GB2526630A - Amplifier for induction loop - Google Patents

Amplifier for induction loop Download PDF

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Publication number
GB2526630A
GB2526630A GB1409729.9A GB201409729A GB2526630A GB 2526630 A GB2526630 A GB 2526630A GB 201409729 A GB201409729 A GB 201409729A GB 2526630 A GB2526630 A GB 2526630A
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resistor
resistance value
amplifier
ratio
amplifier circuit
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GB201409729D0 (en
GB2526630B (en
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Richard Dungan
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CONTACTA SYSTEMS Ltd
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CONTACTA SYSTEMS Ltd
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    • H—ELECTRICITY
    • H03—ELECTRONIC CIRCUITRY
    • H03F—AMPLIFIERS
    • H03F3/00—Amplifiers with only discharge tubes or only semiconductor devices as amplifying elements
    • H03F3/181—Low-frequency amplifiers, e.g. audio preamplifiers
    • H03F3/183—Low-frequency amplifiers, e.g. audio preamplifiers with semiconductor devices only
    • H03F3/187—Low-frequency amplifiers, e.g. audio preamplifiers with semiconductor devices only in integrated circuits
    • H—ELECTRICITY
    • H03—ELECTRONIC CIRCUITRY
    • H03F—AMPLIFIERS
    • H03F3/00—Amplifiers with only discharge tubes or only semiconductor devices as amplifying elements
    • H03F3/181—Low-frequency amplifiers, e.g. audio preamplifiers
    • H03F3/183—Low-frequency amplifiers, e.g. audio preamplifiers with semiconductor devices only
    • G—PHYSICS
    • G05—CONTROLLING; REGULATING
    • G05F—SYSTEMS FOR REGULATING ELECTRIC OR MAGNETIC VARIABLES
    • G05F1/00—Automatic systems in which deviations of an electric quantity from one or more predetermined values are detected at the output of the system and fed back to a device within the system to restore the detected quantity to its predetermined value or values, i.e. retroactive systems
    • G05F1/10—Regulating voltage or current
    • G05F1/12—Regulating voltage or current wherein the variable actually regulated by the final control device is ac
    • G05F1/40—Regulating voltage or current wherein the variable actually regulated by the final control device is ac using discharge tubes or semiconductor devices as final control devices
    • G05F1/44—Regulating voltage or current wherein the variable actually regulated by the final control device is ac using discharge tubes or semiconductor devices as final control devices semiconductor devices only
    • G05F1/445—Regulating voltage or current wherein the variable actually regulated by the final control device is ac using discharge tubes or semiconductor devices as final control devices semiconductor devices only being transistors in series with the load
    • H—ELECTRICITY
    • H03—ELECTRONIC CIRCUITRY
    • H03F—AMPLIFIERS
    • H03F3/00—Amplifiers with only discharge tubes or only semiconductor devices as amplifying elements
    • H03F3/45—Differential amplifiers
    • H03F3/45071—Differential amplifiers with semiconductor devices only
    • H03F3/45076—Differential amplifiers with semiconductor devices only characterised by the way of implementation of the active amplifying circuit in the differential amplifier
    • H03F3/45475—Differential amplifiers with semiconductor devices only characterised by the way of implementation of the active amplifying circuit in the differential amplifier using IC blocks as the active amplifying circuit
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04B—TRANSMISSION
    • H04B5/00—Near-field transmission systems, e.g. inductive or capacitive transmission systems
    • H04B5/20—Near-field transmission systems, e.g. inductive or capacitive transmission systems characterised by the transmission technique; characterised by the transmission medium
    • H04B5/24—Inductive coupling
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04R—LOUDSPEAKERS, MICROPHONES, GRAMOPHONE PICK-UPS OR LIKE ACOUSTIC ELECTROMECHANICAL TRANSDUCERS; DEAF-AID SETS; PUBLIC ADDRESS SYSTEMS
    • H04R25/00—Deaf-aid sets, i.e. electro-acoustic or electro-mechanical hearing aids; Electric tinnitus maskers providing an auditory perception
    • H04R25/55—Deaf-aid sets, i.e. electro-acoustic or electro-mechanical hearing aids; Electric tinnitus maskers providing an auditory perception using an external connection, either wireless or wired
    • H04R25/554—Deaf-aid sets, i.e. electro-acoustic or electro-mechanical hearing aids; Electric tinnitus maskers providing an auditory perception using an external connection, either wireless or wired using a wireless connection, e.g. between microphone and amplifier or using Tcoils
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04R—LOUDSPEAKERS, MICROPHONES, GRAMOPHONE PICK-UPS OR LIKE ACOUSTIC ELECTROMECHANICAL TRANSDUCERS; DEAF-AID SETS; PUBLIC ADDRESS SYSTEMS
    • H04R27/00—Public address systems
    • H04R27/02—Amplifying systems for the deaf
    • H—ELECTRICITY
    • H03—ELECTRONIC CIRCUITRY
    • H03F—AMPLIFIERS
    • H03F2200/00—Indexing scheme relating to amplifiers
    • H03F2200/03—Indexing scheme relating to amplifiers the amplifier being designed for audio applications
    • H—ELECTRICITY
    • H03—ELECTRONIC CIRCUITRY
    • H03F—AMPLIFIERS
    • H03F2200/00—Indexing scheme relating to amplifiers
    • H03F2200/301—Indexing scheme relating to amplifiers the loading circuit of an amplifying stage comprising a coil
    • H—ELECTRICITY
    • H03—ELECTRONIC CIRCUITRY
    • H03F—AMPLIFIERS
    • H03F2203/00—Indexing scheme relating to amplifiers with only discharge tubes or only semiconductor devices as amplifying elements covered by H03F3/00
    • H03F2203/45—Indexing scheme relating to differential amplifiers
    • H03F2203/45138—Two or more differential amplifiers in IC-block form are combined, e.g. measuring amplifiers

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  • Engineering & Computer Science (AREA)
  • Computer Networks & Wireless Communication (AREA)
  • Signal Processing (AREA)
  • Physics & Mathematics (AREA)
  • Acoustics & Sound (AREA)
  • Power Engineering (AREA)
  • General Health & Medical Sciences (AREA)
  • Health & Medical Sciences (AREA)
  • Multimedia (AREA)
  • Neurosurgery (AREA)
  • Otolaryngology (AREA)
  • Electromagnetism (AREA)
  • General Physics & Mathematics (AREA)
  • Radar, Positioning & Navigation (AREA)
  • Automation & Control Theory (AREA)
  • Microelectronics & Electronic Packaging (AREA)
  • Amplifiers (AREA)

Abstract

Conventional current output amplifiers for driving induction loops 3, such as loops for coupling to telecoils connected to hearing aids, may become unstable if the induction loop becomes broken or is disconnected. To improve stability the resistor values in a Howland current source may be modified so that the negative feedback is increased over the positive feedback. This imbalance causes a drop in gain at higher audio frequencies, but the drop is small and tolerable. The modified current source may be incorporated in a bridge circuit which drives both ends of the coil 3 (figure 4).

Description

AMPLIFIER FOR INDUCTION LOOP
TECHNICAL FIELD
This invention relates to amplifiers for use with induction loops. In particular, the invention relates to current amplifiers for use with large area induction loops.
BACKGROUND
Hearing aids are in common use to assist persons having impaired hearing. As is well known, hearing aids generally operate by using a microphone to detect ambient sounds and providing an amplified version of the ambient sounds to a user. Most commonly the amplified version of the ambient sounds is provided through a transducer, such as a speaker or speakers located in or adjacent the users ear canal.
In other forms of hearing aids a perceived version of the ambient sounds may also be provided to a user by supplying electrical signals directly to a users aural nerves. This approach is used, for example, in cochlear implants.
Most hearing aids are also able to operate in an assisted listening mode in which, instead of using a microphone to detect ambient sounds, the hearing aid instead detects a modulated magnetic field which is modulated with a desired sound signal. The hearing aid then recovers the sound signal from the modulated magnetic field and provides an amplified version of the sound signal to a user through the speaker or speakers located in or adjacent the users ear canal. This assisted listening mode may be usefbl in situations where there may be a large amount of ambient noise, for example when carrying out transactions in a commercial setting, or where a user is remote from a sound source, for example in a cinema or concert hall.
Many hearing aids are provided with an integral telecoil to allow the modulated magnetic field to be detected, or are arranged to be able to cooperate with an external telecoil, such as a telecoil worn around a users neck.
In general the modulated magnetic field is provided by an induction loop driven with a current modulated with the desired sound signal. These induction loops are also referred to as hearing loops and 1-loops.
Induction loops generally have both an ohmic resistance and an inductance which varies with frequency, and as a result it is usual for induction loops to be driven by a constant current driver amplifier in order to provide a flat frequency response and so prevent distortions of the sound signal provided by the hearing aid to the user.
However, in practice there is a problem that such constant current amplifiers may become unstable in some circumstances, potentially leading to the amplifier being damaged or destroyed, and having to be repaired or replaced. This may occur, for example, if the induction loop is broken or disconnected from the amplifier while the amplifier is operating. This is a particular problem in situations where the area covered by the induction loop is large, as a larger induction loop will generally require a more powerful amplifier, and the consequences of an amplifier becoming unstable may be more severe for more powerful amplifiers. Further, in practice it has been found that larger induction loops are more likely to be broken or disconnected while operating.
Therefore, an improved constant current amplifier for use with an induction loop is desired.
SUMMARY OF THE INVENTION
In a first aspect, the invention provides an amplif'ing circuit having a similar circuit topology to a Howland Current Pump, but having the values of the resistors unbalanced.
In a second aspect, the invention provides an amplifying circuit comprising two anipli1iing circuits according to the first aspect arranged in a bridge tied load topology.
In a third aspect the invention provides a current amplifier comprising one or more amplifying circuits according to the first or second aspects.
0 In a fourth aspect, the invention provides a driver for an induction loop system comprising one or more amplifying circuits according to the first or second aspects.
In a fifth aspect, the invention provides an induction ioop system comprising the driver according to the fourth aspect together with an induction loop.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 is a perspective view showing an example of a system comprising an amplifier according to an embodiment of the present invention in use; Figure 2 is a diagrammatic view of components of the amplifier of figure 1; Figure 3 is a circuit diagram of an amplifier according to an embodiment of the invention; Figure 4 is a circuit diagram of an amplifier according to another embodiment of the invention; Figure 5 is a circuit diagram of an amplifier according to another embodiment of the invention; and Figure 6 is a circuit diagram of a known amplifier.
DETAILED DESCRIPTION
One known arrangement of a constant current amplifier circuit is shown in figure 6. The illustrated current amplifier is a Howland current pump 100, also referred to as an Improved lowland Current Pump.
The Howland current pump 100 comprises a differential amplifier 101 having a positive input terminal, a negative input terminal, and an output terminal. For clarity the power supply to the differential amplifier 101 is not illustrated in figure 2.
The Howland current pump 100 has first and second input terminals 1 02a and 102b, and first and second output terminals 103a and 103b. In operation of the current amplifier an input electrical signal is applied across the first and second input terminals 102a and 102b, and a load 109 is connected between the first and second output terminals 103a and 103b.
The first input terminal 1 02a is connected to a first end of a resistor 104 having a resistance value RI. A second end of the resistor 104 is connected to the negative input terminal of the differential amplifier 101, and also to a first end of a resistor having a resistance value R2. A second end of the resistor 105 is connected to the output terminal of the differential amplifier 101.
The second input terminal 1 02b is connected to the second output terminal 1 03b, and also to the first end of a resistor 106 having a resistance value R3. A second end of the resistor 106 is connected to the positive input terminal of the differential amplifier 101, and also to a first end of a resistor 107 having a resistance value R4. A second end of the resistor 107 is connected to the first output terminal 103 a.
The output terminal of the differential amplifier 101 is connected to a first end of a sensing resistor 108 having a resistance value R5, in addition to the second end of the resistor 105. The second end of the sensing resistor 108 is connected to the first output terminal 103a and to the second end of the resistor 107.
The sensing resistor 108 typically has a much smaller resistance value than the resistors 104 to 107. Typically the sensing resistor 108 has a resistance value RiO several orders of magnitude smaller than the resistance values Ri to R4 of the resistors 104 to 107.
In the Howland current pump the resistors 104 to 108 must have resistance values Ri to R5 such that the ratio RI:R2 and the ratio R3:(R4+R5) are equal.
However, in practice, it is usually the case that the resistance value RIO of the sensing resistor 108 is negligibly small compared to the resistance values R3 and R4 of the resistors 106 and 107. Accordingly, the required relationship of the resistance values is often simplified to be that the resistors 104 to 107 must have resistance values Ri to R4 such that the ratio Ri:R2 and the ratio R3:R4 are equal.
Commonly, for simplicity, in a Howland current pump all of the resistance values Ri to R4 are the same, and accordingly the ratios R1:R2 and R3:R4 are equal.
In theory, a Rowland current pump should provide a perfect constant current drive with an infinite output impedance. If a Howland current pump was used as constant current amplifier to drive an induction loop this would suffer from the instability problem discussed above, becoming unstable, and possibly destructively unstable resulting in the components of the Rowland current pump being damaged or destroyed. It is believed that this is because the theoretically infinite inductive reactance of a broken or disconnected induction loop cannot be driven by an infinite output impedance current source, potentially resulting in instability of the amplifier.
FIG. I is a general perspective view of an example of an induction loop system 1 according to an embodiment of the present invention. In Figure 1 the induction loop system 1 is arranged for use in a lecture hall 2.
The induction loop system I comprises an induction loop 3 mnning around the perimeter of the lecture hall 2 and a driver unit 4 able to apply an electrical signal to the induction loop 3. The induction loop system I further comprises a microphone 5 connected to the driver unit 4.
Figure 2 shows a schematic diagram of the main components of the induction loop system 1 of figure 1. As is shown in figure 2, the driver unit 4 comprises an automatic gain control module 6 and a current amplifier 10.
In operation of the induction loop system 1, a speaker 7 may speak into the microphone 5 so that the microphone 5 receives an audio speech signal. The microphone 5 uses the received audio speech signal to generate an electrical signal modulated by the audio speech signal, and then supplies this audio modulated electrical signal to the automatic gain control module 6 of the driver unit 4. The automatic gain control module 6 converts the audio modulated electrical signal into a gain controlled audio modulated electrical signal, and supplies this gain controlled audio modulated electrical signal to the current amplifier 10 of the driver unit 4. The current amplifier 10 amplifies the received gain controlled audio modulated electrical signal to produce an output audio modulated current signal. The driver unit 4 then supplies the output audio modulated current signal to the induction loop 3.
The driver unit 4 may further include a power supply unit. The power supply unit may be a DC power supply. The power supply unit may for example be a battery, a mains rectifier, or a DC to DC inverter.
Typically, the output audio modulated current signal is modulated directly with the speech signal, that is, the speech signal is not mixed or combined with any carrier signal. Accordingly, the output audio modulated current signal is in the audio baseband frequency range, typically in the frequency range 100Hz to 5kHz.
The audio modulated current signal supplied to the induction loop 3 generates an audio modulated magnetic field within the induction loop 3, and thus throughout the lecture hall 2. As a result, the audio modulated magnetic field is also modulated directly with the speech signal.
In the illustrated example the induction loop 3 comprises a single looped strand of wire. In other examples the induction loop may be farmed by other forms of conductor, such as multiple strands of wire or a coiled conductor.
1 5 In operation of the induction loop system I, a hearing aid user 8 using a telecoil equipped heating aid 9 and located in the lecture hall 2 inside the induction loop 3 can hear the speaker 7 use their hearing aid 9 in an assisted listening mode. The telecoil equipped hearing aid 9 may have an integral telecoil, or may be connected to a separate telecoil.
The telecoil of the hearing aid 9 will detect the audio modulated magnetic field modulated with the speech signal. The hearing aid 9 hearing aid will then recover the speech signal from the audio modulated magnetic field and provide an amplified version of the speech signal to the user 8 through a speaker or speakers of the hearing aid 9 located in or adjacent the users ear canal. The recovery and amplification of the speech signal by the hearing aid 9 is relatively simple because the audio modulated magnetic field is directly modulated with the speech signal Telecoil equipped hearing aids are well known and need not be described in detail herein. The present invention may be used with any type of telecoil equipped hearing aid. A telecoil equipped hearing aid may have an integral telecoil, or may As a result, a hearing aid user S located within the lecture hall 2 using a hearing aid 9 fitted with a telecoil will be able to hear the speaker 7, even if the acoustic conditions are poor, for example due to ambient noise, poor acoustics in the lecture hail, or the like.
in other examples, the driver unit 4 may be provided with an audio modulated electrical signal from a remote or recorded signal source such as a radio, telephone line, voice over IP connection, DVD player, MP3 player, or the like, instead of from a live microphone 7.
As discussed above, the driver unit 4 comprises a current amplifier 10 which amplifies the received gain controlled audio modulated electrical signal and produces an output audio modulated current signal.
The induction loop 3 will have both an ohmic resistance and an inductance, and this inductance will generally vary with frequency. A current amplifier 10 is used in order to provide a substantially constant or flat frequency response for the system and so minimise distortion of the final sound signal provided by the hearing aid to the user.
If a known current amplifier circuit, such as a Howland current pump 100, was used as the current amplifier, the current amplifier could suffer from the problem discussed above that the current amplifier may become unstable in some circumstances, potentially leading to the current amplifier being damaged or destroyed. In particular, the current amplifier could become unstable if the induction loop 3 were broken or disconnected from the driver unit 4 while the loop system 1 was operating.
Figure 3 shows a current amplifier circuit 20 according to an embodiment of the present invention which may be used as a current amplifier 10 in a loop system 1 such as that illustrated above.
The current amplifier circuit 20 comprises a differential amplifier 21 having a positive input terminal, a negative input terminal, and an output terminal. The differential amplifier 21 also has positive and negative power supply input temiinals, but for clarity the power supply to the differential amplifier 21 is not illustrated in figure 3.
The current amplifier circuit 20 comprises first and second input terminals 22a and 22b, and first and second output terminals 23a and 23b. In operation of the current amplifier the audio modulated electrical signal is applied across the first and second input terminals 22a and 22b, and the induction loop 3 is connected between the first and second output terminals 23a and 23b. The induction loop 3 is the load driven by the current amplifier circuit 20. In figure 3 the audio modulated electrical signal is denoted by AC voltage source 29.
The first input terminal 22a is connected to a first end of a first resistor 24 having a resistance value Ri 1. A second end of the first resistor 24 is connected to the negative input terminal of the differential amplifier 21, and also to a first end of a second resistor 25 having a resistance value R12. A second end of the second resistor 25 is connected to the output terminal of the differential amplifier 21.
The second input terminal 22b is connected to the second output terminal 23b, and also to a first end of a third resistor 26 having a resistance value R13. A second end of the third resistor 26 is connected to the positive input terminal of the differential amplifier 21, and also to a first end of a fourth resistor 27 having a resistance value Rl4. A second end of the fourth resistor 27 is connected to the first output terminal 23a.
The output tenninal of the differential amplifier 21 is connected to a first end of a fifth resistor 28 having a resistance value R5, in addition to the second end of the second resistor 25. The second end of the fifth resistor 28 is connected to the first output terminal 23a and to the second end of the fourth resistor 27.
The fifth resistor 28 acts as a sensing resistor and typically has a much smaller resistance value than the first to fourth resistors 24 to 27. The fifth resistor 28 is effectively in series with the induction loop 3, that is, with the load. As a result, the greater the resistance value RI 5 of the fifth resistor 28, the greater the amount of energy dissipated as heat in the fifth resistor 28. This energy dissipated as heat in the fifth resistor 28 is wasted energy which may reduce the amount of power that the current amplifier circuit 20 is able to supply to the induction loop 3 load.
Accordingly, in general, it is desirable for the resistance value of the fifth resistor 28 to have a resistance value R15 as low as possible consistent with the correct operation of the current amplifier circuit 20. Typically, the fifth resistor 28 has a resistance value RI S several orders of magnitude smaller than the resistance values Ri Ito R14 of the first to fourth resistors 24 to 27.
In the current amplifier circuit 20 the resistance values Rl 1 to Ri 5 of the first to fifth resistors 24 to 28 are selected such that the ratio Ri I:Rl2 is higher than the ratio R13:(Rl4+R15).
The current amplifier circuit 20 acts as a near constant current amplifier.
The differential amplifier 21 may be a feedback amplifier or a differential feedback amplifier. In some examples the differential amplifier 21 may be a single chip amplifier such as an operational amplifier. In other examples the differential amplifier may be a hybrid amplifier comprising an operational amplifier chip together with other discrete components. It is expected that a hybrid amplifier may be able to provide a greater output power than a single chip amplifier.
When the current amplifier circuit 20 is used as the current amplifier 10 the imbalance between the ratios of the different resistance values has the effect of providing a residual level of negative voltage feedback to the differential amplifier 21 when the current amplifier 10 has no load connected. As a result, if the load is removed when the current amplifier 10 is operating, for example due to inadvertent or unintended disconnection of the induction loop 3 from the current amplifier 10, this negative voltage feedback will tend to stabilise the current amplifier circuit 20 and so prevent any possible damage occurring to the current amplifier circuit 20 and the rest of the current amplifier 10.
This may be understood by considering that the current amplifier circuit 20 effectively comprises two separate feedback loops. There is a positive current feedback ioop, which keeps the output current from the current amplifier circuit near constant. Further, the imbalance of the ratios of the resistance values provides a small amount of negative voltage feedback.
When the current amplifier circuit 20 is used as the current amplifier 10 and the current amplifier 10 has a load connected, at audio frequencies the negative voltage feedback is generally much smaller than the positive current feedback and effectively negligible, so that the positive current feedback dominates and the current amplifier circuit acts as a near constant current amplifier. At higher frequencies, where the inductive reactance of the load becomes large and where the current feedback would tend to lose control and could make the current amplifier circuit 20 unstable, the negative voltage feedback becomes dominant and acts to stabilise the current amplifier circuit 20.
The negative voltage feedback will tend to cause the output or gain of the amplifier circuit to drop slightly at higher audio frequencies, but in operation, provided that the imbalance between the ratios of the different resistance values is not too large, the high frequency drop will be small enough to be acceptable and will not significantly effect the sound quality perceived by a hearing aid user.
As is explained above, in the current amplifier circuit 20 the resistance values Ri 1 to R15 of the first to fifth resistors 24 to 28 are selected such that the ratio Rll:R12 is higher than the ratio R13:(R14+R15). In general, the degree of stability of the current amplifier circuit 20 is related to the amount by which the ratio Rl 1:R12 is higher than the ratio R13:(R14+R15). The greater the amount by which the ratio Ril:Rl2 is higher than the ratio R13:(R14+R15), the higher the stability. However, in general the efficiency of the current amplifier circuit 20 and the amount of loss of gain at high frequencies is also related to the amount by which the ratio RI I:R12 is higher than the ratio R13:(R14+R15). The greater the amount by which the ratio Rll:R12 is higher than the ratio R13:(R14+R15), the lower the efficiency, as more power will be dissipated as heat within the current amplifier circuit 20, and the greater the amount of loss of gain at high frequencies.
Preferably, the ratio Ri 1:R12 is higher than the ratio R13:(R14+R15) by a value in the range 1% to 15%. More preferably, the ratio R1l:R12 is higher than the ratio R13:(R14+R15) by a value in the range 2% to 15%. More preferably, the ratio Rl 1:R12 is higher than the ratio R13:(R14+R15) by a value in the range 2% to 10%. Still more preferably, the ratio Ri 1:R12 is higher than the ratio RI 3:(Ri 4+Ri 5) by approximately 3%.
In practice, in examples where the resistance value RI 5 of the fifth resistor 28 is negligibly small compared to the resistance values R13 and Rl4 of the third and fourth resistors 26 and 27, the required relationship of the resistance values may be simplified to be that the resistance values RI I to R14 are selected such that the ratio RI 1:R12 is higher than the ratio Ri3:Ri4.
It is generally desirable to keep the amount of the imbalance between the ratios of the resistance values relatively small in order to ensure that the current feedback is not dominated by the voltage feedback in the audio frequency range where near constant current drive is required and that any distortion produced by the voltage feedback at audio frequencies is small enough to be acceptable.
in practice, the resistance values of real resistors are defined as nominal values with tolerances. As a result, the resistance value of a resistor may be known to lie within a defined range, but the exact resistance value of the resistor may not be known. The sizes of these ranges and tolerances will vary from case to case depending on the specific components used.
As a result, the nominal resistance values should be selected so that the amount of the imbalance between the ratios of the resistance values is sufficiently large that, taking into account the resistance value tolerances, there is no likelihood that the imbalance in the ratios of the resistance values could be reversed. Such a reversal could render the voltage feedback positive rather than negative, making the current amplifier circuit 20 unstable.
In one example where R15 is negligible, resistance values Ri 1 to R13 may have the same value, while the resistance value Rl4 is higher. In another such example, resistance values R12 to R14 may have the same value, while the resistance value Ru is higher. In a first example resistance values RIl to R13 may be lOkQ and resistance value R14 may be 1 lkI»=. In a second example resistance values Ri I to R13 may be 33kE»= and resistance value Rl4 may be 34k0. In a third example resistance values Rl 2 to RI 4 may be I OkQ and resistance value Ri I may be 1 lkCI. In a fourth example resistance values R12 to R14 may be 33kfl and resistance value Rl 1 may be 34kU. In all of these first to fourth examples the resistance value RI 5 may be in the range 0.05 to 0.211, and typically about 0.1 111.
In these examples the amount of reduction of high frequency output will be no more than about 1dB across the audio frequency range of 100Hz to 5kHz. This is within acceptable limits for an induction loop system for use with hearing aids, and complies with the relevant standards.
In the illustrated embodiment of figure 3 the second input terminal 22b, the second output terminal 23b, and the third resistor 26 are all connected together. In some examples the second input terminal 22b, the second output tenninal 23b, and the third resistor 26 may all be earthed or grounded, for example by all being connected to a ground plane conductor.
Figure 4 shows a current amplifier circuit 30 according to an embodiment of the present invention which may be used as a current amplifier 10 in a loop system 1 such as that illustrated above.
The current amplifier circuit 30 may be regarded as a bridge tied load (BTL) amplifier circuit effectively comprising a push/pull arrangement of two of the amplifier circuits according to the previous embodiment.
The current amplifier circuit 30 comprises a first differential amplifier 31 and a second differential amplifier 32. Each of the first and second differential amplifiers has a respective positive input terminal, negative input terminal, and output tenninal. The first and second differential amplifiers 31 and 32 also have respective positive and negative power supply input terminals, but for clarity the power supply to the differential amplifiers 31 and 32 is not illustrated in figure 4.
The first and second differential amplifiers 31 and 32 have the same nominal open loop gain. In the current amplifier circuit 30 the first differential amplifier 31 is the "master" amplifier which determines the current and voltage transconductance and gain of the current amplifier circuit 30 as a whole, and the second differential amplifier 32 is "slaved" to the first differential amplifier 31 and effectively configured as a unity voltage gain inverter to the output of the first differential amplifier 31. As is usual in bridge tied type amplifier arrangements, the addition of the second differential amplifier 32 allows the total voltage swing or output range of the current amplifier circuit 30 to be doubled, and so allows the maximum drive power to the load to be quadrupled.
The current amplifier circuit 30 comprises first and second input terminals 33 a and 33b, and first and second output terminals 34a and 34b. In operation of the current amplifier the audio modulated electrical signal is applied across the first and second input terminals 33a and 33b, and the induction loop 3 is connected between the first and second output terminals 34a and 34b as the load. In figure 4 the audio modulated electrical signal is denoted by AC voltage source 35.
The first input terminal 33a is connected to a first end of a first resistor 36 having a resistance value R21. A second end of the first resistor 36 is connected to the negative input terminal of the first differential amplifier 31, and also to a first end of a second resistor 37 having a resistance value R22. A second end of the second resistor 37 is connected to the output terminal of the first differential amplifier 31.
The second input terminal 3 3b is connected to a first end of a third resistor 38 having a resistance value R23, and also to the positive input terminal of the second differential amplifier 32. A second end of the third resistor 38 is connected to the positive input terminal of the first differential amplifier 31, and also to a first end of a fourth resistor 39 having a resistance value R24. A second end of the fourth resistor 39 is connected to the first output terminal 33 a.
The output terminal of the first differential amplifier 31 is connected to a first end of a fifth resistor 40 having a resistance value P.25, in addition to the second end of the second resistor 37. The second end of the fifth resistor 40 is connected to the first output terminal 33a, and to the second end of the fourth resistor 39.
The output tenninal of the first differential amplifier 31 and the second end of the second resistor 37 are connected to a first end of a sixth resistor 41 having a resistance value R26. The second end of the sixth resistor 41 is connected to a first end of a seventh resistor 42 having a resistance value R27, and to the negative input terminal of the second differential amplifier 32.
The second end of the seventh resistor 42 is connected to the second output terminal 33b, and to the output terminal of the second differential amplifier 32.
Accordingly, the output terminal of the second differential amplifier 32 is connected to the second output terminal 33b.
The fifth resistor 40 acts as a sensing resistor and typically has a much smaller resistance value than the first to fourth resistors 36 to 39 and the sixth and seventh resistors 41 and 42. Typically the fifth resistor 40 has a resistance value R25 several orders of magnitude smaller than the resistance values R21 to R24, R26 and R27 of the first to fourth, sixth and seventh resistors 36 to 39, 41 and 42. The fifth resistor 40 is effectively in series with the induction loop 3, that is, with the load. As a result, the greater the resistance value R25 of the fifth resistor 40, the greater the amount of energy dissipated as heat in the fifth resistor 40. This energy dissipated as heat in the fifth resistor 40 is wasted energy which may reduce the amount of power that the current amplifier circuit 30 is able to supply to the induction loop 3 load. Accordingly, in general, it is desirable for the resistance value of the fifth resistor 40 to have a resistance value 1(25 as low as possible consistent with the correct operation of the current amplifier circuit 30.
The differential amplifiers 31 and 32 are arranged to sense the voltage across the fifth, sensing, resistor 40 in an instrumentation amplifier or balunamp configuration which will sense and respond to the voltage differential across the sensing resistor, while effectively ignoring the output voltage across the load, which is rejected by the common-mode rejection of the balunamp configuration.
In the current amplifier circuit 30 the resistance values R21 to R25 of the first to fifth resistors 36 to 40 are selected such that the ratio R21:R22 is higher than the ratio R23:(R24+R25). The resistance values P.26 and P.27 of the sixth and seventh resistors should be equal to give the second differential amplifier 32 inverting unity gain so that it is effectively "slaved" to the first differential amplifier 31, producing an output voltage equal in value but opposite in polarity to that of the first differential amplifier 31.
The current amplifier circuit 30 acts as a near constant current amplifier.
The differential amplifiers 31 and 32 may be feedback amplifiers or differential feedback amplifiers. In some examples the differential amplifiers 31 and 32 may be a single chip amplifiers such as operational amplifiers. In other examples the differential amplifiers may be hybrid amplifiers each comprising an operational amplifier chip together with other discrete components. It is expected that a hybrid amplifier may be able to provide a greater output power than a single chip amplifier.
When the current amplifier circuit 30 is used as the current amplifier 10 the imbalance between the ratios of the different resistance values has the effect of providing a residual level of negative voltage feedback to the differential amplifiers 31 and 32 when the current amplifier 10 has no load connected. As a result, if the load is removed when the current amplifier 10 is operating, for example due to inadvertent or unintended disconnection of the induction loop 3 from the current amplifier 10, this negative voltage feedback will tend to stabilise the current amplifier circuit 30 arid so prevent any possible damage occurring to the current amplifier circuit 30 and the rest of the current amplifier 10.
When the current amplifier circuit 30 is used as the current amplifier 10 and the current amplifier 10 has a load connected, at audio frequencies the negative voltage feedback is generally much smaller than the positive current feedback, so that the current amplifier circuit acts as a near constant current amplifier. The negative voltage feedback will tend to cause the output or gain of the amplifier circuit to drop slightly at higher frequencies, but in operation, provided that the imbalance between the ratios of the different resistance values is not too large, the high frequency drop will be small enough to be acceptable and will not significantly effect the sound quality perceived by a hearing aid user.
Similarly to the previous example, the current amplifier circuit 30 effectively comprises two separate feedback loops for the first differential amplifier 31, a positive current feedback loop, which keeps the output current from the first differential amplifier 31, and the second differential amplifier 32 which is slaved to the first differential amplifier 31, and thus the current amplifier circuit 30 as a whole, near constant, and a small amount of negative voltage feedback due to the imbalance of the ratios of the resistance values.
As is explained above, in the current amplifier circuit 30 the resistance values R21 to R25 of the first to fifth resistors 36 to 40 are selected such that the ratio R21:R22 is higher than the ratio R23:(R24+R25). In general, the degree of stability of the current amplifier circuit 20 is related to the amount by which the ratio R2l:R22 is higher than the ratio R23:(R24+R25). The greater the amount by which the ratio R2l:R22 is higher than the ratio R23:(R24+R25), the higher the stability. However, in general the efficiency of the current amplifier circuit 30 and the amount of loss of gain at high frequencies are also related to the amount by which the ratio R21:R22 is higher than the ratio R23:(R24+R25). The greater the amount by which the ratio R21:R22 is are higher than the ratio R23:(R24+R25), the lower the efficiency, as more power will be dissipated as heat within the current amplifier circuit 30, and the greater the amount of loss of gain at high frequencies.
S Preferably, the ratio R21;R22 is higher than the ratio R23:(R24+R25) by a value in the range 1% to 15%. More preferably, the ratio R2 1:R22 is higher than the ratio R23:(R24+R25) by a value in the range 2% to 15%. More preferably, the ratio R21:R22 is higher than the ratio R23:(R24+R25) by a value in the range 2% to 10%. Still more preferably, the ratio R2l:R22 is are higher than the ratio R23:(R24+R25) by approximately 3%.
In practice, in examples where the resistance value R25 of the fifth resistor 40 is negligibly small compared to the resistance values R23 and R24 of the third and fourth resistors 38 and 39, the required relationship of the resistance values may be simplified to be that the resistance values R2 I to R24 are selected such that the ratio R21:R22 is higher than the ratio R23:R24.
Similarly to the previous embodiment, it is generally desirable to keep the amount of the imbalance between the ratios of the resistance values relatively small in order to ensure that the current feedback is not dominated by the voltage feedback in the audio frequency range where near constant current drive is required and that any distortion produced by the voltage feedback at audio frequencies is small enough to be acceptable.
However, the nominal resistance values should be selected so that the amount of the imbalance between the ratios of the resistance values is sufficiently large that, taking into account the resistance value tolerances, there is no likelihood that the imbalance in the ratios of the resistance values could be reversed. Such a reversal could render the voltage feedback positive rather than negative, making the current amplifier circuit 30 unstable.
In one example where R25 is negligible, resistance values R2 1 to R23 may have the same value, while the resistance value P.24 is higher. In another example, resistance values R21 to R23, P.26 and R27 may have the same value, while the resistance value P.24 is higher. In one example resistance values R21 to R23, P26 and R27 may be 10kK2 and resistance value R24 may be 1 lkQ. In another example resistance values P.21 to P23, P.26 and P.27 may be 33kfl and resistance value P.24 may be 34kQ. In these examples the resistance value R25 may be in the range 0.05 to 0.2Q, and typically about 0.1 11.
In these examples the amount of reduction of high frequency output will be no more than about 1dB across the audio frequency range of 100Hz to 5kHz. This is within acceptable limits for an induction loop system for use with hearing aids, and complies with the relevant standards.
In the illustrated embodiment of figure 4 the two differential amplifiers are arranged back to back to drive the load, such as the induction loop 3, in a push/pull type arrangement. Compared to the embodiment of figure 3, in addition to providing a current amplifier circuit with improved stability, the embodiment of figure 4 may provide a further advantage of an increased voltage output range.
Because the two differential amplifiers are arranged back to back to drive the load, such as the induction loop 3, from opposite ends with voltage signals in anti-phase, the voltage range which can be applied across the load is double the voltage range of the individual differential amplifiers. As is well known, such a doubling of the voltage range may allow the output drive power to be quadrupled.
This increased voltage output range may provide advantages of allowing lower power and/or lower voltage power supplies to be used to provide power to the current amplifier 20. This may allow the cost and complexity of the loop system 1 to be reduced.
In the illustrated embodiment of figure 4 the second input terminal 33b, the third resistor 38, and the positive input terminal of the second differential amplifier 32 are all connected together. In some examples the second input terminal 33b, third resistor 38, and positive input terminal of the second differential amplifier 32 may all be earthed or grounded, for example by all being connected to a ground plane conductor.
As is explained above, in the illustrated embodiment of the current amplifier circuit 30 in figure 4 the power supply to the differential amplifiers 31 and 32 is not illustrated, for clarity.
Figure 5 shows a current amplifier circuit 50 according to an embodiment of the present invention which may be used as a current amplifier 10 in a loop system 1 such as that illustrated above.
The current amplifier circuit 50 of figure 5 corresponds to the current amplifier circuit 30 illustrated in figure 4, but with a power supply arrangement shown.
Accordingly, common parts of the current amplifier circuits 40 and 30 have the same reference numbers, for clarity. The common parts of the current amplifier circuits 40 and 30 will not be described in detail again, to avoid unnecessary repetition.
The current amplifier circuit 50 further comprises a DC power supply unit 41 having positive and negative terminals, and a voltage regulator 42 having an input terminal, an output terminal, and a ground tern inal.
The negative terminal of the power supply unit 41 is connected to the second input terminal 33b, and also to the negative power supply input terminals of each of the first and second differential amplifiers 31 and 32. The positive terminal of the power supply unit 41 is connected to the positive power supply input terminals of each of the first and second differential amplifiers 31 and 32.
The input terminal of the voltage regulator 42 is connected to the positive terminal of the power supply unit 41, and also to the positive power supply input terminals of each of the first and second differential amplifiers 31 and 32. The ground terminal of the voltage regulator 42 is connected to the negative terminal of the power supply unit 41, and also to the second input terminal 33b.
Unlike the current amplifier circuit 30, in the current amplifier circuit 50 the first end of the third resistor 38 and the positive input terminal of the second differential amplifier 32 are not connected directly to the second input terminal 33b. Instead, the first end of the third resistor 38 and the positive input terminal of the second differential amplifier 32 are connected to the output terminal of the voltage regulator 42.
The current amplifier circuit 50 operates in a similar manner to the current amplifier circuit 30 described above. In the current amplifier circuit 40 the voltage regulator 42 sets the central voltage of the first and second differential amplifiers 31 and 32, that is, the output voltage of the first and second differential amplifiers 31 and 32 when there is no input signal.
In the above embodiments, an induction loop system comprising a single current amplifier is used. In other embodiments an induction loop system could comprise multiple current amplifiers.
In the above embodiments, a driver unit comprising an automatic gain control module and a current amplifier is used. In other embodiments the automatic gain control module may be omitted. In still other embodiments additional amplifier or signal conditioning stages may be used to process the input electrical signal before it is provided to the current amplifier.
The above embodiments describe the use of an induction loop system to support assisted listening for hearing aid users located inside the induction ioop. In other embodiments some or all users may not be located inside the induction loop. For example, the induction loop could be located in a wall adjacent to a user location.
In the above embodiments the use of an induction ioop system in a lecture hall is described. This is described by way of example only. The induction loop system according to the present invention may be used in any situation where it is desired to support assisted listening. These situations may include for example, a cinema, concert hall, bank, station, retail outlet, place of worship, public transport vehicle, or the like.
The different embodiments described above may be combined and features disclosed in relation to one embodiment may be used in different ones of the embodiments.
The above description relates to exemplary embodiments of the invention. The skilled person will be able to envisage alternatives within the scope of the present invention as set out in the appended claims.

Claims (28)

  1. CLAIMS1. An amplifier circuit comprising: an amplifier element having a positive input terminal, a negative input terminal, and an output terminal; first and second input terminals to receive an input signal; first and second output terminals to provide an output signal to a load; and first to fifth resistors having respective resistance values and each having respective first and second ends; wherein: the first end of the first resistor is connected to the first input terminal, and the second end of the first resistor is connected to the negative input terminal of the amplifier element and to the first end of the second resistor; the second end of the second resistor is connected to the output terminal of the amplifier element and to the first end of the fifth resistor; the second input terminal is connected to the second output terminal and to the first end of the third resistor; the second end of the third resistor is connected to the positive input terminal of the amplifier element and to the first end of the fourth resistor; the second end of the fourth resistor is connected to the first output terminal and to the second end of the fifih resistor; and the ratio of the resistance value of the first resistor to the resistance value of the second resistor is higher than the ratio of the resistance value of the third resistor to the sum of the values of the fourth and fifth resistors.
  2. 2. The amplifier circuit of claim 1, wherein the amplifying element is a differential amplifier.
  3. 3. The amplifier circuit of claim 1 or claim 2, wherein the second input tenninal, the second output terminal, and the first end of the third resistor are all connected to ground.
  4. 4. The amplifier circuit of any preceding claim, wherein the ratio of the resistance value of the first resistor to the resistance value of the second resistor is in the range 1% to 15% higher than the ratio of the resistance value of the third resistor to the sum of the values of the fourth and fifth resistors.
  5. 5. The amplifier circuit of claim 4, wherein the ratio of the resistance value of the first resistor to the resistance value of the second resistor is in the range 2% to 15% higher than the ratio of the resistance value of the third resistor to the sum of the values of the fourth and fifth resistors.
  6. 6. The amplifier circuit of claim 5, wherein the ratio of the resistance value of the first resistor to the resistance value of the second resistor is in the range 2% to 10% higher than the ratio of the resistance value of the third resistor to the sum of the values of the fourth and fifth resistors.
  7. 7. The amplifier circuit of claim 6, wherein the ratio of the resistance value of the first resistor to the resistance value of the second resistor is about 3% higher than the ratio of the resistance value of the third resistor to the sum of the values of the fourth and fifth resistors.
  8. 8. The amplifier circuit of any preceding claim, wherein the resistance values of the first to third resistors are the same.
  9. 9. The amplifier circuit of any preceding claim, wherein the resistance value of the fifth resistor is two or more orders of magnitude smaller than the resistance values of the first to fourth resistors.
  10. 10. An amplifier circuit comprising: first and second amplifier elements each having a positive input terminal, a negative input terminal, and an output terminal; first and second input terminals to receive an input signal; first and second output terminals to provide an output signal to a load; and first to seventh resistors having respective resistance values and each having respective first and second ends; wherein: the first end of the first resistor is connected to the first input terminal, and the second end of the first resistor is connected to the negative input terminal of the first amplifier element and to the first end of the second resistor; the second end of the second resistor is connected to the output terminal of the first amplifier element, to the first end of the fifth resistor, and to the first end of the sixth resistor; the second input terminal is connected to the positive input terminal of the second amplifier element and to the first end of the third resistor; the second end of the third resistor is connected to the positive input terminal of the first amplifier element and to the first end of the fourth resistor; the second end of the fourth resistor is connected to the first output terminal and to the second end of the fifth resistor; the second end of the sixth resistor is connected to the first end of a seventh resistor and to the negative input terminal of the second amplifier element; the second end of the seventh resistor is connected to the second output terminal and to the output terminal of the second amplifier element; and the ratio of the resistance value of the first resistor to the resistance value of the second resistor is higher than the ratio of the resistance value of the third resistor to the sum of the values of the fourth and fifth resistors.
  11. 11. The amplifier circuit of claim 10, wherein the resistance value of the sixth resistor is equal to the resistance value of the seventh resistor.
  12. 12. The amplifier circuit of claim 10 or claim 11, wherein the second input terminal, the first end of the third resistor, and the positive input terminal of the second amplifying element are all connected to ground.
  13. 13. The amplifier circuit of any one of claims 10 to 12, wherein each of the first and second amplifier elements has respective positive and negative power tenninals; and further comprising a DC power supply comprising a positive terminal and a negative terminal, and a voltage regulator comprising an input terminal an output tenninal and a ground terminal; wherein the first end of the third resistor and the positive input terminal of the second amplifying element are connected to the second input terminal through the voltage regulator; the positive tenninal of the power supply is connected to the positive power terminals of each of the first and second amplifier elements and to the input terminal of the voltage regulator, and the negative terminal of the power supply is connected to the second input terminal, the ground temiinal of the voltage regulator and the negative power terminals of each of the first and second amplifier elements; the output terminal of the voltage regulator is connected to the first end of the third resistor and the positive input terminal of the second amplifier element.
  14. 14. The amplifier circuit of any one of claims 10 to 13, wherein each of the first and second ampIifring elements is a differential amplifier.
  15. 15. The amplifier circuit of any one of claims 10 to 14, wherein the ratio of the resistance value of the first resistor to the resistance value of the second resistor is in the range 1% to 15% higher than the ratio of the resistance value of the third resistor to the sum of the values of the fourth and fifth resistors.
  16. 16. The amplifier circuit of claim 15, wherein the ratio of the resistance value of the first resistor to the resistance value of the second resistor is in the range 2% to 15% higher than the ratio of the resistance value of the third resistor to the sum of the values of the fourth and fifth resistors.
  17. 17. The amplifier circuit of claim 16, wherein the ratio of the resistance value of the first resistor to the resistance value of the second resistor is in the range 2% to 10% higher than the ratio of the resistance value of the third resistor to the sum of the values of the fourth and fifth resistors.
  18. 18. The amplifier circuit of claim 17, wherein the ratio of the resistance value of the first resistor to the resistance value of the second resistor is about 3% higher than the ratio of the resistance value of the third resistor to the sum of the values of the fourth and fifth resistors.
  19. 19. The amplifier circuit of any one of claims 10 to 18, wherein the resistance values of the first to third resistors are the same.
  20. 20. The amplifier circuit of claim 19, wherein the resistance values of the first to third, sixth and seventh resistors are the same.
  21. 21. The amplifier circuit of any one of claims 10 to 20, wherein the resistance value of the fifth resistor is two or more orders of magnitude smaller than the resistance values of the first to fourth, sixth and seventh resistors.
  22. 22. A cunent amplifier comprising one or more amplifier circuits according to any preceding claim.
  23. 23. A driver for an induction loop system comprising one or more amplifier circuits according to any one of claims 1 to 21.
  24. 24. A driver according to claim 23, and further comprising an automatic gain control unit ananged to condition an input electrical signal and provide the conditioned signal to the one or more amplifier circuits.
  25. 25. An induction loop system comprising: a driver according to claim 23 or claim 24, together with an induction loop.
  26. 26. An induction loop system substantially as shown in or as described with reference to the figures 1 to S of the accompanying drawings.
  27. 27. A driver for an induction loop system substantially as shown in or as described with reference to the figures 2 to 5 of the accompanying drawings.
  28. 28. An amplifier circuit substantially as shown in or as described with reference to the figures 3 to 5 of the accompanying drawings.
GB1409729.9A 2014-05-30 2014-05-30 Amplifier for induction loop Expired - Fee Related GB2526630B (en)

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Cited By (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
EP3813254A1 (en) 2019-10-22 2021-04-28 Avantgarde Acoustic Amplifier circuit with variable current source

Citations (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
WO1989008970A1 (en) * 1988-03-18 1989-09-21 Leon Andre Pieters Induction loop driving amplifiers
US6501255B2 (en) * 2000-11-21 2002-12-31 Lake Shore Cryotronics, Inc. Differential current source with active common mode reduction
GB2449904A (en) * 2007-06-07 2008-12-10 Univ Montfort A high-frequency current source for Electrical impedance Tomography (EIT), with compensation for amplifier gain error
US7821496B2 (en) * 1995-01-18 2010-10-26 Immersion Corporation Computer interface apparatus including linkage having flex
US7937109B2 (en) * 2008-01-24 2011-05-03 Hewlett-Packard Development Company, L.P. Current source driver for common ground signal interface

Patent Citations (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
WO1989008970A1 (en) * 1988-03-18 1989-09-21 Leon Andre Pieters Induction loop driving amplifiers
US7821496B2 (en) * 1995-01-18 2010-10-26 Immersion Corporation Computer interface apparatus including linkage having flex
US6501255B2 (en) * 2000-11-21 2002-12-31 Lake Shore Cryotronics, Inc. Differential current source with active common mode reduction
GB2449904A (en) * 2007-06-07 2008-12-10 Univ Montfort A high-frequency current source for Electrical impedance Tomography (EIT), with compensation for amplifier gain error
US7937109B2 (en) * 2008-01-24 2011-05-03 Hewlett-Packard Development Company, L.P. Current source driver for common ground signal interface

Cited By (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
EP3813254A1 (en) 2019-10-22 2021-04-28 Avantgarde Acoustic Amplifier circuit with variable current source

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GB2526630B (en) 2016-05-04

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