Patent Publication Number: US-6904156-B1

Title: System and method for reducing hearing aid squeal

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to hearing aid amplifiers. 
   2. Description of Related Art 
   A hearing aid is typically comprised of a microphone which receives an acoustic input signal and converts it into an electrical signal, a filter which processes the signal, an amplifier which produces an amplified output signal, and a speaker which delivers the output signal. 
   With many hearing aid designs, especially those including Class D amplifiers, the amplifier will lose gain control under low battery conditions which may produce a loud squeal or gun shot noise. This may occur as the battery charge depletes with use, and also when a loud tone enters the hearing aid at a low frequency, causing the battery voltage to momentarily drop. Some hearing aids have been equipped with a latch which disables the hearing aid output when the amplifier oscillator frequency reaches a certain level. These prior art latches have not been completely effective, as latch activation can erroneously disable the hearing aid during a power supply transient in addition to a true low battery condition. 
   SUMMARY OF THE INVENTION 
   In one embodiment, the invention includes a circuit for reducing squeal comprising a battery supplying a battery output voltage, a voltage sensor having an output dependent on said battery output voltage, an audio amplifier, a cutoff circuit connected to substantially disable the audio amplifier in response to the voltage sensor output, and a crowbar circuit connected to load the battery in response to the voltage sensor output. 
   In another embodiment, the invention includes a hearing aid comprising an audio amplifier having an audio input, a power supply input, and an amplification input, and having an output. The hearing aid further comprises a microphone connected to the audio input of the audio amplifier, a speaker connected to the output of the audio amplifier, a battery having an output voltage, a voltage sensor having an input receiving the modified voltage and having a cutoff output, and a crowbar output, wherein the voltage sensor determines if the battery output voltage is below a threshold voltage. The hearing aid further comprises a cutoff circuit having an input connected to the cutoff output, and having an amplification output connected to the amplification input of the audio amplifier, such that in response to the cutoff output indicating the battery output voltage is below the threshold voltage the amplification output substantially disables the audio amplifier and a crowbar circuit having an input connected to the crowbar output, and connected to load the battery with a circuit element in response to the crowbar output indicating the battery output voltage is below the threshold voltage. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a block diagram of the components within a hearing aid, including a squeal stopper circuit. 
       FIG. 2  is a high level block diagram of the components within the squeal stopper circuit of FIG.  1 . 
       FIG. 3A  is a schematic of a squeal stopper circuit in one embodiment of the invention. 
       FIG. 3B  is a schematic of a second embodiment of a squeal stopper circuit. 
       FIG. 4A  is a schematic showing a first embodiment of a squeal stopper output coupled to a hearing aid. 
       FIG. 4B  is a schematic showing a second embodiment of a squeal stopper output coupled to a hearing aid. 
       FIG. 5  is a graph illustrating operational characteristics of the squeal stopper circuit of FIG.  3 A. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Embodiments of the invention will now be described with reference to the accompanying Figures, wherein like numerals refer to like elements throughout. The terminology used in the description presented herein is not intended to be interpreted in any limited or restrictive manner, simply because it is being utilized in conjunction with a detailed description of certain specific embodiments of the invention. Furthermore, embodiments of the invention may include several novel features, no single one of which is solely responsible for its desirable attributes or which is essential to practicing the inventions herein described. 
     FIG. 1  is a block diagram of the components within a hearing aid, including a squeal stopper circuit  20 . Microphone  22  receives audible sounds and converts the sounds to electrical signals which are transmitted to audio amplifier  18 . Audio amplifier  18  increases the strength of the electrical signals received from microphone  22  and sends the amplified signals to speaker  16 . Speaker  16  converts the electrical signals to audible sounds which the hearing aid user may hear. Battery  12 , which may be any suitable hearing aid battery, is coupled to a power supply  14 . A typical hearing aid battery  12  may produce an output voltage at node  13  of about 1.4 volts with ESR of approximately 22 ohms when new, and may drop to less than 1.0 volt during its useful life of approximately one month (depending on the battery quality and hearing aid power requirements). Power supply  14  typically includes a charge pump which provides a voltage  15 , which is a multiple of the battery  12  voltage, to the squeal stopper circuit  20 . In one embodiment, power supply  14  provides a voltage that is three to four times larger than the battery  12  voltage, however, it is contemplated that different multiples of the battery  12  voltage may be provided by power supply  14 . It will also be appreciated that in some embodiments the power supply  14  may be omitted entirely. 
   The squeal stopper circuit  20  is coupled directly to the battery output at node  13 , and has an output  24  coupled to the audio amplifier  18 . The squeal stopper circuit  20  reduces squeal generally caused by a low battery in two ways. First, when the battery voltage, and thus the power supply voltage, drops to a first threshold level the output  24  to audio amplifier  18  changes state. In response, the audio amplifier  18  is disabled, thus preventing any signal from reaching the speaker  16 , and preventing squeal. In addition, when the battery voltage drops to a second threshold level, the squeal stopper circuit loads the battery with, for example, a current source or resistive element. This load prevents a bad battery from momentarily recovering voltage and causing the hearing aid to turn back on, potentially causing a squeal. However, a good battery will be relatively unaffected by the additional load and may continue to provide power to the hearing aid for its remaining useful life. 
   Due to size considerations, the audio amplifier, power supply, and squeal stopper circuitry will normally be fabricated as part of a common integrated circuit, although separate circuits may be used. The input and output transducers and battery are typically separate electromechanical components that attach to the integrated circuit via contact pads and/or wires. 
     FIG. 2  is a block diagram of the components within the squeal stopper circuit  20  of FIG.  1 . In one embodiment, the squeal stopper circuit  20  comprises three functional components: a voltage sensor  32 , a cutoff circuit  34 , and a crowbar circuit  36 . Voltage sensor  32  determines if the battery  12  voltage has dropped below one or more threshold levels. In one embodiment, voltage sensor  32  includes a cutoff output  33  coupled to a cutoff circuit  34  that changes state when a first battery threshold level of 0.8 volts is reached. In addition, a crowbar output  35  which is couple to a crowbar circuit  36  changes state when a second battery threshold level of 0.75 volts is reached. 
   As shown in  FIG. 2 , in this embodiment the voltage sensor  32  receives an input from power supply  14 , which provides an amplification of battery  12  voltage. As such, in the embodiment of  FIG. 2 , the voltage sensor determines when the battery  12  voltage has dropped below the determined battery threshold voltages (e.g. 0.8 and 0.75 volts) by monitoring the power supply  14  voltage. For example, in an embodiment where the power supply voltage  15  is four times the battery  12  voltage, the voltage sensor circuit  32  may determine that the battery  12  voltage is at the 0.8 volt first threshold when the power supply voltage is at 3.2 volts (i.e. 4×0.8 volts=3.2 volts). In another embodiment, the first and second threshold levels are reached when the power supply voltage is at 3.0 and 2.75 volts, respectively. It is contemplated that other threshold levels may be set according to the specific hearing aid characteristics, battery magnitude and quality, and other factors specific to the hearing aid. 
   Cutoff circuit  34  has an input coupled to the voltage sensor  32  and an output  24  coupled to the audio amplifier  18 . When the cutoff input  33  indicates that the battery  12  voltage has dropped below a first threshold voltage (0.8 volts in one embodiment) the output  24  of the cutoff circuit  34  changes state. This state change disables the amplifier  18 . The cutoff circuit may include simply a transistor or as a controlled current source. Depending on amplifier design, the cutoff output  24  may interact with the amplifier in different ways. In one embodiment, a transconductance preamplifier is disabled by opening the bias current path. In another embodiment, a “digitally controlled” amplifier is disabled by the output  24  changing state from a non-asserted to an asserted state. These embodiments are illustrated in  FIGS. 4A and 4B . 
   Crowbar circuit  36  has an input coupled to voltage sensor  32  and an output coupled to the battery  12 . As indicated above, the crowbar signal  35  indicates whether the battery  12  has dropped below a second threshold level (0.75 volts in one embodiment). When crowbar circuit  36  senses an input  35  indicating that the battery  12  has dropped below the threshold level, the crowbar output  26  loads the battery with a current source or a resistive element. In one embodiment, a resistor in the 1-10 kohm range has been found suitable for use in the crowbar circuit  36  to load the battery  12 . In an advantageous embodiment of the invention, the battery loading circuit element is a 2000 ohm resistive element. If a current source is used, it may be configured to draw about 500 microamps from the battery to provide the desired loading. As mentioned above, this load prevents a bad battery from momentarily recovering and potentially causing a squeal on the hearing aid speaker  16 . 
   It will be appreciated by those in the art that all voltages and resistor values discussed herein are examples only. It is contemplated that any battery voltage may be implemented in the present invention. In addition, the first and second threshold levels may be set to any value, and the threshold levels may be in reference to the battery voltage, the power supply voltage, or any combination of the two. 
     FIG. 3A  is an electrical schematic showing one embodiment of a squeal stopper circuit. The squeal stopper circuit  20  provides an output  24  to the audio amplifier  18  (not shown in  FIG. 3 ) and receives as inputs the voltage of battery  12  (at node  13 ), and the power supply voltage  15 . Audio amplifier  18  is enabled and disabled according to the state of output line  24 . As shown in  FIG. 3 , the power supply  14  is coupled to battery  12  at node  13 , As such, the power supply  14  receives the battery voltage and amplifies the battery output voltage by a predetermined multiplier. The power supply output at node  15  is coupled to a diode bank  42  and resistor  44  at node  41 . Diode bank  42  is comprised of one or more series diodes to obtain a desired voltage drop. Five diodes are shown but more or fewer diodes may be employed depending on the application. 
   When a fresh battery  12  is in the hearing aid (e.g. a battery with voltage above both the first and second threshold levels), the audio amplifier  18  is operating at a determined amplification level, and the squeal stopper circuit draws only a small amount of current from the battery. With a fresh battery, the voltage at node  43  is still high enough after the drop across the diode bank  42  to force cutoff transistor  54  and inverting transistor  52  to the on state. When inverting transistor  52  is in the on state, crowbar transistor  50  is turned off because the on state of inverting transistor  52  ties the gate of crowbar transistor  50  to ground  60 . In this state of normal, fresh battery operation, the crowbar circuit is open such that the battery is not affected, and the state of the output line  24  is grounded through cutoff transistor  54 . It may be desirable, but not necessary, to include a filter capacitor  53  on the gate of crowbar transistor  50 . 
   During normal operation, current is drawn from the power supply to ground through two pathways. One is through the diode bank  42  and resistor  46 . The other is through resistor  44  and inverting transistor  52 . In advantageous embodiments of the invention, resistors  44  and  46  are each at least about 100 megohms, thus limiting the total current draw from a 5 V power supply to no more than about 100 nanio-amps. In one suitable embodiment, approximately 200 megohm resistors are used for both resistors, whereby with a 5 V power supply output, each resistor draws about 25 nanoamps from the power supply. In another embodiment, resistor  44  is about 200 megohms, and resistor  46  is about 100 megohms. In these embodiments, therefore, during normal operation of the hearing aid when the power supply voltage  15  is about 2.5-5 volts, the squeal stopper circuit  20  draws a total of less than about 50-75 nanoamps from the battery  12 , making it substantially transparent to the remainder of the circuit, and not causing a significant reduction in battery life. 
   When the battery  12  voltage decreases over time to a first threshold level (e.g. about 0.8 volts), the cutoff transistor  54  begins to turn off as its gate receives a decreased voltage through diode bank  42 . At the same time, inverting transistor  52  begins to turn off. As the battery voltage reaches a second threshold level (e.g. about 0.75 volts), the increase in voltage on the gate of crowbar transistor  50  due to the increase in impedance of inverting transistor  52  turns on the crowbar transistor  50 . When the crowbar transistor  50  is fully on, the battery is shunted to ground through resistor  48 , which as discussed above, may be about 1000-10,000 ohms, with 2000 ohms having been found suitable in one embodiment. In addition to a resistor  48 , other battery loading circuit elements are possible to be used in place of the resistive element shown in  FIG. 3A , such as a current source. In one embodiment, the crowbar circuit stresses the battery  12  by drawing approximately  500  microamperes. By loading the battery with the resistor  48 , the crowbar circuit  36  causes the voltage of a weak battery to drop even further, and thus will advantageously prevent a weak battery from momentarily recovering. However, if a fresh battery is installed, and the battery voltage drop is due solely to a particular audio input signal, the additional load provided by the crowbar circuit will have no substantial effect. With this design, an old battery is prevented from repeatedly dropping and recovering in response to audio inputs with the associated risk of repeated squealing or gunshot noise. Instead, an old and weak battery will be loaded by the crowbar circuit such that recovery is impossible, and will remain loaded until the battery is replaced with a battery that is strong enough to raise the potential at node  43  of  FIG. 3A  enough to turn on the inverting and cutoff transistors, and thus to turn off the crowbar transistor  52 . It can be seen that if the battery voltage drops below the threshold of transistors  52 ,  54 , then no current will flow either. 
   In the embodiment of  FIG. 3A , the threshold levels are determined mainly by the characteristics of the diode bank  42 , the resistors  44  and  46 , and the transistors  52  and  54 . When implemented on an integrated circuit, the diode type (which is PIN type in one suitable embodiment) as well as the areas, lengths, etc. of these components can be designed to provide a desired relationship between the battery output voltage and the voltages on the gates of the cutoff, inverting, and crowbar transistors. The resistors may, for example, be fabricated from high resistance polysilicon regions of the integrated circuit. The diode characteristics are particularly significant, and may be modified within standard fabrication techniques to produce the desired voltage drops between the power supply output  15  and node  43 . Response characteristics for the crowbar portion of the circuit will also be affected by the dimensions of the inverting transistor and crowbar transistor. The dimensions of the cutoff transistor  54  are advantageously selected to produce a low source-drain on state resistance. In one embodiment, the circuitry of squeal stopper  20  may be implemented on a 200 micrometer by 300 micrometer area of a semiconductor chip. 
     FIG. 3B  illustrates a second embodiment of a squeal stopper circuit. This embodiment shares many characteristics with the embodiment of  FIG. 3A , but in this case, the gate of the cutoff transistor  54  is held at a slightly higher voltage than the gate of the inverting transistor  52  by the diode drop from diode. 59 . This provides further time separation between the on state of the crowbar transistor  50  and the off state of the cutoff transistor as the battery voltage drops. In addition, in the embodiment of  FIG. 3B , current draw in normal operation is controlled by 25-50 nanoamnp current sources  61 ,  63 , rather than by simple resistive elements. As mentioned above, cutoff circuit element  54  may be embodied as a current source. 
     FIG. 4A  is a schematic illustrating one embodiment of a squeal stopper output  24  coupled to an audio amplifier  18 . In the embodiment of  FIG. 4A , cutoff output  24  provides the ground path for the bias current of a transconductance amplifier. In this embodiment, the gain of audio amplifier  18  is controlled by a bias current to ground through current source  66 . As the impedance of the cutoff transistor  54  increases, reducing the gain of the audio amplifier  18  to essentially zero when the cutoff transistor  54  is completely off. 
     FIG. 4B  is a schematic illustrating a second embodiment of a squeal stopper output  24  coupled to an audio amplifier  18 . In the embodiment of  FIG. 4B , the audio amplifier has a digital enable input  65  that enables amplifier operation when low, and disables amplifier operation when high. The cutoff line  24  is tied to the power supply output  15  through a pull-up resistor  61 . During normal fresh battery operation, cutoff transistor is open, and ties the cutoff line  24  to ground. As cutoff transistor  54  becomes closed due to decreasing power supply voltage  15 , the cutoff line is pulled high to the power supply output through resistor  61 . 
     FIG. 5  is a graph showing the voltage levels at various nodes of the circuit of  FIG. 3A , over a period of time as the battery voltage drops. The graph illustrates for the sake of demonstration the behavior of the circuit when node  15  and node  24  are resistively coupled. The vertical axis of  FIG. 5  is in units of volts, while the horizontal axis is in time units. This graph illustrates a rapid drop in battery voltage over the course of about 100 milliseconds, which may be due, for example, to attempting to amplify a high amplitude, low frequency input signal. It will be appreciated, however, that the time scale of voltage drop may be different, and at least portions of the voltage drop can occur slowly over days or weeks of battery use. In addition, while the voltages shown in  FIG. 5  approximate actual voltages used in some embodiments of the invention, they are intended only to serve as exemplars of the operation of the present invention, and not to limit the invention to the specified voltages and/or time scales. 
   Turning back to both  FIGS. 3 and 5 , curve  70  shows the voltage at node  15 , which is the output voltage generated by the power supply  14 . Curve  72  shows the voltage at node  24 , which is the voltage level on the cutoff line. Curve  74  shows the voltage at node  13 , which is the battery output voltage. Finally, curve  76  shows the voltage at node  40 , which is the voltage on the grounded side of crowbar resistor  48 . 
   As seen in  FIG. 5 , at t=0 node  13  is at approximately 1.35 volts, node  15  is approximately 4.5 volts, and node  24  is approximately  0  volts (since it is tied to ground through closed cutoff resistor  54 ). Node  15  is a multiple of node  13 , through the operation of power supply  14 , and, thus, the voltages at the two nodes change proportionally. When node  15  approaches approximately 3 volts, the cutoff transistor  54  begins to open, and the voltage at node  24  (or output signal  24 ) begins to increase. As the power supply voltage  15  continues to decline, transistor  54  continues opening until it is completely open, causing output signal  24  to have a voltage substantially equal to power supply voltage  15 . Nodes  24  and  15  then decrease simultaneously. 
   Shortly after cutoff transistor  54  begins to approach a completely off high impedance state when power supply  15  voltage reaches approximately 2.6 volts, crowbar transistor  50  begins going to low impedance due to the operation of inverter transistor  52 . Node  40  then begins to decrease in voltage as crowbar transistor  50  continues closing and grounding this node. The voltage at node  40  continues to decrease, as transistor  50  continues to close, until the voltage at node  40  reaches approximately zero volts. 
   Once the crowbar transistor  50  is fully on, the behavior of the system will depend on whether the battery is old and weak or fresh. If the battery is fresh, the load from crowbar resistor  48  will not prevent the battery voltage from recovering and going back up after a transient load. 
   On the other hand, with a weak battery, the voltage at node  13  will continue to drop due to the additional crowbar resistor load as illustrated by curve  74   a . This will also reduce the power supply output voltage curve  70  at node  15 . It can be recognized that eventually curve  70  meets curve  74  and eventually transistor  50  shuts off beyond the useful range of interest. 
   The foregoing description details certain embodiments of the invention. It will be appreciated, however, that no matter how detailed the foregoing appears in text, the invention can be practiced in many ways. As is also stated above, it should be noted that the use of particular terminology when describing certain features or aspects of the invention should not be taken to imply that the terminology is being re-defined herein to be restricted to including any specific characteristics of the features or aspects of the invention with which that terminology is associated. The scope of the invention should therefore be construed in accordance with the appended claims and any equivalents thereof.