Patent Publication Number: US-2006008110-A1

Title: Receiver with multiple drive coils

Description:
CROSS-REFERENCE TO RELATED PATENT APPLICATIONS  
      This application claims the benefit under 35 USC 119(e) to U.S. Application 60/585,572, filed Jul. 7, 2004, incorporated herein by reference in its entirety. 
    
    
     BACKGROUND OF THE INVENTION  
      The present invention relates to moving armature receivers. More specifically, the invention relates to moving armature receivers adapted to provide a high acoustic output at high frequencies when driven by an amplifier with a low output impedance. In addition, the invention relates to a hearing aid output stage adapted to drive a multiple drive coil moving armature receiver.  
      Presently, miniatures moving armature loudspeakers, or receivers, for hearing aid use are more and more often applied in voltage-driven applications driven by digital switching amplifiers or class D amplifier such as a digitally modulated PWM or PDM based output amplifier. Such digitally modulated amplifiers have output stage characteristics that typically will drive the moving armature loudspeakers from a very low source impedance, typically 10-50 Ω at audio frequencies, from a pair of differential amplifier terminals at audio frequencies.  
      A prior art miniature moving armature receiver comprises a single elongate drive coil forming a central tunnel or aperture with a central longitudinal axis. A pair of plane quadratic permanent magnet members is oppositely arranged within a magnet housing so as to form a substantially rectangular air gap there between. The magnet housing is arranged in abutment with the drive coil and positioned in a manner so that a central axis of the rectangular air gap is substantially aligned with the central axis of the drive coil tunnel. A flat U-shaped armature made of a magnetically permeable material comprises a deflectable portion that extends longitudinally and centrally through the drive coil tunnel and the air gap along their common central longitudinal axis.  
      Since the armature is magnetized by the pair of permanent magnets, an alternating signal current applied to the drive coil will cause a drive force being applied to the armature in a direction substantially perpendicular to the common central longitudinal axis of the drive coil tunnel and the air gap. The movement of the armature causes a corresponding movement of a diaphragm through a drive rod or pin rigidly connected to the armature. The drive coil is electrically connected to a pair of externally accessible drive terminals positioned on a housing of the miniature moving armature receiver. The pair of differential amplifier terminals of the digitally modulated PWM or PDM based output amplifier is accordingly connected to these coil terminals.  
      Unfortunately, the impedance of a moving armature receiver according to the described prior art single drive coil designs increases rapidly with frequency within the audio frequency range. This increase of impedance is caused by inherent inductive characteristics of the drive coil. A result of this rising impedance curve is that a lower than desired net input electrical power can be supplied to the drive coil in an upper portion of the audio frequency range, i.e. frequencies above about 2000 Hz or 3000 Hz.  
      The increasing impedance of the moving armature receiver leads to an attenuated acoustical high frequency response under small signal operating conditions and to a less than desired maximum output power capability in the high frequency range. Both of these drawbacks are particularly unfortunate since it should be realised that the vast majority of hearing impaired individuals have the largest hearing loss at high frequencies, and as such the greatest need for amplification and output capability in the upper portion of the audio frequency range to adequately compensate for their respective hearing losses.  
      EP 1 154 673 A1 describes a hearing aid with a moving armature receiver comprising first and second drive coils fed by first and second electrical signal. The two drive coils serve to solve the problem of adding an analog and a digital electrical signals by magnetical superposition of the respective magnetic fields generated by the first and second drive coils.  
      GB 2 301 728 A describes a moving armature loudspeaker with two drive coils each connected to separate electrical input terminals. The loudspeaker is intended for use in a headset and the two separate drive coils serve to connect the loudspeaker to two different communication networks, a local network and a command net. The two drive coils may have different impedances, thus allowing both coils to individually match the electrical characteristics of the network to which they are connected.  
     SUMMARY OF THE INVENTION  
      It is an object of the present invention to provide a moving armature receiver suitable for hearing aids. The receiver must be capable of providing a high acoustic output in the upper portion of the audio frequency range.  
      The object is complied with by providing in a first aspect, a moving armature receiver suitable for a hearing aid, comprising  
      a housing comprising a sound aperture for delivering an acoustical signal in response to an input signal,  
      a coil tunnel comprising first and second drive coils,  
      a deflectable armature comprising an armature portion extending through the coil tunnel, and  
      frequency dividing means adapted to split the audio input signal into a first audio signal of a first predetermined frequency range for the first drive coil and into a second audio signal of a second predetermined frequency range for the second drive coil.  
      By driving the receiver using two separate drive coils driven in different frequency ranges of the audio spectrum it is possible to compensate for the rising impedance characteristics towards higher frequencies that a single drive coil exhibits. A moving armature receiver according to the invention is capable of providing a superior maximum output capability and a more suitable response in the upper portion of the audio frequency range.  
      It is to be understood that the first and second drive coils may be physically positioned in any suitable position relative to each other. In a non-exhaustive list of positions, the first and second drive coils are positioned: behind each other, around each other, and on top of each other. The first and second drive coils may be selected to have either low impedance or high impedance, and if preferred, the first and second drive coils may have different impedances. In preferred embodiments, the first and second drive coils have a DC impedance in the range of 50-500Ω, more preferably in the range of 50-100Ω. Alternatively, the drive coils may have a DC impedance in the range of 1-50 kΩ, more preferably in the range of 1-10 kΩ.  
      The frequency dividing means may comprise one or more passive components, preferably a capacitor.  
      The first and second drive coils may be electrically connected in a number of different ways. They may be partly electrically interconnected, or they may be separately electrically connected to the frequency dividing means, more alternatively the frequency dividing means may comprise separate first and second parts adapted for connection to respective first and second drive coils. The first and second drive coils may be connected in cascade between a pair of externally accessible input terminals on the moving armature receiver housing. Alternatively, respective first ends of the first and second drive coils are connected to a common node operatively connected to a first externally accessible input terminal and second ends of the first and second drive coils are connected to a second and third externally accessible input terminals, respectively, so as to form a three terminal receiver. An inductance ratio between the first and second drive coils is between 1 and 5, preferably between 2 and 3.  
      The receiver may further comprise an amplifier operatively coupled between a receiver input terminal and the frequency dividing means.  
      The receiver may further comprise an amplifier operatively coupled between an output of the frequency dividing means and the first or the second drive coil.  
      In a preferred embodiment the receiver comprises:  
      a first amplifier operatively coupled to a first output of the frequency dividing means to receive the first audio signal and adapted to drive the first drive coil in the first predetermined frequency range, and  
      a second amplifier operatively coupled to a second output of the frequency dividing means to receive the second audio signal and adapted to drive the second drive coil in the second predetermined frequency range.  
      The frequency dividing means is preferably adapted to receive and process a digitally coded input signal and generate the first and second audio signals in form of respective digitally coded audio signals and the first and second amplifiers comprise respective digital amplifiers.  
      Referring to all mentioned embodiments, the frequency dividing means preferably has a crossover frequency between 1 and 5 kHz. The first predetermined frequency range may be limited to substantially extend above the crossover frequency. The second predetermined frequency range may be limited to substantially extend below the crossover frequency.  
      The frequency dividing means may be adapted to provide the first audio signal of a first frequency range that covers all or most of the operating range of the miniature moving armature receiver such as a frequency range between 100-10 kHz or 200 Hz-8 kHz. According to this embodiment, the second audio frequency signal is applied to the second drive coil across frequencies 1-10 kHz or more preferably 2 kHz-10 kHz or even more preferably 3 kHz-10 kHz. Alternatively, the audio frequency range may be divided into two or more frequency ranges without any substantial overlap. The operating frequency ranges of the drive coils may be split so that the first drive coil is supplied with a drive signal up to about 3 kHz, while the second drive coil is supplied with a drive signal above 3 kHz. The frequency dividing means may control the split of drive voltage or current between the first and second coils and may use roll-off rates of 6 or 12 or 18 dB below and above the crossover frequency.  
      The receiver may further comprise a third drive coil and frequency dividing means adapted to split the input signal into a third electrical audio signal of a third predetermined audio frequency range for the third drive coil.  
      Referring to all mentioned embodiments of the first aspect, it is understood that the moving armature receiver is applicable not only for hearing aids but in general for portable communication equipment, including e.g.: mobile phones, in-ear monitors and headsets.  
      In a second aspect, the invention provides a hearing aid output stage, comprising:  
      an output stage input terminal adapted to receive a digitally coded input signal,  
      digital frequency dividing means operatively connected to the output stage input terminal and adapted to split the digitally coded audio signal into first and second audio signals of first and second predetermined frequency ranges, respectively,  
      a first receiver coil driver operatively connected to a first output of the digital frequency dividing means to receive the first audio signal,  
      a second receiver coil driver operatively connected to a second output of the digital frequency dividing means to receive the second audio signal.  
      Thus, the output stage is adapted for connection to respective drive coils of a dual coil moving armature transducer, e.g. a transducer with  3  or  4  external terminals, and thus achieve the advantages mentioned in connection with the first aspect of the invention.  
      Preferably, the first and second receiver coil drivers comprise a respective pair of differential output terminals.  
      In a third aspect, the invention provides a portable communication device comprising a moving armature receiver according to the first aspect. The portable communication device may be such as a hearing aid, a mobile phone, an in-ear monitors or a headset. The same advantages and embodiments mentioned above in connection with the first aspect also apply to the third aspect.  
      In a fourth aspect, the invention provides a hearing aid comprising an output stage according to the second aspect. The same advantages and embodiments mentioned above for the second aspect apply for the fourth aspect.  
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      In the following, the invention will be described in detail with reference to the accompanying figures, of which  
       FIG. 1  shows a schematic diagram of a receiver with two drive coils,  
       FIG. 2  shows diagrams of two embodiments where frequency dividing between two drive coils is implemented by a capacitor,  
       FIG. 3  shows a diagram of an embodiment where frequency dividing between three drive coils is implemented by two capacitors,  
       FIG. 4  shows a diagram of two drive coils with a three terminal connection to a set of differential amplifier outputs.  
       FIG. 5  shows a schematic diagram of a preferred digital receiver with two drive coils driven by separate digital amplifiers, and  
       FIG. 6  shows a schematic diagram of a preferred digital output stage adapted to drive a receiver with two drive coils. 
    
    
      While the invention is susceptible to various modifications and alternative forms, specific embodiments have been shown by way of example in the drawings and will be described in detail herein. It should be understood, however, that the invention is not intended to be limited to the particular forms disclosed. Rather, the invention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims.  
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
      The moving armature receivers that will be described in the following are especially suited for hearing aid miniature receivers. However, the principles may be applied also within other miniature portable devices, such as headsets.  
       FIG. 1  shows a schematic diagram illustrating basic principles of a moving armature receiver  1  according to the invention. The receiver  1  comprises a moving armature transducer  2  with two drive coils L 1  and L 2 . The receiver  1  is adapted to receive an audio input signal  3  and convert it to an according acoustic output signal via a sound aperture  6 . The audio input signal  3  may be provided as an analogue or digitally modulated signal such as PWM or PDM signals wherein the audio signal has been modulated by an ultra-sonic carrier wave. For example, the audio input signal  3  could be a differential output signal from a class D amplifier.  
      By means of a frequency divider FD the audio input signal  3  is split into a first audio signal  4  of a first frequency range and a second audio signal  5  of a second frequency range. The two resulting audio signals  4 ,  5  are then fed to the two drive coils L 1  and L 2 , respectively. By proper selection of the first and second audio frequency ranges it is possible to “design” an effective impedance characteristics for the receiver  1  so that it exhibits a frequency characteristics with enhanced response in the upper portion of the audio frequency range than is the case for a single drive coil.  
      For example the audio input signal  3  may be fed through the frequency divider FD without filtering, so that the first audio signal  4  extends throughout the entire frequency range. The frequency divider FD may then be adapted to highpass filter the audio input signal  3  and provide it as the second audio signal  5 . Hereby an effective impedance as seen from the input of the receiver  1  will be the impedance of the first drive coil L 1  at low frequencies. In the upper audio frequency range, i.e. in the pass band of the highpass filter of the frequency divider FD, the receiver  1  will exhibit an input impedance being a parallel of both drive coils L 1 , L 2 , and thus the effective impedance in the upper audio frequency range becomes lower than for each of the coils L 1 , L 2  separately. As a result the receiver will exhibit an enhanced response in the upper audio frequency range.  
      In general the frequency divider FD can be implemented either by passive components or the frequency divider FD can be implemented by active means. In the following  FIGS. 2 and 3  illustrate simple passive embodiments.  
       FIG. 2 , upper part, illustrates a diagram of an embodiment with the first L 1  and second L 2  drive coils arranged in cascade, and a class D amplifier A applies a signal to the drive coils L 1 , L 2 . The class D amplifier A may be an analog PWM or PDM type amplifier or a digital PWM or PDM type amplifier. A capacitor C 1  operates as the frequency dividing network, and it is connected across the first drive coil L 1  to bypass the first coil L 1  in the second audio frequency range and supply substantially all drive current to the second coil. The ratio of inductance between the first L 1  and the second L 2  drive coils is preferably about 2-3.  
      By suitably selecting the capacitor C 1  in accordance with the chosen inductance value of L 1 , C 1  will function as an effective short circuit of the first coil L 1  in the upper portion of the audio frequency range. Accordingly, substantially the entire audio signal, i.e. drive voltage of the class D amplifier A, will be applied to the second coil L 2  within the second audio frequency range leading to a correspondingly larger drive current through the second drive coil L 2  as compared to the situation with a single drive coil.  
      At low frequencies, i.e. in the first audio frequency range, the bypass capacitor C 1  presents a relatively high impedance and the drive current will flow equally through the first L 1  and second L 2  drive coils which means that the cascade of the first L 1  and second L 2  drive coils operate as a single coil with an impedance equal to the sum of their individual coil impedances.  
       FIG. 2 , lower part, illustrates a diagram of an embodiment with the first L 1  and second L 2  drive coils arranged in parallel, and a class D amplifier, of a type as mentioned above, applies a signal to the drive coils L 1 , L 2 . By suitably selecting the capacitor C 1  that operates as a frequency dividing network, it will function as an effective short circuit in the upper audio frequency range by placing the coils L 1 , L 2  in parallel to lower the total impedance of the receiver. Preferably, the second coil L 2  has a much lower inductance than the first coil L 1 . Thus, the majority of the drive current delivered by the class D amplifier A will flow through the second drive coil. At low audio frequencies, the capacitor C 1  will ensure that drive current flows mainly through the first drive coil L 1  due to the rising impedance of the capacitor C 1 .  
       FIG. 3  shows a diagram of an embodiment illustrating that the general inventive principle of using more that one drive coil can be extended to use many drive coils. In  FIG. 3 , three drive coils L 1 , L 2 , L 3  are coupled in parallel. Preferably, L 2  has an inductance much higher than L 3 , and L 1  has an inductance higher than L 2 . L 2  is cascaded by a capacitor C 1 , and L 3  is cascaded by another capacitor C 2 . Following the principles described above, L 1  will be active in the entire frequency range, whereas suitable inductance values of L 2  and L 3  can be combined with suitable values for capacitors C 1 , C 2  so that L 2  is active in the frequency range above a first crossover frequency, whereas L 3  is active above a second crossover frequency higher than the first crossover frequency. For example first and second crossover frequency of 1 kHz and 5 kHz may be chosen. Thus,  FIG. 3  illustrates the general principle that respective frequency dividing networks are used to split the audio drive signal from the class D amplifier A into a number of adjacent frequency bands that each has a dedicated drive coil associated therewith. By using more than two drive coils, such as three, four, five or more drive coils, and a proper selection of drive coil impedances and crossover frequencies an even more suitable resulting impedance curve can be obtained than using two coils.  
      In a preferred embodiment, a class D amplifier, of a type as mentioned above, is integrated into the miniature moving armature receiver. The class D amplifier is preferably a digital amplifier and adapted for receipt of a digitally formatted audio signal. The class D amplifier comprises a pair of differential output terminals operatively coupled to each end portion of the coil networks shown in  FIG. 1 .  
       FIG. 4  shows another embodiment of the invention with a class D amplifier having a pair of differential outputs A 1 , A 2  operatively coupled to first L 1  and second L 2  drive coils. The drive coils L 1 , L 2  are arranged in parallel and each of the drive coils L 1 , L 2  has an associated capacitor C 1 , C 2  in series with it. Each drive coil L 1 , L 2  has one end connected to a common ground terminal. This embodiment requires a three-terminal miniature moving armature receiver in those variants that do not have an integral class D amplifier. Such a three-terminal receiver is illustrated by the dashed box. For applications, that use an amplifier with a pair of oppositely phase drive signals A 1 , A 2 , the first L 1  and second L 2  coils are wound along the coil tunnel in opposite phase to ensure that the drive forces applied to the deflectable armature portion are in-phase.  
      In  FIG. 4  C 1  is preferably much larger than C 2 , and preferably C 1  is larger than 1μ. In addition, L 1  has an inductance preferably being approximately twice the inductance of L 2 . As a result, C 1  in series with the first drive coil L 1  will ensure that the first drive coil L 1  receives substantial all drive current from the A 1  terminal of the class D amplifier through the entire audio frequency range. Conversely, the smaller capacitor C 2  in series with the second drive coil L 2  ensures that only an upper audio frequency range portion of the signal drive current delivered by the A 2  terminal of the class D amplifier is applied to the second drive coil L 2 . At audio frequencies where the impedance of capacitor C 2  is of about equal value or smaller than the impedance of the second drive coil L 2 , a main portion of the drive current available to the miniature moving armature receiver will flow through the second coil L 2  since the impedance of this coil L 2  is lower than the impedance of the first drive coil L 1 .  
       FIG. 5  shows a schematic illustration of another preferred embodiment. In  FIG. 5 , the miniature moving armature receiver comprises two integrally positioned class D amplifiers A 1  and A 2 . A pair of differential output terminals of a first class D amplifier A 1  is operatively connected to a the first drive coil L 1  and pair of differential output terminals of a second class D amplifier A 2  is operatively connected to a the second drive coil L 2 .  
      The embodiment of  FIG. 5  is illustrated with an active digital frequency dividing network FD that splits the incoming digitally coded input signal into two or more frequency ranges that are applied to a respective digital class D amplifier A 1 , A 2 . An advantage of this embodiment is that the previously mentioned components of the frequency dividing network such as capacitors can be entirely omitted since their function is replaced by digital logic that can be integrated together with one or several digital Class D amplifiers on a common CMOS integrated circuit.  
      However, the frequency dividing network of  FIG. 5  could alternatively be implemented as either a passive or an active network or even a combination of both. The passive network may comprise a capacitor in series with the second coil L 2  to ensure that the second coil receives substantial drive current solely in the upper audio frequency range, see description in relation to  FIG. 2  for further details. The drive first coil L 1  may advantageously be driven across the entire audio frequency range by direct connection to the first class D amplifier. In the upper audio frequency range the first L 1  and second L 2  coils cooperate to apply drive force to the armature.  
      The receiver embodiment illustrated in  FIG. 5  comprises a digital data input section I 2 S adapted to receive a digitally coded input signal according to a I 2 S digital audio protocol. Alternatively, a digital data input section may be adapted to receive digital audio signals coded according to a serial data protocol such as IIC, SPI or other digital audio protocols, for example SPDIF.  
       FIG. 6  illustrates a hearing aid output stage, solid box, adapted to drive a hearing aid moving armature receiver, dashed box, with first L 1  and second L 2  drive coils. Preferably, the output stage is formed integral with the hearing aid circuitry that will normally comprise a DSP (Digital Signal Processor). Thus, the output stage is adapted to receive a digital audio input signal. The output stage comprises a digital frequency divider circuit FD adapted to split the received digital audio input signal into first and second digital audio signals of first and second frequency ranges, respectively. The first and second digital audio signals are fed to digital class D amplifiers A 1 , A 2 , respectively. These amplifiers A 1 , A 2  amplify the received first and second digital audio signals and apply first and second amplified audio signals to first and second output terminals of the output stage, respectively. The schematic illustration indicates a four-terminal connection between the output stage and the receiver. However, using the principles illustrated in  FIG. 4 , a three-terminal version may be implemented.