DE2517993A1 - AMPLIFIER WITH EXTREMELY LOW POWER CONSUMPTION - Google Patents

Description

G 49 851 / K *

DAHLBERG ELECTRONICS, Inc., 7731 Country Club Drive, MINNEAPOLIS, Minnesota / USA

Extremely low power consumption amplifier

The invention relates generally to the field of electronics Amplifier circuits for hearing aids and, in particular, a power amplifier with extremely low power consumption for operation of the receiver of a hearing aid. Such an amplifier circuit must have a high efficiency and an extremely small current flow without adversely affecting the output volume.

The main goal of the hearing aid industry is to create such devices that can cure the suffering of various types of hearing loss reproduce speech hearing ability to sufferers at normal speech levels, with the devices requiring the lowest possible cost and cause the user the least possible level of discomfort. To this end, much effort has been made, to reduce the size of the hearing aid and to improve the intelligibility of normal speech for the user. Numerous previous ones Further developments in the formation of a hearing aid circuit unfortunately led to a greater number of electronic ones Components, both physical size and operating current of the circle enlarged. This also led to an increased output volume and improved intelligibility circles designed by cheaper signal processing techniques greater power consumption and shorter battery life.

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Of course, there is a decrease in power consumption for the purpose of enlargement The battery life is desirable in order to keep the battery cost to the user as low as possible, in some cases can amount to around 2 - 4 DM per week. Not only is the cost of replacing the battery important, but also the necessity Frequent battery replacement can be annoying or inconvenient for the user. With some known hearing aids The battery may need to be replaced every few days, and this need can be a concern for the user because he has to constantly check the availability of replacement batteries in order to avoid unpleasant situations in the event of a dead battery to avoid.

In a known type of hearing aid amplifier, an output transistor operating in A mode is used, the receiver being connected as the collector load of the transistor. In such a circuit, with a moderately high output power (approx. 115 db sound pressure level at saturation with 38 db acoustic gain), a current of approx. 0.5 milliamps is drawn, of which 80% is consumed in the output stage. If the output transistor is biased so that it operates somewhere near its center point to avoid clipping and distortion at high output levels, the transistor must be biased at its quiescent point so that about half the battery voltage at the output transistor drops while the other half lies on the recipient. In order to avoid the flow of excessively large quiescent currents, it is necessary to use a receiver with a relatively large DC resistance of typically 900 ohms. However, this leads to a circuit with low efficiency, since when amplifying a hearing signal, a large part of the power is consumed in the ohmic component of the receiver impedance. Such a circle gives a life of about 350 to 400 hours for a certain type of Mercury battery.

In another type of hearing aid circuit, power consumption is reduced to about 0.25 milliamps, but only at the expense of a reduced maximum sound pressure level output. This kind of

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Kreisen also uses an output transistor that works in Α mode and can run for around 800 hours on the same type of battery, yet the maximum output is at a sound level of about 10 db with an acoustic gain of 27 db. This known circle can therefore be useful for people who do not require a large amount of gain, but of course this circuit does not solve the problem of excessive battery drain in more powerful amplifiers.

The object of the present invention is to avoid of the disadvantages described to create an improved hearing aid amplifier of the type mentioned.

According to the invention, a power amplifier with extremely low power consumption for operating the receiver of a hearing aid is proposed, using a complementary pair of transistors, each transistor being a dynamic load on the represents another transistor. The collectors of the transistors are connected together, and their emitters lead to the corresponding ones Pole connections of a battery so that a current can flow through the transistor pair in the forward direction. Furthermore are Means are provided for controlling the bias current of the transistors. A partially amplified signal from a preamplifier is called the Base terminal of the transistors supplied. The receiver and a blocking capacitor are between a signal ground and the common Connection between the collectors of the transistor pair connected in series. In detail, according to the present invention, there is a power amplifier Proposed with extremely low power consumption for operating the receiver of a hearing aid, which is characterized by a DC power source, by a pair of complementary transistors, their collectors with each other and their emitters are connected to the respective pole terminals of the DC power source to allow current to flow through the pair may, by means connected to at least one of the transistors for controlling the bias current through the pair, by means for Applying an AC voltage signal to be amplified to at least one of the transistors, through a blocking capacitor and

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by means of connecting the blocking capacitor and the Receiver in series with the couple's collectors.

In a preferred embodiment, the voltage generated at the blocking capacitor can be used for negative feedback, this negative feedback voltage to the input of the preamplifier can be fed. The negative feedback is not only used to adjust the gain of the amplifier, but it forms also a particular degree of low-frequency attenuation or a reduced response behavior, which is necessary for correcting certain Types of hearing loss is beneficial.

The amplifier circuit according to the invention has such a low power consumption that it is more than three times as long with a battery how known hearing aids with comparable output levels work. The reduced power consumption of the hearing aid circuit does not mean anything Disadvantage for the maximum sound pressure level output. For example, in a circle according to the invention, a maximum average Sound pressure level of 116 db (with peaks of 12o db) achieved with a current of only 0.15 milliamps (1.35 volts battery voltage). With the same battery as in the previous examples, this results in a lifespan of 1300 hours, which is more than corresponds to three times the time of conventional, comparable hearing aids.

The invention is illustrated below on the basis of exemplary embodiments shown in the drawings explained in more detail. Show it:

Figure 1 - a schematic circuit diagram of a preferred embodiment of the present invention, Figure 2 - a schematic circuit diagram of an alternative embodiment of the circuit of Figure 1 and Figure 3 - a graphical representation of the output characteristics of the Circle from Figure 1.

In the extremely low-power hearing aid amplifier from FIG. 1, the Circuit powered by a battery Io as a DC voltage source. The negative connection of the battery Io is connected to a signal ground

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se serving line 11 is connected, and the positive terminal of the battery Io leads via an on-off switch 12 to one as B + Supply for the circuit serving line 13. With the reference number 14 is a microphone that picks up the noises to be amplified with which they are converted into electrical signals appearing on a line 15. The microphone is also with the Signal ground line 11 connected to form a reference potential for the signal on line 15. In addition, the microphone be an electret or ceramic microphone, with a bias potential being fed from line 13 to microphone 14 will. This bias voltage is used to supply an internal preamplifier used with this type of microphone.

The output line 15 of the microphone is through a capacitor 16 coupled to an adjustable tap of a potentiometer 17, which is used for volume control. The two ends of the potentiometer 17 are on the one hand with the signal ground on the line 11 and on the other hand via a capacitor 2o to the base an npn transistor 21 is connected. The emitter of the transistor leads to signal ground, and its collector is connected to the positive voltage of line 13 via a resistor 22. The collector of the transistor 21 is also connected via a line 23 to the base of a transistor 24, the emitter of which for Signal ground leads and whose collector is connected to line 13 via a resistor 25.

The output stage contains a complementary pair of transistors 3o and 31. The emitter of the pnp transistor 3o leads to line 13, while the emitter of the npn transistor 31 to the signal ground, namely the line 11 is connected. The collectors of the two transistors are connected to one another by a line 32, and a bias resistor 33 connects the base terminals of the Transistors 3o and 31. The collector of transistor 24 leads directly via a line 34 to the base of transistor 31 and also Via a capacitor 35 to the base of the transistor 3o, the capacitor 35 effectively bridging the bias resistor 33.

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A receiver 40, which is preferably a low impedance device as discussed below, is connected to the output connected to the amplifier. The receiver is the converter for converting the amplified electrical signals into amplified ones acoustic signals to be picked up by the respective person. One connection of the receiver 4o is connected to the line 32, while the other terminal is connected to a line 41-. A capacitor 43 is parallel to the receiver 4o between lines 32 and 41, and another capacitor is connected between line 41 and signal ground. A Resistor 44 connects line .41 to the base of transistor 21, while resistor 45 connects the base of transistor 21 connects to the signal ground.

In general, form the transistors 21 and 24 and the one assigned to them Circuit a preamplifier that amplifies the signals from the microphone. The transistors 3o and 31 and the associated circuit generally form the output amplifier for operating the receiver 4o. In practice, the sizes or values of the electrical Circle selected so that transistors 21 and 24 are biased for A operation. The resistor 33 is selected accordingly, to bias the transistors 3ο and 31 through the current path to the emitter-base junction of the transistor 3o, resistor 33 and base-emitter junction of transistor 31 to control

The voltage on the line 41 is via a voltage divider to Input of the preamplifier fed back. This feedback voltage consists of a direct voltage component passing through the receiver 4o from the line 32 and one across the capacitor falling AC voltage component. The DC voltage component is used to determine the operating point of the output transistors

while the AC component is used to control the overall gain of the circuit and the desired To form frequency dependence. The feedback tension texler contains the resistors 44 and 45, the ratio of which is the proportion of the Voltage on line 41 is determined, which is fed to the base of transistor 21. Although changing the value of any of the

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These resistances lead to a change in the overall operating characteristics leads, in practice it is appropriate to adjust or select resistor 45 to the desired quiescent operating point voltage of the output on line 32. The feedback resistor 44 can then be selected or adjusted to adjust the overall AC gain of the circuit. Of course, the person using the hearing aid can adjust the volume of the Set the sound output in a given situation by actuating the potentiometer 17.

The open circuit voltage on line 32 is set to half the supply voltage. The capacitor 42 performs the function of a Blocking any steady state DC current through while providing an AC signal ground to the receiver represents. Therefore, there is no DC voltage path through the receiver to ground (with the exception of the negligible current through resistors 44 and 45), and only a very small current flows under idle conditions. Since by the recipient no significant Direct current flows, the efficiency of the circuit is improved.

if there is an alternating voltage signal to be amplified, it is coupled via the capacitors 16 and 20, amplified by the transistors 21 and 24, in each case with phase reversal, and then fed directly to the base of transistor 31. The capacitor 35 lying parallel to the resistor 33 forms

AC voltage bypass for the resistor. Therefore, the signal applied to the base of the transistor 31 becomes through the capacitor 35 also fed to the base of the transistor 3o, which applies at least to the important AC voltage frequencies. This bypass capacitor is not necessary for operation, but useful for reducing the distortion or the THD. The transistors 3o and 31 are operated in such a way that one transistor continues to be conductive, while the other during a half-cycle of the alternating voltage cycle conducts less strongly what happens during the other half cycle of the cycle is reversed. Therefore, each transistor forms a dynamic load for the other transistor, and

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this factor also leads to an improved efficiency. The voltage at the collectors connected together via the line 32 fluctuates accordingly as a function of the amount supplied Signal. This amplified signal on line 32 passes through receiver 40 and capacitor 42 to signal ground. The value of the capacitor 43 is chosen so large that it is not noticeable Influences the signals in the desired audible range, which is between approximately 25o Hz and 4,000 Hz. Forms over 4ooo-5ooo Hz the capacitor 43 provides a shunt path for the receiver 4o, whereby the gain of the circle at high frequencies is greatly reduced in order to increase the circle stability.

As in contrast to the well-known circles described above No direct current flows into the receiver, there is no need to maintain a large impedance in the vaccine. On the other hand, a recipient can be small Lower impedance result in greater output power and greater efficiency by reducing the losses part of the signal due to the resistance component of the receiver winding. In practice, receivers have low impedance of about 1,000 ohms and high quality or a high Q value has been found to be satisfactory. In addition, the complementary forms Output circuit of the transistors 3o and 31 a source of low impedance for operating the receiver, thereby increasing the reproduction quality improved and distortion reduced.

In Figure 3, as a graphical operational representation of a circle like that of Figure 1, curve 50 shows the output as a function the frequency for an input field of 60 db, while curve 51 applies to an input field of 80 db. The curve 5o shows the rapid drop in response behavior above 4ooo-5ooo Hz and further the strong damping below 8oo Hz. Elevations due to resonance phenomena of the receiver when operating a normal hearing aid coupling are more balanced. The area of interest is that of the fast or strong bass attenuation below about 8oo Hz, where the response characteristic drops by up to about 2o db at 5oo Hz. The recipient alone would

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produce a drop of about 6 db per octave. By taking advantage of it the negative feedback voltage across capacitor 42 from FIG. 1 however, a faster drop of around 12 db per octave is achieved. If necessary, even a greater drop can be achieved, for example by suitable choice of the capacitor 2o or the receiver impedance.

In many common types of hearing loss, hearing at higher frequencies is severely impaired, while hearing sensitivity is impaired at lower frequencies is the same as for normal people. If the response gain is uniform over the frequency is used, the gain required to make the high frequencies audible results in the low-frequency noise can be raised to an unbearable level. Of course, this would be a major discomfort for them user mean and also severely impair the intelligibility of the signal, due to excessively louder Hum components in the amplified sound information. The starting circle of Figure 1 accordingly leads to two desirable results, namely to a reduction in power consumption and to the desired frequency response through frequency selective feedback. This is all done by a minimal number achieved by components.

FIG. 2 shows an alternative output connection for the basic circuit for an application in the case of very severe hearing loss where it is necessary to achieve a large gain over the entire usable frequency band, including the lower frequencies. In FIG. 2, the components marked with apostrophes correspond to the corresponding components in FIG. 1. Thus, the complementary output transistors ³ o 1 and 31 ″ generate the output signal on line 32 ′, which is connected to the two collectors connected together. In the circle from FIG However, the blocking capacitor 42 "is only used to block the DC bias component and not to form the frequency-selective feedback voltage. The feedback voltage is instead applied to a simple

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Voltage divider removed from resistors 62 and 63. The from voltage taken from the resistor connection on line 41 ' is fed back to the preamplifier via resistor 44 '. Naturally affects the size ratio of the resistors 62 and the gain of this circuit, as it does for the ratio of the Feedback resistor 44 "and resistor 45 'applies. Da the feedback is removed from an ohmic voltage divider from FIG the feedback is not frequency dependent. The low frequency dependence of the circuit is mainly due to determines the properties of the receiver 4o '. However, if it is desired for a given type of hearing loss, a dashed line can be used Wise connected capacitor 64 can be used to short the resistor 63 at higher frequencies. The value of Feedback resistances are set as required is to achieve the desired belt center reinforcement. Then, at low frequencies, the degree of negative feedback or negative feedback due to the increasing impedance begins of capacitor 64 will rise and the overall gain of the loop will decrease.

In the following, to explain a typical circle from FIG 1 whose exemplary component values are given: R17 - loKOhm; R22 - looKOhm; R25 - looKOhm; R33 - loKOhm to 1 MOhm, selection of preload; R45 - looKOhm to 1 MOhm, selection the DC operating point; R49 - about 27o KOhm, select the desired gain; all capacitors - 1 microfarad. In figure 2, for example, R63 is 100K ohms and R62 is used to select the desired gain.

If the circle from FIG. 1 is suitably set, it has during the idle mode on lines 23 and 34 a voltage of about 0.55 volts. The voltage at the base of transistor 3o is about 0.75 volts, and the voltage on line 32 is about half the battery voltage.

While the circuit is normally with the output transistors in A mode is working, it is possible to operate the circuit in AB mode

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to drive. This is achieved by changing the value of the resistance 33 is decreased to increase the bias current. In In this operating mode, the output circuit can draw additional current from the battery in the event of large AC signals. The AB operation enables a greater maximum output power and a greater degree of efficiency with this power.

In Figure 1 it is possible to remove the resistor 33 and the capacitor 35, the base connections of the transistors 31 and 3o would be directly connected to each other. In this case, the bias current can be achieved by adding a resistor in the line 13 is arranged in series with the positive terminal of the battery and the remaining part of the amplifier circuit. The voltage drop across this resistor as a result of the current drawn by the amplifier gives the bias current for the output transistors. However, the efficiency is somewhat reduced by the loss in resistance of the power supply.

Overall, the present invention includes a highly effective, Low-current amplifier circuit with improved selective frequency dependence for use in hearing aids.

- patent claims -

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Claims (1)

Claims 1J power amplifier with extremely low power consumption for operating the receiver of a hearing aid, characterized by a direct current supply source (Ιο), by a pair of complementary transistors (3o _f 31; 3ο ¹ , 31 ') r their collectors with each other and their emitters with the corresponding pole connections of the direct current supply source are connected so that current can flow through the pair, by means (33, 33 ') connected to at least one of the transistors for controlling the bias current through the pair, by means (21, 24) for applying an alternating voltage signal to be amplified to at least one of the Transistors, through a blocking capacitor (42, 42 ') and by means (32, 32') for connecting the blocking capacitor and the receiver (4o, 4o ") in series with the collectors of the pair. 2. An amplifier according to claim 1, characterized in that the means for controlling the bias current has one between the base terminals of the transistors (3o, 31; 3o ', 31 ") switched resistor (33, 33 '). 3. Amplifier according to claim 2, characterized by means (35, 35 ') for bridging the resistor (33, 33') with respect to the AC voltage signals to be amplified. 4. Amplifier in particular according to claim 1 for a hearing aid with a microphone and a receiver, characterized in that that a preamplifier (21, 24) is connected to the microphone (14) for amplifying the signals generated by this, that the pair of complementary transistors form an output stage that has means for coupling the signals from the preamplifier to the Base connections of the complementary transistors are present, furthermore means with an output capacitor (42, 42 ·) for connection of the receiver (4o) from the collectors of the pair to signal ground and means (44, 44 ') for forming a negative feedback from the output of the amplifier circuit to the preamplifier. - 13 - 509848/0942 5. Amplifier according to claim 4, characterized in that the negative feedback means a direct current negative feedback from the collectors of the transistor pair (3o _f 31; 3o ', 31') to the preamplifier (21, 24) for controlling the direct current operating point of the transistor pair and an alternating current negative feedback from the output capacitor ( 42, 42 ') to the preamplifier to form a frequency-selective negative feedback in order to reduce the response behavior at low frequencies. 6. Amplifier according to claim 4, characterized in that one terminal of the output capacitor (42) is connected to the signal ground and the receiver (4o) in series between the collectors of the transistor pair (3o, 31) and the output capacitor (42), the negative feedback means (44) being connected to the common connection of the receiver and the output capacitor are connected. 7. An amplifier according to claim 6, characterized in that the means for controlling the bias current has one between the base terminals of the two transistors (3o, 31) have connected bias resistor (33). 8. Amplifier according to claim 7, characterized by a bridging capacitor connected via the bias resistor (33) (35, 35 ') 9. Amplifier according to claim 4, characterized by one of the collectors of the transistor pair (3o, 31; 3ο ¹ , 31 ") leading to the preamplifier ohmic voltage divider (44, 45; 44 ', 45') .O. Amplifier in particular according to one of claims 1-9 with a high degree of efficiency, characterized in that the receiver (4o, 4ο ¹ ) converts the amplified signals into sound signals, that means (21, 24) form the base of a first transistor (3o, 3o "; 31, 31 ') provide the AC voltage signals to be amplified that means are provided with a load for connecting the first transistor to the DC power supply source - 14 - 509848/0942 are that a blocking capacitor (42, 42 ') in series with the Receiver is between the first transistor and a terminal of the DC power source and that the load is one to the first transistor-complementary second transistor (31, 31 '; 3o, 3o '), the base of which is to receive the to be amplified AC voltage signal is connected, whereby the second transistor for the first transistor a dynamic Load represents. 11. Amplifier according to claim lo, characterized by a bias resistor (33, 33 ') connecting the base terminals of the first and second transistors (3o, 3ο ¹ ; 31, 31') for controlling the bias current flowing through. 12. Amplifier according to claim Io, characterized in that the Means for supplying the alternating voltage signals to be amplified have a preamplifier (21, 24) and that feedback means (44, 44 ') for returning part of the on the condenser (42, 42 ') appearing signal are provided to the preamplifier, whereby an improved low-frequency Response drop or damping behavior is achieved. 509848/0942