Audio amplifier with integrated filter

Embodiments provide an audio amplifier circuit with integrated (built-in) filter (e.g., a digital-to-analog converter (DAC) filter). The audio amplifier circuit may have a non-flat (e.g., low-pass) closed loop frequency response. The audio amplifier circuit may include a low pass filter coupled between an input terminal that receives the input analog audio signal and the input of the gain stage of the amplifier. In some embodiments, additional impedance networks may be included to produce a desired low-pass filter response, such as a second order filter, a third order filter, and/or another suitable filter response. Other embodiments may be described and/or claimed.

TECHNICAL FIELD

Embodiments herein relate to the field of electronic circuits, and, more specifically, to an audio amplifier with integrated filter.

BACKGROUND

Some audio reproduction devices include a digital-to-analog converter (DAC) circuit to convert a digital audio signal to an analog audio signal. A DAC filter may be included at the output of the DAC circuit to roll-off the response of the circuit above the audio band and thereby attenuate the level of high frequency switching components present at the DAC output. A separate amplifier circuit may be included after the DAC filter to amplify the analog audio signal and drive one or more audio output devices (e.g., headphones and/or speakers).

DETAILED DESCRIPTION OF DISCLOSED EMBODIMENTS

The terms “coupled” and “connected,” along with their derivatives, may be used. It should be understood that these terms are not intended as synonyms for each other. Rather, in particular embodiments, “connected” may be used to indicate that two or more elements are in direct physical or electrical contact with each other. “Coupled” may mean that two or more elements are in direct physical or electrical contact.

However, “coupled” may also mean that two or more elements are not in direct contact with each other, but yet still cooperate or interact with each other.

The description may use the terms “embodiment” or “embodiments,” which may each refer to one or more of the same or different embodiments. Furthermore, the terms “comprising,” “including,” “having,” and the like, as used with respect to embodiments, are synonymous, and are generally intended as “open” terms (e.g., the term “including” should be interpreted as “including but not limited to,” the term “having” should be interpreted as “having at least,” the term “includes” should be interpreted as “includes but is not limited to,” etc.).

Embodiments herein provide an audio amplifier circuit with integrated (built-in) filter (e.g., a digital-to-analog converter (DAC) filter). The audio amplifier circuit may have a non-flat (e.g., low-pass) closed loop frequency response. The audio amplifier circuit described herein may have reduced circuit complexity/cost, power consumption, and/or distortion compared with circuits that include separate filter and audio amplifier circuits. While the audio amplifier circuit is described herein with reference to a DAC filter, the audio amplifier circuit may be used in any suitable implementation in which the provided filter response is desired.

In some embodiments, the audio amplifier circuit may include a low pass filter coupled between an input terminal that receives the input analog audio signal and the input of the gain stage of the amplifier (e.g., the inverting input of an operational amplifier (op-amp) of the gain stage). In some embodiments, additional impedance networks may be included to produce a desired low-pass filter response, such as a second order filter, a third order filter, and/or another suitable filter response.

The audio amplifier circuit with integrated filter may be implemented in a single-ended or differential amplifier arrangement.

The audio amplifier circuit may include a plurality of amplifier stages, including a first amplifier stage (e.g., a gain/driver stage) and a second amplifier stage (e.g., an output stage). In some embodiments, the audio amplifier circuit may include additional amplifier stages, such as one or more intermediate amplifier stages coupled between the first amplifier stage and the second amplifier stage. The audio amplifier circuit may further include impedance networks to provide feedback and/or feedforward paths for the amplifier stages. For example, the impedance networks may include an impedance network to provide a feedback path from the output of the first amplifier stage to an input of the first amplifier stage, an impedance network to provide a feedback path from the output of the second amplifier stage to an input of the first amplifier stage, an impedance network coupled between the output of the second amplifier stage and the output terminal of the amplifier circuit (e.g., that is coupled to the load), and/or an impedance network coupled between the output of the first amplifier stage and the output terminal of the amplifier circuit. An additional impedance network may provide a feedback path from the output terminal of the circuit (coupled to the load) to the input of the first amplifier stage. The impedance networks may include one or more resistors and/or capacitors. In some embodiments, the impedance networks may be exclusively composed of resistors and/or capacitors.

The amplifier circuit described herein may have low power dissipation and low distortion compared with prior designs. The amplifier stages may be operated in any suitable operating mode, as appropriate, such as Class A, Class AB, Class B, Class G, Class H, etc. In some embodiments, different amplifier stages may be operated in the same or different operating modes.

FIG. 1illustrates an audio amplifier circuit100(hereinafter “circuit100”) with built-in DAC filter, in accordance with various embodiments. The circuit100receives an analog input audio signal Vin at an input terminal102and generates an output audio signal Vout at an output terminal104. The output audio signal Vout may drive a load106coupled to the output terminal104. The load106may be one or more audio output devices, such as headphones and/or speakers.

The circuit100may include a gain stage108(also referred to as driver stage108) and an output stage110. The gain stage108and output stage110may be coupled in series between the input terminal102and output terminal104to process/amplify the audio signal. As such, the gain stage108and output stage110may both be referred to as amplifier stages. However, in some embodiments, the output stage110may be configured to provide unity gain. In other embodiments, the output stage110may provide another suitable gain.

The gain stage108may include one or more amplifiers to amplify the analog input audio signal. For example, gain stage108is shown inFIG. 1to include an amplifier X1with an input terminal coupled to receive the input audio signal (e.g., via impedance networks Zin and Z5, which will be discussed further below). The output stage110may be coupled between the output of the gain stage108and the output terminal104of the circuit100. For example, the output stage110may include a pair of transistors Q1and Q2. The collector of Q1may be coupled to a positive supply rail112to receive a positive supply voltage +V, and the collector of Q2may be coupled to a negative supply rail114to receive a negative supply voltage −V. The bases of the transistors Q1and Q2may be coupled together at the input of the output stage110to receive the output signal from the gain stage108. The emitters of the transistors Q1and Q2may be coupled together at the output of the output stage110to pass the output audio signal to the output terminal104(e.g., via impedance network Z1). The transistors Q1and Q2may be biased by a bias circuit (not shown) to operate in a desired operating mode. While the transistors Q1and Q2are shown as bipolar junction transistors, it will be apparent that other embodiments may include another type of transistor, such as field-effect transistors (FETs) (e.g., metal-oxide-semiconductor FETs (MOSFETs)).

Additionally, it will be apparent that the circuit100may include different designs of the gain stage108and/or output stage110, more or fewer amplifier stages, and/or other modifications as known to those skilled in the art. For example, in some embodiments, the output stage110may include a compound amplifier (e.g., op-amp) in addition to or instead of the complementary emitter-follower arrangement of transistors Q1and Q2depicted inFIG. 1.

The circuit100may further include a plurality of impedance networks, such as impedance networks Z1, Z2, Z3, Z4, and/or Z5. Impedance network Z1may be coupled between the output of the output stage110and the output terminal104of the circuit100. The impedance network Z2may be coupled between the input terminal of the gain stage108(at which the gain stage108receives the audio input signal, such as the inverting input of op-amp X1as shown inFIG. 1) and a node116at which the output of the gain stage108is coupled with the input of the output stage110. Impedance network Z3may be coupled between the node116and the output terminal104(e.g., in parallel with the combination of output stage110and impedance network Z1). Impedance network Z4may be coupled between the input of the gain stage108and the output of the output stage110.

The impedance networks Z1, Z2, Z3, and Z4may be balanced (e.g., in what is referred to as a bridge arrangement) to provide low distortion. For example, the impedance networks Z1, Z2, Z3, and Z4may be designed to satisfy the relationship Z1(s)·Z2(s)=Z3(s)·Z4(s), where Z(s) refers to the impedance as a function of frequency. Additionally, imperfect characteristics of gain stage108may be compensated for by suitably altering impedance network Z2(for instance by compensating for a dominant pole in gain stage108by reducing the value of Z2when Z2is a capacitor).

The impedance networks may include one or more impedance elements, such as one or more resistors and/or capacitors, to provide the desired impedance and/or frequency response. For example, impedance network Z1may include a resistor114coupled between the output of the output stage110and the output terminal104of the circuit100. The impedance network Z2may include a capacitor coupled between the input terminal of the gain stage108the node116at which the output of the gain stage108is coupled with the input of the output stage110. Impedance network Z3may include a resistor in parallel with a capacitor. Impedance network Z4may include a resistor in series with a capacitor.

In various embodiments, impedance network Z5may be coupled between the input terminal102and the output terminal104. The impedance network Z5is shown to include a resistor R5and resistor R6coupled in series between the input terminal102and output terminal104. The impedance network Z5may further include a capacitor C7coupled between a node118(that is between the resistors R5and R6) and ground.

In various embodiments, the circuit100may further include an input impedance network Zin coupled between the input terminal102and the input of the gain stage108. For example, in some embodiments, the impedance network Zin may be coupled between the node118of the impedance network Z5and the input of the gain stage108. The input impedance network Zin may form a low pass filter. For example, the input impedance network Zin may filter the signal present at node118, which is based on the input audio signal and the feedback received from the output node104via the impedance network Z5.

Without the input impedance network Zin and/or impedance network Z5, the closed loop response of the circuit including the impedance networks Z1-Z4, the gain stage108, and the output circuit110would be such that, below a certain frequency (e.g., determined by the time constant of impedance network Z4), the gain of the circuit increases at 6 dB per octave as frequency decreases. Accordingly, the circuit would have a zero in its response at the frequency determined by the time constant of impedance network Z4. Therefore, the circuit would be a partial integrator. The low pass filter provided by the impedance network Zin counterbalances the zero in impedance network Z4, thereby causing the closed loop response of the circuit100to be that of a full integrator. In other embodiments, the impedance network Z5may counterbalance impedance network Z4instead of, or in cooperation with, impedance network Zin. The input impedance network Zin and/or impedance network Z5will not significantly disturb the bridge balance of impedance networks Z1-Z4since they are coupled outside of the bridge circuit formed by impedance networks Z1-Z4. In some embodiments, the impedance network Z2, which provides feedback for the gain stage108, may be modified from the design shown inFIG. 1if the open loop gain of the gain stage108(e.g., operational amplifier X1) is not sufficiently high to otherwise avoid disturbing the bridge balance. For example, as described above, the value of the capacitor of Z2may be reduced.

In various embodiments, impedance network Zin may form an integrator with gain stage108(e.g., amplifier X1) and impedance networks Z2and Z4. The integrator may provide the circuit100with an overall frequency response that is a low-pass filter. Accordingly, the circuit100may be used as a DAC filter and/or in another implementation in which the provided filter response is desired.

The input impedance network Zin may include any suitable components to provide the low pass filter. For example, as shown inFIG. 1, the impedance network Zin may include resistors Rin1and Rin2coupled in series between the input terminal102of the circuit100and the input of the gain stage108. A capacitor Cin may be coupled between a node120(that is between the resistors Rin1and Rin2) and ground.

The feedback loop formed by impedance network Z5may not disturb the balance of the bridge circuit formed by impedance networks Z1-Z4, since the feedback path is outside the bridge arrangement formed by impedance networks Z1-Z4(e.g., because impedance network Z5derives feedback from the output voltage signal at output terminal104). Additionally, because the closed loop response of the internal amplifier arrangement is that of an integrator, as described above, the feedback loop formed by the impedance network Z5may reduce the closed loop output impedance of the overall circuit100.

In various embodiments, the circuit100may implement a nominal 2nd order low pass filter, for example, similar to an infinite gain multiple feedback filter (referred to as an MFB filter). However, in contrast to an MFB filter, the input impedance network Zin may provide a loading on the node118of impedance network Z5that corresponds to the parallel combination of resistor Rin2and capacitor Cin in series with resistor Rin1. This loading may alter the response of the filter away from a true 2nd order filter. In some embodiments, values of the components of impedance network Z5(e.g., R5, R6, and C7), input impedance network Zin (e.g., Rin1, Rin2, and/or Cin), and/or other components of circuit100may be selected to approximate a 2nd order filter over a wide frequency range. Alternatively, or additionally, the impedance network Z5may be modified from the design shown inFIG. 1to implement a true 2nd order filter or a closer approximation of a true 2nd order filter. For example, a suitable resistance-capacitance network may be added across both resistor R5and resistor R6.

FIG. 2illustrates another audio amplifier circuit200(hereinafter “circuit200”) with built-in DAC filter, in accordance with various embodiments. The circuit200uses a differential input, in contrast to the single-ended input of circuit100. The circuit200receives a differential input audio signal (e.g., Vin+ and Vin−) at input terminals202aand202b, and generates an output audio signal Vout at an output terminal204. The output audio signal Vout may drive a load206coupled to the output terminal204. The load206may be one or more audio output devices, such as headphones and/or speakers.

As discussed above with respect to circuit100, the impedance networks Z2and Z4may provide feedback for the gain stage208at a first input terminal of the gain stage208(e.g., at the inverting input of op-amp X1). The circuit200may further include an impedance network230coupled between the second input terminal of the gain stage208and ground. The impedance network230may approximately match the operation of impedance networks Z4, Z2, gain stage208, and output stage210. For example, the impedance network230may provide an impedance that corresponds to the impedance of Z2in parallel with Z4, as shown (e.g., with capacitor C2pin parallel with the series combination of capacitor C4pand resistor R4p).

The values of capacitor C4pand resistor R4pmay be determined based on the gain provided by output stage210(e.g., unity gain or non-unity gain). For example, if the gain of the output stage210is less than one, then the impedance of various elements of impedance network230should be increased compared with the impedance for unity gain in output stage210. Additionally, or alternatively, in some embodiments, a limited gain bandwidth product of the op-amp X1may be taken into account by increasing the capacitance of capacitor C2p.

In various embodiments, a first input terminal of the gain stage108(e.g., the inverting input terminal of op-amp X1) may be coupled to the input terminal202ato receive one component (Vin−) of the differential input signal. For example, the first input terminal of the gain stage108may be coupled to the input terminal202avia an input impedance network Zin and an impedance network Z5. Input impedance network Zin and impedance network Z5may be similar to the corresponding impedance networks of circuit100.

A second input terminal of the gain stage108(e.g., the non-inverting input terminal of op-amp X1) may be coupled to the input terminal202bto receive the other component (Vin+) of the differential input signal. For example, the second input terminal of the gain stage108may be coupled to the input terminal202bvia a second input impedance network Zinp and an input impedance network Z6. The second input impedance network Zinp may

Impedance networks Z5and Z6may both provide a low-pass filter. In some embodiments, a capacitor C7may be shared between the impedance networks Z5and Z6, as shown. The capacitor C7may thus be considered to be part of both impedance networks Z5and Z6. In other embodiments, impedance networks Z5and Z6may include separate capacitors coupled between the respective internal nodes and ground. In some embodiments, the impedance network Z6may include a resistor R5pcoupled in series between the input terminal202band the second input terminal of the gain stage208, and another resistor R6pcoupled in shunt with the signal path. It will be apparent that other designs are possible.

The input impedance networks Zin and Zinp are shown inFIG. 2to include separate capacitors Cin and Cinp. In some embodiments, the input impedance networks Zin and Zinp may share a capacitor instead of using separate capacitors, similar to the sharing of capacitor C7by impedance networks Z5and Z6.

FIG. 3illustrates another audio amplifier circuit300(hereinafter “circuit300”) with built-in DAC filter, in accordance with various embodiments. Circuit300may be similar to the circuit200, except that impedance network230is omitted, and the second input impedance network Zinp is replaced with an input impedance network Zopt. The circuit300may have lower common-mode rejection performance than the circuit200, but with a simpler circuit layout. The input impedance network Zopt may provide a low-pass filter and may be designed to provide suitable (e.g., optimized) common-mode rejection and differential frequency response. For example, the impedance network Zopt may include a resistor Ropt in series with the signal path between input terminal302band the second input terminal of the gain stage308, and a capacitor Copt coupled in shunt with the signal path (between the signal path and ground). The values of the resistor Ropt and capacitor Copt may be selected to optimize the common-mode rejection and differential frequency response of the circuit300.

In some embodiments, the input impedance network Zin and the input impedance network Zopt may share a capacitor instead of including separate capacitors Cin and Copt.

In some embodiments, the amplifier circuit with built-in DAC filter may be designed to provide a third order filter (e.g., third order roll-off of the frequency response). For example,FIG. 4illustrates an audio amplifier circuit400(hereinafter “circuit400”) with built-in DAC filter that provides a third order filter response, in accordance with various embodiments. The circuit400may be similar to the circuit200. However, the impedance network Z4may be modified from circuit200to obtain the third order filter. For example, the impedance network Z4may include an additional capacitor C3rdcoupled in parallel with capacitor C3. In some embodiments, a resistor Rst may be coupled in series with capacitor C3rd(e.g., with resistor Rst between the capacitor C3rdand the output terminal404), with the series combination of Rst and C3rdcoupled in parallel with capacitor C3. The resistor Rst may facilitate stability of the closed loop in circuit400, for example, depending on the gain-bandwidth of the output stage410. In other embodiments, the resistor Rst may be omitted. In addition to, or instead of, including resistor Rst, an additional low-pass filter may be included between the output of the gain stage408and the input of the output stage410(e.g., coupled between the output of op-amp X1and the input to transistors Q1and Q2).

The impedance network Z3may be modified from circuit300to balance the bridge arrangement with the impedance network Z4. For example, a resistor R3smay be coupled in series with capacitor C3(e.g., with the series combination of R3sand C3coupled in parallel with resistor R3.

The impedance network430may also be modified from the impedance network230of circuit200, to balance the common-mode rejection and differential filter response in light of the modifications to impedance network Z4. For example, an additional capacitor C3rdbmay be coupled in parallel with resistor R4pin the impedance network430.

In some embodiments, the circuit400may be simplified in a similar manner to the way circuit300is a simplified version of circuit200. For example,FIG. 5illustrates another audio amplifier circuit500(hereinafter “circuit500”) with built-in DAC filter, in accordance with various embodiments. Circuit500may be similar to the circuit400, except that impedance network430is omitted, and the second input impedance network Zinp is replaced with an input impedance network Zopt.

Accordingly, the circuit500may be partially balanced, and have reduced common-mode rejection ratio compared to the fully balanced configuration of circuit400. However, the common-mode rejection ratio of the circuit500may still be sufficient for some applications.

In various embodiments, the circuits100,200,300,400, and/or500may be modified from the specific configurations/components shown inFIGS. 1-5. For example, the output stage, which is shown inFIGS. 1-5to include transistors Q1and Q2, may include another suitable configuration and/or may include other components.

Additionally, or alternatively, the inverting input of the op-amp X1may be either voltage-sensing or current-sensing. When the inverting input is current-sensing, the balance is less sensitive to gain bandwidth (GBW) as input network impedance is altered.

While the amplifier circuits described herein have focused on implementing second or third order filters, in some embodiments, the amplifier circuit may implement a higher or lower order of filter.

The amplifier circuit described herein (e.g., circuit100,200,300,400, and/or500) may be included in any suitable audio reproduction system.FIG. 6schematically illustrates one example of a system600that includes an audio amplifier circuit602that may correspond to any one of the amplifier circuits described herein (e.g., circuit100,200,300,400, and/or500). The system600may include a DAC604to receive a digital audio signal and generate an analog audio signal based on the received digital audio signal. The digital audio signal may be received from another component of the system600(e.g., a processor, media player, digital signal processor, etc.) and/or another device that is communicatively coupled with the system600(e.g., via a wired connection (e.g., Universal Serial Bus (USB), optical digital, coaxial digital, high definition media interconnect (HDMI), wired local area network (LAN), etc.) and/or wireless connection (e.g., Bluetooth, wireless local area network (WLAN, such as WiFi), cellular, etc.).

The amplifier circuit602may amplify the analog audio signal received from the DAC604to generate an amplified audio signal. The amplifier circuit602may include a built-in DAC filter, as described herein. The amplifier circuit602may pass the amplified audio signal to one or more audio output devices606. The audio output devices606may include any suitable devices to generate an audible sound based on the amplified audio signal, such as one or more headphones and/or speakers.

The system600may be included in any suitable device, such as a mobile phone, a computer, an outboard USB DAC device, an audio/video receiver, an integrated amplifier, a standalone audio amplifier, a powered speaker (e.g., a smart speaker or a non-smart powered speaker), etc.

In some embodiments, the system600may include an audio processor circuit to process the digital audio signal prior to passing it to the DAC602. For example, the audio processor may include a digital signal processor to implement audio processing such as filtering and delays. Additionally, or alternatively, the system600may include one or more additional components, such as one or more processors, memory (e.g., random access memory (RAM), mass storage (e.g., flash memory, hard-disk drive (HDD), etc.), antennas, a display, etc.