DE2353696A1 - Hearing aid with transistor amplifier - has automatic gain control with certain response and oscillation decay time - Google Patents

Abstract

Means are provided for the response and for oscillation decay time control. The multistage transistor amplifier AGC is operated by the working point of a preamplifier transistor, or by a d.c. control voltage varying a negative feed-back. This control voltage is obtained by collecting a part of the signal a.c. voltage from the output amplifier output stage, or from a preceding stage. This is rectified and smoothed by an RC network affecting the AGC response and oscillation decay time. The above means control the RC network time constant. The RC network consists of at least one series resistor and one shunt resistor, and the means control the capacitance of at least one shunt capacitor.

Description

37/73 PLI Fs / Li

10/25/1973

ROBERT BOSCH ELEKTRONIK GMBH Berlin and Stuttgart

Device for the hearing impaired with an automatic gain control

The invention relates to a hearing aid device with a transistor amplifier, with an automatic gain control having a certain settling time and a certain settling time is provided.

High quality devices for the hearing impaired often come with an automatic Equipped gain control, which is to protect the wearer of the device, so the hearing impaired, from that the listener of the The hearing impaired device supplies the hearing impaired's ear with an output sound pressure that is too high and has an uncomfortable effect. Also can an automatic gain control for certain types of Hearing impairment improve speech intelligibility.

In a hearing aid device with automatic gain control, the transistor amplifier has a maximum gain as long as no or only a low input sound pressure falls on the microphone of the device; if the input sound pressure and thus also the output sound pressure exceed a certain threshold, the gain is regulated down according to a certain law. The different types of automatic gain control differ in the threshold of the input sound pressure or the output sound pressure at which the

Down regulation 509819/0 * 56

37/73 _m - 2 -

Down regulation sets in, and by law, according to the gain above this threshold depending on the output sound pressure or the input sound pressure is changed. However, these differences do not play a role in connection with the invention, since the invention is fundamental is suitable for all types of gain control.

If the input sound pressure hitting the microphone is suddenly increased, the amplification increases with a certain amount of time Delay turned down. This time delay in regulating the gain down is referred to below as the settling time the gain control. For the settling time, between about 10 and Values lying 50 ms have been proven.

Even with a decrease in the input sound pressure, the gain should be of the transistor amplifier can be increased with a certain delay in order to allow short pauses when speaking bridge and avoid other distortions. This delay in increasing the gain is said to be the decay time are designated. Values between 100 and 500 ms have proven to be favorable for the decay time.

Settling time and decay time of the gain control are determined by appropriate during manufacture of the hearing aid device Dimensioning of the components is fixed and cannot be changed after the completion of the device.

509819/0456

37/73 - 3 -

In contrast to this, the invention provides that on the hearing aid device Setting means for changing the settling time and / or the settling time of the gain control are provided are.

This is because tests have surprisingly shown that the most favorable values for the settling time and for the settling time in many cases are different for the individual hearing impaired and that you can adjust them individually Values can achieve an improvement in speech intelligibility compared to the normal values specified during production, Nothing definitive could yet be determined about the reasons for this phenomenon. But it seems certain that'in in many cases by customizing both the settling time as well as the decay time improves the performance of the hearing impaired device for the individual hearing impaired can be. The different types of hearing loss should also play a role here.

The invention now gives the audiologist and the hearing aid acoustician the opportunity to determine each time by trial and error when adjusting the hearing aid whether a further improvement in the effect of the hearing aid and speech intelligibility can be achieved by changing the settling time and / or the settling time of the gain control. Certain rules can not yet be drawn up for this. The most favorable values for the settling time and / or the settling time must be determined each time through tests.

509819/0456 -.

37/73 - 4 -

In most cases, automatic gain control takes place of the multi-stage transistor amplifier by means of a control DC voltage, which is obtained by the output stage or a previous stage of the amplifier, a part of the signal voltage is taken, rectified and after the Rectification by an RC network with a gain control that influences the settling time and the settling time Time constants is smoothed.

The DC control voltage can be used in various ways to regulate the gain. For example, it can use the Change the operating point of one or more transistors of the initial amplifier. According to this type, for example the so-called AVC devices are regulated.

The DC control voltage can also be the measure of a signal voltage negative feedback change, for example by influencing a controllable impedance in the negative feedback path. This kind The control can be found, for example, in devices with linear dynamic compression (so-called DRC devices).

The invention is applicable to both types of automatic gain control applicable in that the time constant of the RC network can be set using the setting means.

This setting of the time constant of the RC network can be carried out in various ways. One possibility is, for example _t that, in a series resistance of at least one 509819/0456

37/73 "- '5 -

resistance and at least one shunt capacitor RC network the effective capacitance of at least one shunt capacitor can be changed by the setting means.

The effective capacitance of the shunt capacitor can be changed, for example, by adding several for the shunt capacitor Capacitors of various capacities are provided, which can optionally be switched into the RC network using the setting means.

The time constant of the RC network can also be changed by adding a second capacitor to the shunt capacitor and a resistor existing series circuit is in parallel and the resistance of this series circuit by the setting means is changeable. The smaller you make the resistance of the series connection, the greater the time constant of the RC network, the settling time and settling time are the greater.

The invention is to be explained in more detail with reference to the drawing.

In Fig. 1 is a known circuit of a four-stage Tran " Sistor amplifier with automatic gain control for hearing impaired devices is shown. This circuit should be used here will be explained as far as is necessary for an understanding of the invention is required.

The input sound pressure is converted by a microphone 2 into a signal voltage, which is the basis of a first transistor

level 8

509819/0456.

37/73 - 6 -

stage 8 is fed. The amplified alternating signal voltage is picked up at the collector of an output stage transistor 7 and fed to a listener 3, the output sound pressure of which reaches the ear of the hearing impaired person.

The transit gate amplifier is switched via a sin and off switch powered by a battery 4. :

For the automatic gain control, a part of the signal alternating voltage is used to obtain the control DC voltage Collector of the output stage transistor 7 branched off and rectified by a transistor 6 acting as a rectifier diode.

The rectified voltage arrives at one of the series resistors R., R ₂ and R, as well as from the shunt capacitors C | and C, existing RC network. The DC control voltage smoothed by this RC network is at the base of the first transistor stage 8 and determines its operating point.

In the idle state, i.e. when no or only a small input «- If the sound pressure reaches the microphone 2 and there is no control DC voltage at the rectifier 6, the capacitors are

C. and C, charged approximately to the base voltage of transistor 8.

If there is a greater input sound pressure and a control DC voltage is generated at the rectifier 6

by

509819/0456

37/73 - 7 -

This DC control voltage charges the capacitors C- and C ₂ in opposite directions. The voltage across the capacitors C ₁ and C 2 thus decreases, and the potential at the base of the first transistor stage 8 is shifted in the negative direction. This reduces the gain factor of the first transistor stage 8.

For the delayed onset of the downward regulation of the gain of the first transistor stage 8, that is to say for the settling time, the charging time constant of the capacitors C and C2 is decisive. Since the charging of the capacitors C ₁ and C ₂ is carried out via the resistors R ₃ and L, the values of C, Co, Ro are ^unc * ^R 3 decisive ^out ~ for the charging time constant. For the values of these four components "entered in FIG. 1, there is a settling time t. Of approximately 50 ms.

The decay time, that is to say the delay when the gain of the first transistor stage 8 is increased again when the control DC voltage becomes smaller, on the other hand, is determined by the discharge time constant of the capacitors C and C ₂ . The discharge of these two capacitors takes place essentially via the resistor R ₁ and the base-emitter path of the first transistor stage 8. With the values entered in FIG. 1, a decay time t of approximately 500 ms results.

. In

9819/0458

37/73 - 8 -

In Figures 2, 3, 4 and 5 the transistor amplifier is inclusive the microphone 2, the receiver 3, the battery 4 and the rectifier 6 are no longer shown in detail, but only represented by a dashed box 1, also indicated in FIG. 1, because it refers to these details does not matter for the invention. The figures mentioned only illustrate the settling time and the Decay time decisive RC networks for smoothing the control DC voltage.

The circuit shown in Fig. 2, also known, differs from that of Fig. 1 by a less complex design of the RC network. This only consists of the two series resistors R. and R ₂ and a shunt capacitor C ,. This less demanding circuit is particularly suitable for transistor amplifiers with a lower gain.

The circuit of FIG. 1 differs from the circuit which is also known Circuit of FIG. 3 only in that the smoothed DC control voltage for gain control of two transistor stages is used.

For example, similar to FIG. 1, the first stage of the amplifier can be controlled via the resistor R., the second or a later stage of the amplifier can be controlled via a resistor R.

Although the invention of course applies to each of the circuits of the RC network shown in FIGS. 1, 2 and 3

applied

509819/0456

37/73 - 9 -

can be used, there are only two options here of the invention applied to the circuit of the Fig. 1 will be discussed. The application to the circuits of FIGS. 2 and 3 then results automatically.

According to a first embodiment of the invention, as shown in FIG. 4, the shunt capacitors C ₁ and C _{2 are} each replaced by a plurality of capacitors of different capacities, one of which can always be switched on as required.

Instead of a single capacitor C. of 10 pF, five different capacitors 13, 14, 15, 16 and 17 are provided in FIG. 4, which, for example, can have a capacitance of 2 pF, 5 pF, 10 pF, 15 pF and 20 pF. In the same way, instead of the capacitor C _2, five capacitors 23, 24, 25, 26 and 27 of different capacities are provided, of which only one can be switched on at a time. A switch 11 is used to select one of the capacitors 13 to 17, while a switch 12 is intended for the selection of one of the capacitors 23 to 27. The two switches 11 and 12 can be mechanically coupled to one another and represent the setting means within the meaning of the invention. The switches 11 and 12 can, however, also be adjustable independently of one another.

With the help of these setting means it is possible to set a capacitance of 2 μΡ, 5 pF, 10 pF, 15 pF or 20 pF for the shunt capacitors C and C. For the values of R ₁ , R ₂ and R ^ entered in FIG. 4 , the result is then

following 509819/0456

37/73

- 10 -

following settling times t. and decay times t

the end

: ^C l ^C 2 tem the end 2 ] iF . 2 jiF 10 ms 100 ms 10 yF 10 yF 50 ms 500 ms 20 yF 20 mF 100 ms 1000 ras

According to the second exemplary embodiment of the invention shown in FIG. 5, the two shunt capacitors C ₁ and C 1, which here each have a capacitance of 2 μΡ, each have a series circuit of a capacitor C 1. or Cj ^ ¹¹¹ ^ k a capacitance of 4 yF and a variable resistance Rg or Rg with a maximum value of 2.5 kfl in parallel. '

Due to the variable resistances R «and

that through the

Setting means can be actuated within the meaning of the invention, the charging time constant and thus the settling time can be influenced. The settling time is shortest when Rg and Rq have the value 0 and longest when Rg and R _{g are set} to their maximum value of 2.5 kn.

With the values given in FIG. 5 for the individual components is obtained for the smallest and for the largest word from Rg and Rq the. the following settling times and settling times:

R ₉

'a

"the end

0 Ω
2.5

On

2.5

30 ms 60 ms

300 ms

300 ms

509819/0456

the

37/73 - 11 -

The on the basis of Fig. 4 and. 5, on the invention Measures based on this can easily be applied to the circuits of FIGS. 2 and 3 without the need for further Explanations are required. ·

In Fig. 6 the circuit of a hearing aid device is shown, in which the automatic gain control by influencing a signal alternating voltage negative feedback is achieved. This is a device for the hearing impaired, above all Suitable for the hearing impaired with a high level of recruitment 1st.

The signal alternating voltage negative feedback arises from the fact that from the collector of a third pre-stage transistor 9 im Point 30 a part of the preamplified alternating signal voltage via a voltage divider consisting of resistors 31 and 32 tapped and via the collector-emitter path of a control transistor 33 and one for the AC signal voltage Permeable capacitor 10 is fed back to the base of the first pre-stage transistor 8. The measure of this Negative feedback and thus the gain depends on the impedance of the collector-emitter path of the regulating transistor 33. These Impedance can be increased by one of the base of the control transistor 33 controlled DC voltage can be changed.

To obtain the control DC voltage, the Collector of the third pre-stage transistor 9 at point 34 Part of the preamplified signal alternating voltage is removed and the removed signal alternating voltage is

tension

509819/0456

37/73 - 12 -

voltage rectifying transistor 35 supplied. The DC control voltage rectified by the transistor 35 reaches _{the base of the control transistor 33 via an RC network consisting of the two transverse capacitors C β} and Cg and a series resistor R-, which serves to smooth the DC control voltage.

The circuit shown in a dashed box 36 serves to stabilize the control DC voltage and is intended to do so Make it as independent as possible of fluctuations in temperature and battery voltage.

The settling time and settling time of the gain control depend here in exactly the same way as it does on the basis of the Fig. 1 was explained, from the charging time constants and the discharge time constants of the RC network. The discharge of the Shunt capacitors C, and Cg takes place here, however, via the Base-emitter path of the regulating transistor 33 and the resistor 32.

The invention can be applied to the circuit of FIG. 6 in the same way as was described with reference to FIGS. 4 and 5. One way of setting the settling time and the settling time is that instead of a single capacitor C _g and Cq, several capacitors of different capacities are provided, each of which can be switched into the RC network by a switch. Settling time and settling time result

509819/0 ^ 56

- 13 -

then from the respective set capacities for C _ß and Cg. Some examples of how the settling time t. and the decay time t _ due to the change in the capacities

the end

from Cg and C _fl can be seen in the table below.

^C 6 _M F ^C 8 t a t the end O , 47 yF 1.5 yF 5 ms 8th ms O , 47 up 10 μ F 10 ms 40 ms 10 10 yF 40 ms 100 ms

Another possibility for setting the settling time and the settling time is obtained, quite analogously to FIG. 5, in that a series circuit of a capacitor and a variable resistor is connected in parallel _{to each of the capacitors C g and Cg.} This embodiment is illustrated in Fig. 7, where the capacitor C _{g is} a series circuit of a capacitor C _g and a variable resistor

the capacitor Cg a series circuit of a capacitor C and a variable resistor R ₁₂ is connected in parallel. The two resistors R ₁₁ and R ₁₂ can be changed by the setting means in accordance with the invention.

. At the indicated in Figure 7 values for the individual components of the RC network are obtained for the extreme settings (Q Ω and 2.5 Kij) given below settling times t _and t decay _aug:

- 14 -

509819/04 56

- 14 -

11

'a

the end

O Ω

O Ω

2.5 kΩ

50 ms

100 ms

150 ms 200 ms

It should be noted that all in the drawing and in the Description of the values given for the RC networks smoothing the DC control voltage should only be regarded as examples and of course can be changed each time as required so that predetermined ranges for both the settling time as well as for the decay time can be detected by the adjustment means provided on the hearing aid device. For this are in each case only simple calculations are required, which any person skilled in the art can easily carry out with knowledge of the above description can.

Another possibility for influencing the settling time and the settling time would be to change the values in the RC networks lying series resistances. This does not need to be discussed further either, because the change in the Series resistances also affect the charging time constants and the discharge time constants and the calculations of these Constants are familiar to the person skilled in the art.

- 15 -

60 9819/0456

Claims (5)

37/73 - 15 - PLI Fs / Li • October 25, 1973 Claims <1. / hearing aid device with a transistor amplifier which is provided with an automatic gain control having a certain settling time and a certain settling time, characterized in that setting means for changing the settling time and / or the settling time of the gain control are provided on the hearing aid device. 2. Hearing aid device according to claim 1, wherein the automatic. Gain control of the multistage transistor amplifier by means of a control voltage that changes, for example, the operating point of a pre-stage transistor or the amount of a signal alternating voltage negative feedback, which is obtained by taking a part of the signal alternating voltage from the output stage or a preceding stage of the output amplifier, rectifying it and then rectifying it an RC network is smoothed with a time constant influencing the settling time and the settling time of the gain control, characterized in that the time constant of the RC network can be set by the setting means. 3. hearing aid device according to claim, characterized ^, that with an existing of at least one series resistor and at least one shunt capacitor RC network effective capacitance of at least one shunt capacitor can be changed by the setting means. - 16 - 509819/04 5 6 37/73 - 16 - 4. hearing aid device according to claim 3, characterized in that that several capacitors of different capacities are provided for at least one shunt capacitor, the optional can be switched into the RC network by the setting means (FIG. 4). 5. Hearing aid device according to claim 2, characterized in that at least one shunt capacitor of the RC network A series circuit consisting of a second capacitor and a resistor is parallel and the resistance of this Series connection can be changed by the setting means (Fig. 5 and 7). 509819/0456 Blank page