EP3324647A1

EP3324647A1 - An assembly and an amplifier for use in the assembly

Info

Publication number: EP3324647A1
Application number: EP16199655.8A
Authority: EP
Inventors: Michel De Nooij; Adrianus Maria Lafort; Michiel Van Nieuwkerk
Original assignee: Sonion Nederland BV
Current assignee: Sonion Nederland BV
Priority date: 2016-11-18
Filing date: 2016-11-18
Publication date: 2018-05-23

Abstract

An assembly of a capacitive transducer and an amplifier where a bootstrapping is added with a low pass filter and a capacitor between the amplifier output and the amplifier input or the transducer, where the low pass filter has a cut-off frequency at or below a resonance frequency of the transducer. The capacitor capacitance is selected so that it does not destroy the constant charge mode of the transducer.

Description

The present invention relates to an assembly comprising a capacitive transducer feeding a signal to an amplifier with a bootstrap reducing a resonance peak of the transducer.
A first aspect of the invention relates to an assembly comprising:

a capacitive transducer comprising two transducer terminals, the transducer being configured to sense a variation and output a corresponding signal on at least one of the terminals, the transducer having a response with at least one resonance frequency,
an amplifier having an amplifier input and an amplifier output,
a first connection element connecting one of the transducer terminals to the amplifier input,
a second connection element connected to the amplifier output and the connection element (such as amplifier input),

In this respect, the elements of the assembly may be provided in a single housing or even in the same chip. Alternatively, different elements may be provided separately and interconnected, such as by electrical conductors.
A transducer is an element configured to sense or detect a parameter of the transducer or its surroundings, such as vibration or sound. Often, the transducer will have a stationary element and a movable element and will output a signal corresponding to a variation of a distance between the movable element and the stationary element. In this respect, corresponding often means that the frequency contents of the output signal, at least within a predetermined frequency interval, corresponds to that of the parameter detected.
Naturally, which element is stationary and movable will depend on in which coordinate system one views the system. In many situations, the movable element is more resilient and bendable, for example, than the stationary element, so that the stationary element is stationary in relation to a remainder, such as a housing, of the transducer. Naturally, multiple movable elements may be used in addition to or instead of a movable element and a stationary element. Also, multiple stationary elements may be used together with the one or more movable elements.
The transducer has at least two transducer terminals. The output signal is normally output as a difference in voltage between two terminals. Thus, the output may be seen as derived from one of the terminals, if the other is kept at a predetermined, fixed voltage, such as ground.
A capacitive transducer is an electro-acoustical or electro-mechanical transducer, the capacitance of which varies with the parameter sensed. An electrical field may be generated in the transducer by biasing two elements therein (providing a voltage between the elements) and/or by permanently charging an element. When that or another element moves within that field, an output signal may be derived which relates to the change in capacitance due to this movement.
Capacitive transducers have at least one movable element which has a mass and a spring constant and thus has a resonance frequency. This resonance frequency will be a frequency at which the output signal has, compared to an input signal causing the output, a higher output intensity, such as a higher voltage. A resonance peak is a peak in the output characteristics or response of the transducer where the output it higher when normalized to a predetermined input (excitement) value. For e.g. a vibrating element induced at its resonance frequency, the vibration amplitude may be higher, causing the output to be higher.
Naturally, multiple resonance frequencies or peaks may exist in the output response of the transducer.
An amplifier is an element which is configured to receive an input signal and output an output signal where the intensity (voltage/current or the like) of the output signal has been amplified. In this respect, an amplification may be higher than 1, so that the intensity output is higher than that received, or lower than 1, whereby the intensity output is lower than that received. An amplification of 1 outputs the same intensity. This may be desired for other purposes, such as for altering the apparent impedance of a circuit receiving the output of the amplifier compared to the component feeding the signal to the amplifier. The amplification may also be negative, whereby the polarization of the signal output of the amplifier is the opposite of that received.
Naturally, an amplifier may have multiple inputs. Often, when a single input is described, any additional inputs may be provided with predetermined signals or voltages, such as ground.
Thus, between the transducer terminal and the amplifier input, a first transporting element is provided which may simply be a conductor or which may comprise components for altering the signal output of the transducer before inputting a signal to the amplifier. Preferably, any components of the transporting element are passive, i.e. require no separate power supply.
A second connection element is connected to the amplifier output and the first connection element, such as the amplifier input. This second connection element comprises a capacitor and low pass filter.
Low pass filters have a cut-off frequency below which it is not desired to attenuate the signal but above which attenuation is desired. Usually, the filter will have an attenuation increasing with frequency and not a sharp change in its filtering characteristics. Often, a cut-off frequency is described as the frequency at which a predetermined attenuation is seen, such as an attenuation of 3dB (also called a -3dB point). According to the invention, the cut-off frequency of the low-pass filter is lower than the at least one resonance frequency, so that the signal output of the low pass filter will have frequencies at the resonance frequency in question attenuated.
The second transporting element additionally has a capacitor having a capacitance of 50%-800% of a capacitance of the transducer.
For signals from the transducer with a frequency below the cut-off frequency of the filter, the voltage on both sides of capacitor preferably are approximately equal (the capacitor is being "bootstrapped"), so that the capacitor thus does not load the transducer. For signal frequencies above the cut-off, however, the capacitor is preferably not bootstrapped, and thus loads the transducer, and the signal is attenuated.
Thus, the operation of the low pass filter is to control the operation (bootstrapping) of the capacitor, the operation of which is to load the transducer at frequencies above the cut-off frequency of the low pass filter.
When the low pass filter is positioned closer to the amplifier output and the capacitor closer to the first connection element, such as when the capacitor is connected to the first connection element or the transducer, the capacitor is bootstrapped for the low frequencies passed by the filter.
In one embodiment, the cut-off frequency is 30%-100% of the at least one resonance frequency, such as 40-90%, 50-85%, or 60-80% of the resonance frequency.
In one situation, the capacitance of the capacitor is 600% or less, such as 500% or less such 400% or less, such as 300% or less of the capacitance of the transducer. The capacitor capacitance may alternatively be selected in the interval of 50-600%, such as 90-500%, such as 100-400% of the capacitance of the transducer.
Naturally, the properties of the second connection element may be adapted, such as to different types of transducers or different uses thereof, such as different resonance frequencies thereof. This may e.g. be achieved when the capacitor has a programmable or selectable capacitance. A capacitor may have an adaptable capacitance, or multiple capacitors may be provided and chosen from.
Also, the properties, such as the cut-off frequency, the slope and the like of the low pass filter may be adapted to the transducer and/or resonance frequency.
In one embodiment, the assembly further comprises a voltage supply configured to output a voltage, where the transducer comprises:

a movable element,
one or more stationary elements, and
a voltage input configured to receive the voltage and provide the first voltage between two of the movable element and the stationary element(s),

As mentioned above, a transducer terminal may be connected to a predetermined voltage, such as ground. Otherwise, generally, the output from that terminal may also be fed to the amplifier, if desired.
Any number of stationary elements may be provided. Often one or two stationary elements are provided in situations where the movable element is a plane element, where the stationary element(s) is/are also plane element(s) provided parallel to the movable element in a desired distance so that the movable element may move while being in a vicinity of the stationary element(s). Transducers of this type may be microphones, where the movable element may then be a diaphragm.
The voltage is provided between two of the movable element and the stationary element(s). If a single stationary element is provided, the voltage is provided between the stationary element and the movable element. If two stationary elements are provided, the voltage may be provided between the stationary elements or between one stationary element and the movable element. Naturally, different voltages may be provided to all of the stationary elements and the movable element.
Multiple movable elements may also be provided if desired, where any additional movable element may also receive a voltage or output a signal.
An output of the transducer may be derived from any one or more of the stationary element(s) and the movable element. Usually, the output of the transceiver will depend on the movement or position of the movable element in relation to the stationary element(s).
In this embodiment, the first connection element may comprise a capacitor. This capacitor may provide a DC decoupling between the transducer terminal and the amplifier, so as to not affect the charging of the transducer.
A transducer of this type may be a MEMS transducer, i.e. a transducer manufactured using the MEMS technology.
In general, the gain of the amplifier may be selected for a number of reasons. In one situation, the amplifier gain is desired to be around 1 in order to not amplify or attenuate the signal from the transducer but simply adapt the impedance "seen" by circuitry for receiving the output of the present assembly.
Thus, generally, the amplifier gain is preferred in the interval of 0.5 and 2.
Another consideration for defining the amplifier gain is that at a predetermined frequency below the cut-off frequency of the filter, a resulting voltage on both sides of the capacitor - one provided by the transducer and the other from the amplifier output and the low pass filter preferably are within 20%, such as within 10% of each other (the 20/10% being from the higher of the two voltages). Thus, if the low pass filter has an attenuation at that frequency, the amplifier gain may be adjusted to reduce the voltage difference over the capacitor.
Preferably, the low pass filter is positioned in series with the capacitor, the low pass filter being positioned closer to the amplifier output and the capacitor is connected directly to the transport element and/or the transducer.
Preferably, a capacitance of the capacitor is no more than $y = \frac{2 ϕ - \sqrt{27 (1 - x)}}{x \sqrt{27 (1 - x) - 2 ϕ}},$
times a capacitance of the transducer, where x is a ratio between a minimum distance and an unbiased distance of the transducer and ϕ is a ratio between bias voltage and pull-in voltage of the transducer
Thus, from parameters of the transducer (see further below), a maximum capacitance of the capacitor may be determined. Naturally, the capacitance of the capacitor may be selected to be below 90% of this maximum, such as below 80%, such as in the interval of 50-95% of this maximum, such as in the interval of 60-80%.
A second aspect of the invention relates to an amplifier unit for use in the assembly according to any of the preceding claims, the amplifier unit comprising:

a first and a second input terminals,
an amplifier having an amplifier input and an amplifier output,
a first connection element connecting the first input terminal to the amplifier input,
a second connection element connected to the amplifier output and the amplifier input,

Naturally, the above considerations, embodiments and situations are equally relevant in relation to the second aspect. Thus, the adaptability of the capacitance and the cut-off frequency are desired in order to make the unit versatile and useful for combining with a number of different transducers.
Naturally, the second connection element may comprise the above capacitor and any other components desired, such as high-impedance elements and the like.
The unit may also comprise the above voltage supply providing the voltage to the transducer, so that the transducer terminals may simply be connected to the input terminals, where all other elements are provided in the unit. Then, such elements, such as the voltage supply, may also be adaptable to a particular type of transducer.
The present embodiments may be combined with a number of other advantageous improvements of systems, such as the Applicants co-pending applications filed on even date and with the titles: "A CIRCUIT FOR PROVIDING A HIGH AND A LOW IMPEDANCE AND A SYSTEM COMPRISING THE CIRCUIT", "A TRANSDUCER WITH A HIGH SENSITIVITY", "A SENSING CIRCUIT COMPRISING AN AMPLIFYING CIRCUIT AND THE AMPLIFYING CIRCUIT" and "A PHASE CORRECTING SYSTEM AND A PHASE CORRECTABLE TRANSDUCER SYSTEM". These references are hereby incorporated by reference.
In the following, preferred embodiments of the invention will be described with reference to the drawing, wherein:

Figure 1 illustrates a first embodiment of the invention,
Figure 2 illustrates a second embodiment of the invention,
Figure 3 illustrates a third embodiment of the invention,
Figure 4 illustrates the frequency response and the effect of the bootstrapping and
Figure 5 illustrates a manner of determining a maximum value for a ratio between load capacitance and sensor capacitance.

In figure 1, an assembly 10 is illustrated where a capacitive transducer 12 outputs a signal on one terminal (the other terminal is connected to ground) to an input of an amplifier 20. The output of the amplifier is fed to a connection element comprising a low pass filter 25 and a capacitor 28 to feed a low pass filtered signal to the amplifier input.
The present transducer 12 has a response (output intensity/voltage as a function of received frequency) having one or more resonance peaks. Such resonance peaks are frequencies or frequency intervals where the output is higher than at other frequencies. Often, such resonance frequencies are in an upper frequency portion of a relevant frequency interval.
For microphones, the relevant frequency interval is 20Hz - 20kHz, such as 100Hz-10kHz and resonance frequencies often are higher than 10kHz. Often, microphones are based on capacitive microphones, such as MEMS devices, electret devices, or piezo electric devices.
For vibration sensors, the relevant frequency interval is 100Hz-5kHz and resonance frequencies are often seen around 3-4kHz . Often, vibration sensors are obtained using capacitive elements.
Piezo transducers often have resonance frequencies at about 25kHz, where the peak may be more than 15dB higher than a mean value at e.g. a reference frequency of 1 kHz. Thus, a cut-off frequency of between 10kHz and 20kHz and a slope of 6dB/oct or higher could be useful in relation to such transducers.
MEMS transducers often have resonance frequencies at about 15kHz, where the peak may be more than 10 dB higher than a mean value at e.g. 1 kHz. Thus, a cut-off frequency of between 8kHz and 12kHz and a slope of 6dB/oct could be useful in relation to such transducers.
For signals from the transducer with a frequency below the cut-off frequency of the filter, the voltage on both sides of capacitor 28 is approximately equal (the capacitor is being "bootstrapped"), and the capacitor thus doesn't load the transducer. For signal frequency above the cut-off, however, the capacitor is not bootstrapped, and thus loads the transducer, and the signal is attenuated.
A capacitive sensor or transducer may be biased or not. A biased sensor has one or more elements, one being movable in relation to the other, between which a voltage is provided and the relative movement of which is detected.
Ideally, a biased capacitive sensor, such as the transducer of figure 2, is operated in constant charge mode, so that collapse cannot occur.
A capacitive load, such as the capacitor 28, connected to the transducer behaves as a charge buffer, and thus disturbs the constant charge operation and negatively affects the stability of the transducer.
In transducers where the movement of the diaphragm (or other movable element) is mechanically limited, so that the diaphragm cannot touch the backplate (or other stationary element), this mechanical limitation allows some deviation from the strict constant-charge condition.
Depending on (1) the ratio of the minimum distance and the unbiased distance 'x', and (2) the ratio of the operating bias voltage and the pull-in voltage 'ϕ', a certain maximum capacitive load can be allowed. This maximum can be expressed as a ratio with the capacitance of the sensor 'y'.
E.g., using a parallel plate approximation for the sensor, $y = \frac{2 ϕ - \sqrt{27 (1 - x)}}{x \sqrt{27 (1 - x) - 2 ϕ}},$
so that if the minimum distance 'x' is 1/3^rd of the air gap, and the applied bias 'ϕ' is 90% of the pull-in voltage, the maximum load capacitance 'y' is about 4 times the capacitance of the sensor. In practice a lower value may be used in order to allow for parasitic capacitances in the circuit.
In figure 5, the maximum value for the ratio y (load capacitance / sensor capacitance) is illustrated for a variety of values for the ratio x (unbiased distance / minimum distance; X=10, 15, 20, 25 and 30%) as a function of ratio ϕ (bias voltage / pull-in voltage). The total load capacitance connected to the sensor preferably is smaller than this maximum in order to prevent collapse. For the value of the capacitor 28 this means that any parasitic capacitance should be taken in to account, including some margin for manufacturing variance.
In addition, the slope of the low pass filter may be selected so that the resulting filtering of the transducer signal is suitable. Naturally, the order of the filter may be selected in order to obtain the filtering characteristics sought.
The amplifier gain preferably is positive. Naturally, the gain may be adapted to the filtering characteristics, but often, the gain is desired to be around 1 or smaller than 1. Thus, the function of the amplifier may also be that of an impedance adaptation to what is desired by circuitry to receive the output of the present system.
In this manner, the resonance peak may be reduced but the frequencies below the cut-off need not be affected to any significant degree.
This bootstrapping can reduce the resonance peak and thus the requirements on the amplifier. Thus, a larger headroom may be obtained, or the supply voltage to the amplifier may be reduced to save power in battery operated situations.
In figure 4, an example of this result is seen where the upper graph is the output or response of a transducer. The resonance peak is 15dB higher than the output at 1000Hz. The signal fed back to the amplifier input is the middle graph and has the resonance peak reduced to 11dB. The amplifier output is the lowest graph where the resonance peak is reduced to 8dB. This reduction of the resonance peak without to any significant degree affecting the frequencies below a few thousand Hz is obtained using a capacitance of the capacitor 28 being about 1.5 times the capacitance of the transducer, and a filter with -3dB point at 8kHz.
In figure 2, a set-up 10 is illustrated having a transducer 12, preferably a biased MEMS transducer, having a diaphragm d and a back plate bp between which a biasing voltage is provided. An output of the transducer 12 is correlated with a distance between the diaphragm and back plate so that when the diaphragm is vibrated, such as when exposed to sound, a correspondingly varied output is seen.
The output of the transducer 12 is fed through a capacitor 18 to the amplifier 20. The desired operation of the capacitor 18 is to transfer the varying signal from the transducer without creating a DC connection between the transducer and the remainder of the circuit, as this could destroy the biasing of the transducer. Thus, the capacitor preferably has a value being sufficiently high. At present, the capacitor 18 is at least 2, such as at least 4, such as at least 6, such as at least 8 times the capacitance of the transducer.
In usual set-ups of this type, a biasing circuit is provided comprising a charge pump 22 and a high impedance element 24, such as a pair of anti-parallel diodes, generating a high impedance at the terminal of the transducer where the signal is derived. Also, a high impedance element 26, such as a pair of anti-parallel diodes, is preferably provided between ground and the connector between the capacitor 18 and the amplifier input, to provide a high impedance at this position.
The output of the transducer has an output voltage swing defined by at least the biasing voltage 22 but also any resonance peaks of the transducer. This puts requirements on the amplifier, as a maximum output voltage swing may be rather large especially at the resonance frequency.
Again, the bootstrapping is provided feeding the output signal back to another capacitor 28 provided in series with the low pass filter. In this embodiment, the output of the low pass filter 25 is the output of the assembly and thus will be fed to any subsequent circuitry for receiving the output.
In figure 2, the bootstrap (output of capacitor 28) is connected between capacitor 18 and the amplifier input. However, the bootstrap may be connected to any portion of the circuit between the transducer output (diaphragm) and the amplifier input.
One may call the diaphragm, in this embodiment, a high impedance terminal, as the impedance is defined by the element 24 creating a high impedance, and the back plate a low impedance terminal, as in this embodiment, it is connected to ground.
Usually, when biased transducers are used, the biasing voltage of the transducer 12 is much higher than the supply voltage to the amplifier 20 in order to obtain a suitable sensitivity of the transducer. However, the output voltage swing of the transducer is, in fact, even higher, as the diaphragm may move from a position far from its rest position and far away from the back plate to an opposite position very near to the back plate. Thus, in theory, the output of the transducer may be up to twice the biasing voltage. Often, the movement of the diaphragm is physically limited in the direction toward and close to the back plate in order to ensure that the diaphragm does not touch the back plate.
The transducer output voltage swing is partially reduced by the present bootstrapping, but it may nevertheless be desired, in order for the amplifier to be able to handle such an output voltage swing, to supply the amplifier with a comparable supply voltage being at least 40% but preferably at least 50%, 60%, 80%, 90%, 100%, 120%, 125%, 130%, 140% or more times the biasing voltage to the transducer.
In this manner, the input voltage swing of the amplifier is sufficiently large for it to handle the output of the transducer without distorting or removing part of the received signal.
In figure 3, an alternative assembly to that if figure 2 is seen where an additional low-pass filter 251 is seen. In this situation, two low pass filters, 25 and 251 are provided, which may be different, one is provided in the bootstrap/feed back and the other for filtering the output of the amplifier to provide a filtered output of the assembly.
Naturally, all types of capacitive sensors may be used, such as piezo, MEMS, electret, or biased capacitive sensors, like microphones and acceleration sensors.
Above, biased sensors are described having a diaphragm and a single back plate with the diaphragm biased. Naturally, the back plate may be biased, and any of the two elements may be used for generating the output of the transducer.
Also, multiple back plates may be provided. Usually, a back plate on either side of the diaphragm is provided and the biasing voltage provided between the diaphragm and a back plate. Alternatively, the biasing voltage may be provided between the two back plates. The output may be derived from the diaphragm or a back plate or from both back plates such as if these are connected to inputs of a differential amplifier.
The above assemblies may be divided into building blocks, where the amplifier and bootstrapping circuit (low pass filter and capacitor - see hatched block in figure 1) may be made more general and adaptable to multiple transducers or transducer types. This adaptation may be obtained by providing an adaptable low pass filter (variable resistance/capacitance or multiple resistors and/or capacitors to choose from, variable cut-off frequency and/or slope), multiple values of the capacitor 28 or a variable capacitance thereof and where the amplifier may have a variable gain.
If the transducer is a biased transducer, also the capacitor between the amplifier and the transducer may be included in the building unit (see hatched outline in figure 2). In this situation, also the charge pump 22 and the high impedance element 24 may be included in the general building block.
In that situation, the general purpose amplifier unit 11 (amplifier and bootstrapping loop with filter and capacitor) may be connected to a variety of transducers and transducer types and then be programmed or adapted to a particular transducer or transducer type.
This unit may have two terminals for connection to the transducer 12. One terminal may (see hatched line figure 2) simply be one connected to ground. Also, the biasing circuits 22/24 may be provided in the unit 11 if desired.

Claims

An assembly comprising:
- a capacitive transducer comprising two transducer terminals, the transducer being configured to sense a variation and output a corresponding signal on at least one of the terminals, the transducer having a response with at least one resonance frequency,

- an amplifier having an amplifier input and an amplifier output,

- a first connection element connecting one of the transducer terminals to the amplifier input,

- a second connection element connected to the amplifier output and the first connection element,
where the second connection element comprises a capacitor and low pass filter,
the low pass filter having a cut-off frequency of the low-pass filter is lower than the at least one resonance frequency and
the capacitor having a capacitance of no more than 800% of a capacitance of the transducer.
An assembly according to claim 1, wherein the cut-off frequency is 30%-100% of the at least one resonance frequency.
An assembly according to any of claims 1 and 2, wherein the capacitance of the capacitor is no more than 500% of the capacitance of the transducer.
An assembly according to any of the preceding claims, wherein the capacitor has a programmable or selectable capacitance.
An assembly according to any of the preceding claims, further comprising a voltage supply configured to output a voltage, where the transducer comprises:
- a movable element,

- one or more stationary elements, and

- a voltage input configured to receive the voltage and provide the first voltage between two of the movable element and the stationary element(s),
where each transducer terminal is connected to one of the movable element and the stationary element(s).
An assembly according to claim 5, wherein the first connection element comprises a capacitor.
An assembly according to claim 5 or 6, wherein the transducer is a MEMS transducer
An assembly according to any of the preceding claims, wherein the amplifier has a gain in the interval of 0.5 and 2.
An assembly according to any of the preceding claims, wherein the capacitor and low pass filter are connected in series, the low pass filter being provided closer to the amplifier output.
An assembly according to any of the preceding claims, wherein a capacitance of the capacitor is no more than $y = \frac{2 ϕ - \sqrt{27 (1 - x)}}{x \sqrt{27 (1 - x) - 2 ϕ}},$
times a capacitance of the transducer, where x is a ratio between a minimum distance and an unbiased distance of the transducer and ϕ is a ratio between bias voltage and pull-in voltage of the transducer.
An amplifier unit for use in the assembly according to any of the preceding claims, the amplifier unit comprising:
- a first and a second input terminals,

- an amplifier having an amplifier input and an amplifier output,

- a first connection element connecting the first input terminal to the amplifier input,

- a second connection element connected to the amplifier output and the amplifier input,
the second connection element comprising a low pass filter and a capacitor, the low pass filter having a selectable or programmable cut-off frequency and the capacitor having a selectable or programmable capacitance.