IT202000030449A1 - HIGH VOLTAGE AMPLIFIER AND ELECTROSTATIC SPEAKER INCLUDING THE AMPLIFIER - Google Patents

Description

HIGH VOLTAGE AMPLIFIER AND ELECTROSTATIC SPEAKER INCLUDING THE AMPLIFIER

Description

Field of application

The present invention belongs to the technical sector of electrical devices, preferably but not exclusively for use within acoustic devices, as well as an electrostatic speaker including the amplifier.

State of the art

How ? It is well known that electrostatic loudspeakers are particular loudspeakers characterized by the presence of a vibrating membrane placed inside a support frame which guarantees the correct tension and keeps it in position.

In particular, the vibrating membrane ? usually composed of a film of insulating material suitably treated in order to reduce the resistivity? surface and favor the diffusion of a surface electric charge.

As schematized in Fig. 1, the membrane ? inserted between two stators, each generally defined by a perforated panel associated with respective conductive elements, for example suitably insulated electric cables or layers of conductive material, so? to constitute each one an electrostatic cell and in which sar? inserted the membrane.

The essential structure of the speaker ? completed by a voltage generator which generates a continuous polarization voltage Vp of the order of a few kV, applied to the vibrating conductive membrane to distribute an electric charge over it.

The generator ? moreover configured to produce a pair of alternating voltages Vs and in phase opposition, and also of the order of kV, to be applied to the stators to generate an electric field in the space defined between the same stators and in which ? membrane placed.

The combined action of the polarization charge distributed on the membrane and of the electric field in which the same? immersed creates a force that moves the membrane itself.

because voltages that drive the stators are substantially an audio signal suitably amplified, the membrane? stimulated to move according to the audio signal itself, so that the movement of the membrane generates the pressure waves responsible for the generation of sound in the air.

Historically, the voltages for the stators are created with a system based on transformers, schematized in Fig. 2.

The input audio is applied to an amplifier which drives the step-up transformer characterized by a turns ratio of the order of 1:100+100, as audio amplifiers, whether solid state or tube, are designed to drive dynamic loudspeakers, which require voltages in the order of tens of volts.

For a long time the step-up transformer ? Was it the only method to obtain the high voltages required by the electrostatic panel and, at least in principle, the only limit to the voltage that can be supplied? given by the insulation of the wire.

In fact, modern electronic devices, such as MOSFETs or bipolar transistors, support maximum voltages of the order of hundreds of volts, a maximum of a thousand and would not be suitable for such purposes. Only relatively recently have MOSFETs and IGBTs with maximum voltages of a few kV been made available.

However, the transformer, despite being used for a long time, is not the optimal solution since it is ? characterized by at least two contrasting parameters: the maximum permissible magnetic flux from the nucleus and the resonance frequency. The magnetic flux ? given by

in which V1 ? the voltage applied to the primary, f ? the frequency, N1 ? the number of coils of the primary.

The magnetic core that makes up the transformer? characterized by a maximum admissible flow beyond which saturation of the core occurs.

The transformer must for? operate with a flux lower than the saturation value: it follows that as the frequency decreases, the voltage applicable to the transformer primary also decreases and this determines, for practical purposes, a limit to the minimum frequency that can be handled by the driver.

In principle, to reduce the minimum frequency you can? increase the number of turns of the windings, but in this way you increase the capacity? parasite which in turn determines the lowering of the resonant frequency of the transformer which limits, in fact, the maximum frequency that can? be correctly applied to the transformer.

? evident that the coupling transformer limits the bandwidth that can? be reproduced by the entire electrostatic loudspeaker system.

Presentation of the invention

Purpose of the present invention? that of overcoming the drawbacks indicated above, by realizing a high voltage amplifier which is particularly effective and which allows the aforementioned limitations of the solutions using a transformer to be overcome. A particular purpose? that of realizing a high voltage amplifier, suitable to be used for electrostatic loudspeakers.

Yet another purpose? that of realizing such a high voltage amplifier which is characterized by a high extension of the pass band, unlike the transformers.

Yet another purpose? that of realizing such a high voltage amplifier which is compact, efficient and light and which can be replicated in series also possibly using commercially available components.

Yet another purpose? that of realizing a high voltage amplifier suitable for electrostatic loudspeakers which has reduced absorption and compact dimensions to be able to be inserted inside the loudspeaker frame so to have a streamlined and pleasing to the eye assembly, thus avoiding bulky frames and heat sinks.

These purposes, as well as others that will appear more? explained below, are achieved by a high voltage amplifier which, according to claim 1, comprises means for generating voltage suitable for being applied to at least one load and having a driving circuit of the SRPP type made with solid state devices , wherein said driving circuit comprises an input branch provided with a first solid-state active component and with a first resistance for the passage of a first current and an output branch placed in series with said input branch and provided with a second solid-state active component and a second resistance for the passage of a second current.

According to a peculiar characteristic of the invention, said driving circuit SRPP ? of the modified type and has a third branch placed in parallel with the portion of said first circuit comprising said first resistance and ? configured to introduce a non-linearity? adapted to produce an increase in said first current and allow the output portion of said output branch to deliver all the current required by the load.

Advantageous embodiments of the invention are obtained according to the dependent claims.

Brief description of the drawings

Further characteristics and advantages of the invention will become more evident thanks to the aid of the attached figures in which:

FIG. 1 ? a schematic representation of an electrostatic loudspeaker; FIG. 2 ? a schematic representation of a transformer driving circuit according to the prior art;

FIG. 3 ? a schematic representation of a driver circuit according to the present invention;

FIG. 4 ? a graph showing the trend of voltage and current inside a known type SRPP circuit;

FIG. 5 ? a graph showing the trend of voltage and current inside a SRPP circuit according to the present invention.

Detailed description of a preferred embodiment In the following, a particular configuration of a high voltage amplifier according to the present invention is described, intended to be applied to an electrostatic loudspeaker, i.e. a loudspeaker provided with one or more? loudspeakers each comprising a pair of stators connected to the high voltage amplifier for the generation of an electric field and a vibrating conductive membrane placed in the gap between the two stators and also powered by means of a polarization voltage to be set in vibration and generate sound.

However, it is understood that the amplifier according to the present invention can find advantageous application in any electrical or electronic component that requires the use of the amplifier, such as, by way of example but not limited to, a TEM cell to be used in compatibility measurements? electromagnetic.

In general, an electrostatic loudspeaker, generically indicated with 1, will present? a basic structure substantially similar to the known electrostatic speakers, that is, as schematized in Fig.1, it will include? a pair of stators facing each other and spaced apart to define an interspace, a conductive membrane placed in the interspace and means for generating voltage suitable for supplying the conductive membrane with a continuous bias voltage Vp and for applying to the stators a pair of voltages Vs in phase opposition to generate a potential difference suitable for establishing an electric field in the interspace and bringing the conductive membrane into vibration for the generation of sound.

The structure of the two stators can be chosen among those known from the state of the art, without particular limitations, and each of them will have? typically a support structure suitable for defining an anchorage for the membrane and having a front surface suitable for facing, in use, towards the outside, and a rear surface suitable for facing, in use, towards the membrane.

The support structure will be also equipped with means for the conduction of electrical energy connected to the voltage generation means.

However, in a particularly preferred manner, the electric energy conduction means comprise at least one electric conductor provided with an insulating sheath and integral with the rear face of the support structure of the stators.

In a peculiar way, the electric conductor will be? co-molded throughout its extension on the rear surface of the support structure to be integral with the same throughout its longitudinal extension and without the possibility? of deviations.

Conveniently, the coupling between the electric conductor and the support structure could be obtained by preparing a mold having the negative shape of the support structure, which must be be provided with suitable housings for the electrical conductor.

Therefore, the presenter mold? similar housings of complementary shape to those of the support structure and in which it will come? arranged the electrical conductor.

In this way, once the electrical conductor has been positioned in the mold according to the configuration that it will have to have a finished processing, you can? proceed with the co-moulding, preferably an injection co-moulding, of the material from which it will be? formed the support structure and that will be? preferably a thermoplastic polymeric material.

This particular configuration will allow? to have a highly stable and uniform electric field inside the cavity.

Regardless of the structure of the stators and the respective electrical energy conduction means, the high voltage generation means, defined by the high voltage amplifier according to the present invention, are characterized by the fact that they do not require the use of transformers instead of which there will be, for each of the stators, driving circuits of the SRPP type suitably modified to increase the output current and the voltage gain.

In particular, the aforesaid driving circuits, illustrated more? clearly in Fig.3, they are modified to operate in class AB.

To this end, the driving circuits each comprise an input branch provided with a first solid-state active component and a first resistance for the passage of a first current and an output branch, placed in series with the input branch, and provided with a second active component and a second resistance for the passage of a second current.

In a peculiar way, the SRPP driver circuit ? of the modified type and has a third branch placed in parallel with the portion of the first circuit comprising the first resistor and ? configured to introduce a non-linearity? adapted to produce an increase in the first current and allow the output section of the output branch to deliver all the current required by the load.

In this way the SRPP circuit is characterized by the fact that the maximum current that can be supplied ? equal to the quiescent current.

Therefore, in the event that the load requires a high current, sar? It is necessary to provide a suitably high quiescent current, which in turn determines a strong power dissipation and the need? of abundant means for heat dissipation.

The addition of the third branch allows the input branch to increase the quiescent current so as to automatically adapt to the demands of the load. Will the power absorbed at rest be maintained? low and grow? only if required by piloting requirements. This allows the load to be driven perfectly, while keeping the dissipation at rest relatively low.

Preferably, the third branch comprises a third resistance placed in parallel with the first resistance and having a lower value than the same.

Furthermore, the third branch comprises a diode placed in series with the third resistance and a voltage reference capable of supplying a fixed or variable voltage higher than the rest voltage on the first resistance and preferably equal to a value substantially equal to one and a half times the value of that tension.

For example, the voltage reference can? be a voltage generator located downstream of the third resistor.

The active components will be selected from the group comprising bipolar transistors, IGBTs, MOSFETs and the like.

According to a further particularly advantageous aspect, which will appear? more clear in the following, these components will be powered by the source (if mosfet) or by the emitter (if igbt or bipolar transistors) instead? from the gate, creating a structure that you can? define CASCODE-SRPP.

When sizing the driving circuit, it is also necessary to consider that the current I1 develops the voltage R1 * I1 on the first resistance R1

D1 begins to conduct when the current I1 which circulates in the first component X1 causes a voltage drop higher than E1 on R1.

Since D1 begins to conduct, the equivalent resistance seen by the first component X1 will be? given by the parallel between R1 and R3. This entails the possibility to grow the current in X1 much more? faster than with only the first resistor R1.

The addition of the third branch allows a strong increase in the impulsive current in the first active component, which translates, in fact, into the temporary increase in the current at rest which, in turn, allows the circuit to deliver a much higher current than the current of static rest.

As a demonstration of the above, consider a hypothetical classic SRPP biased with 7.5mA at rest, called to deliver sinusoidal voltage on a 2nF capacitor.

With reference to the graph of Fig. 4, in which the green trace represents the voltage supplied to the load and the blue trace represents the current I1 in the lower Mosfet X1 and the red trace ? the current I2 in the upper MOSFET X2, it is observed that the voltage supplied ? strongly distorted with respect to the sinusoid of reference and that the current supplied ? strongly asymmetrical, a consequence of the different capacity? to deliver current to the two MOSFETs that make up the SRPP.

? otherwise? it is clear that the lower MOSFET X1, due to its class A operation, works a sinusoidal current with an amplitude equal to the quiescent current (7.5mA peak).

Instead, the upper MOSFET X2, which substantially functions as a follower of the voltage developed on its source resistance due to the current modulated by the lower MOSFET X1, is able to deliver all the current required by the load.

The result ? a highly distorted output current and, consequently, a highly distorted voltage supplied to the stator and a voltage supplied limited by the ?slew rate? imposed by the maximum current that can be delivered (equal to the current at rest) divided by the capacity? of the stator to drive.

Adding to the classic SRPP the network formed by the diode D1, resistance R3 of much lower value than R1 and voltage E3 equal to approximately one and a half times of the voltage at rest on R1, in the same operating conditions of the previous circuit (therefore same load and same input voltage), the graph of Fig. 5 is obtained in which the green trace represents the voltage supplied to the load, the red trace represents the current I2 in the upper MOSFET X2 and the blue trace represents the current I1 in the lower MOSFET X1.

From this graph it can be observed that the current in the lower MOSFET X1 has a ?sinusoidal-like? only below quiescent current.

For currents higher than the quiescent current, the current in the lower MOSFET can grow very fast, while the upper mosfet continues to work as in the previous case.

The result ? what time is the SRPP stadium ? capable of delivering all the current required by the load.

Considering that the lower MOSFET X1 never reaches the complete interdiction, why? the current must not, however, be completely reset, otherwise the saturation of the? output), you can? to state that the new SRPP essentially works in class AB. The SRPP circuit cos? modified ? nevertheless characterized by a strong non-linearity, due precisely to the presence of the third branch, which introduces a strong distortion which in fact prevents its use in open loop.

It is therefore necessary to close a feedback loop that keeps the gain of the entire stage under control, guaranteeing linearity? of the system. As a non-limiting example, it is proposed to feed the input branch from the source through an operational amplifier.

The circuit is now configured as a CASCODE loaded by the new SRPP, in which the operational amplifier sets the gain, defined as usual as the ratio between the RF1/RF2 reaction resistances, compensates for the non-linearity? and smoothes out the frequency response.

The final result ? a driver that delivers thousands of Volts to the electrostatic panel, with all the current required by the panel and keeping the power dissipated at rest very low.

From the foregoing it can be observed that the object of the present invention achieves the intended objects.

Claims (8)

Claims 1. A high voltage amplifier, comprising means for generating voltage suitable for being applied to at least one load and having a driving circuit of the SRPP type made with solid state devices, in which said driving circuit comprises an input branch provided with a first solid-state active component and a first resistance for the passage of a first current and an output branch placed in series with said input branch and provided with a second solid-state active component and a second resistance for the passage of a second current; characterized in that said driving circuit SRPP ? of the modified type and has a third branch placed in parallel with the portion of said first circuit comprising said first resistance and ? configured to introduce a non-linearity? adapted to produce an increase in said first current and allow the output portion of said output branch to deliver all the current required by the load. 2. Amplifier according to claim 1, characterized in that said third branch comprises a third resistance placed in parallel with said first resistance and having a lower value than the same. 3. Amplifier according to claim 2, characterized in that said third branch comprises a diode placed in series with said third resistance and a voltage reference able to supply a fixed or variable voltage higher than the rest voltage on said first resistance and preferably equal to a value substantially equal to one and a half times the value of said voltage at rest. 4. Amplifier according to claim 3, characterized in that said voltage reference ? a voltage generator placed downstream of said third resistance. 5. Amplifier according to any one of the preceding claims, characterized in that said active components are selected from the group comprising bipolar transistors, IGBTs, MOSFETs and the like. 6. Amplifier according to any one of the preceding claims, characterized in that said active components are MOSFETs. 7. Amplifier according to claim 6, characterized in that said MOSFET ? powered by the source. 8. An electrostatic diffuser, comprising: - a pair of stators facing each other and spaced apart to define a gap; - a conductive membrane placed in said interspace; - means for generating voltage suitable for being applied to at least one load to supply said conductive membrane with a direct bias voltage (Vp) and for applying to said stators a pair of alternating voltages in phase opposition to generate a potential difference suitable establishing an electric field in said interspace and causing said conductive membrane to vibrate for the generation of sound; wherein said generation means comprise, for each of said stators, a respective driving circuit of the SRPP type made with solid-state devices to increase the output current and the voltage gain; wherein said driving circuit comprises an input branch provided with a first solid-state active component and with a first resistance for the passage of a first current and an output branch placed in series with said input branch and provided with a second active component and a second resistance for the passage of a second current; characterized in that said driving circuit SRPP ? of the modified type and has a third branch placed in parallel with the portion of said first circuit comprising said first resistance and ? configured to introduce a non-linearity? adapted to produce an increase in said first current and allow the output portion of said output branch to deliver all the current required by the load.