.2 by a second connecting branch r.sub.2, and to the output U.sub.out. The output U.sub.out is also connected to the second terminal m.sub.2 of the secondary winding L.sub.2.

[0090] In particular, the output U.sub.out is connected to the second input U-through a first impedance Z.sub.1, the second input U.sub.+ is connected to the middle terminal m.sub.3 through a second impedance Z.sub.2 and the second impedance Z.sub.2 is connected to the middle terminal m.sub.3 of the secondary winding L.sub.2 by a ground connection M. The output U.sub.out is connected to the second terminal m.sub.2 of the secondary winding L.sub.2 through a third impedance Z.sub.3.

[0091] The amplifying stage A comprises an additional differential amplifier U, including a first input U.sub.+, defining a non-inverting input for the additional differential amplifier U, a second input U.sub., defining an inverting input for the additional differential amplifier U, and an output U.sub.out for generating an additional differential signal at a second output node OUT.sub..

[0092] The first input U.sub.+ is connected to the first terminal m.sub.1 of the secondary winding L.sub.2 by a first connecting branch r.sub.1, and the second input U.sub. is connected to the middle terminal m.sub.3 of the secondary winding L.sub.2 by a second connecting branch r.sub.2, and to the output U.sub.out.

[0093] The output U.sub.out is also connected to the second terminal m.sub.2 of the additional secondary winding L.sub.2.

[0094] In particular, the output U.sub.out of the additional differential amplifier U is connected to the second input U.sub. of the additional differential amplifier U through an additional first impedance Z.sub.1, the second input U.sub.+ of the additional differential amplifier U is connected to the middle terminal m.sub.3 through an additional second impedance Z.sub.2 and the additional second impedance Z.sub.2 is connected to the middle terminal m.sub.3 of the additional secondary winding L.sub.2 by an additional ground connection M. The output U.sub.out of the additional differential amplifier U is connected to the second terminal m.sub.2 of the additional secondary winding L.sub.2 through an additional third impedance Z.sub.3.

[0095] The amplifier circuit 1 comprises an output circuit O, connected to the amplifying stage A.

[0096] The output circuit O comprises the first output node OUT.sub.+ to receive the output signal U.sub.out from the differential amplifier U. The output circuit O comprises the second output node OUT.sub. to receive the output signal U.sub.out from the additional differential amplifier U.

[0097] The output circuit O comprises a differential output amplifier U1, including a first input U1.sub.+ defining a non-inverting input for the differential output amplifier U1 and connected to the first output node OUT.sub.+ through a first resistor R1. The differential output amplifier U1 includes a second input U1.sub. defining an inverting input for the differential output amplifier U1 and connected to the second output node OUT.sub. through a second resistor R2. The differential output amplifier U1 includes an output U1.sub.ou for generating a differential output signal V.sub.out. The output U1.sub.out is connected to the second input U.sub. of the differential output amplifier U1 through a third resistor R3. The first output U1.sub.+ is connected to a ground output M1 through a fourth resistor R4.

[0098] The numeral 100 in the drawings denotes an audio amplifier.

[0099] The audio amplifier 100 comprises an amplifier circuit 1. The audio amplifier 100 comprises a power amplifier 2. In an example, the audio amplifier 100 comprises a first stage S1, including the amplifier circuit 1 and a second stage S2, including the power amplifier 2. The first stage S1 may be located upstream of the second stage S2, so that the amplifier circuit 1 feeds the balanced differential signal V.sub.out to the audio amplifier 100.

[0100] Alternatively, the first stage S1 may be located downstream of the second stage S2, so as to receive an amplified signal V.sub.a from the power amplifier 2.

[0101] In the case where the first stage S1 is located upstream of the second stage S2, the first stage S1 constitutes a stage for preconditioning the signal (prior to the power amplifying stage). In the case where the first stage S1 is located downstream of the second stage S2, the first stage S1 constitutes a stage for measuring the output signal from the power amplifier 2.

[0102] The audio amplifier 100 comprises a processing stage S4, located between the first stage S1 and the second stage S2. Thus, the processing stage S4 is configured to receive the differential output signal V.sub.out from the first stage S1 and to feed the processed differential output signal V.sub.out to the second stage S2. Preferably, the processing stage S4 includes an analog-to-digital converter ADC1 and a digital signal processor PD; in addition, the processing stage includes a digital-to-analog converter ADC2.

[0103] FIG. 2A illustrates, by way of an example, an audio amplifier 100 where the first stage S1 is located upstream of the second stage S2.

[0104] FIG. 2B illustrates, by way of an example, an audio amplifier 100 where the first stage S1 is located downstream of the second stage S2. The audio amplifier 100 may include a shunt resistor R.sub.sh, which is part of the measuring stage; the amplifier circuit 1 is connected to the ends of the shunt resistor R.sub.sh to perform a measurement of the current and/or voltage at the ends of the shunt resistor R.sub.sh.

[0105] FIG. 2C illustrates, by way of an example, an audio amplifier 100 where the first stage S1 is located upstream of the second stage S2; the audio amplifier 100 is configured to feed an amplified output signal V.sub.a from the power amplifier 2 to a loudspeaker A. In the detail, FIG. 2C illustrates an active audio diffuser comprising an audio amplifier 100 configured to receive an input power supply signal V.sub.i from the mains R through an input connector; the amplifier circuit 1 is configured to receive the input power supply signal V.sub.i and to provide a balanced differential output signal V.sub.out which it feeds to the processing stage S4. The power amplifier 2 is configured to receive the balanced differential output signal V.sub.out from the processing stage S4 and to provide an amplified output signal V.sub.a, which it feeds to the loudspeaker A. Thus, in this case, the amplifier circuit 1, in the first stage S1, constitutes a stage for isolating the input connector from the external mains.

[0106] In an example, the audio amplifier 100 comprises an amplifier circuit 1 located in the first stage S1, a power amplifier 2 located in the second stage S2, downstream of the first stage S1, and an additional amplifier circuit located in a third stage, downstream of the second stage S2 and configured to perform a measurement of output current and/or voltage from the power amplifier 2.

[0107] FIG. 3A shows, by way of an example, an electronic card on which there is an amplifier circuit 1, an analog-to-digital converter ADC1, a plurality of relays RE and pins MO. The transformer T and the additional transformer T are mounted side by side and oriented in the same way. In particular, the turns of the windings of the transformer T and of the additional transformer T are wound along axes which are parallel to each other.

[0108] Additionally, the transformer T and the additional transformer T are located relative to each other at a distance such that an external magnetic flux generates the same current in the transformer T and in the additional transformer T. Thus, each transformer T and T of the transformer pair has the same mutual orientation (as illustrated, for example, in FIG. 3B).