Patent Grant
8680926
The present invention relates to an integrated amplification circuit for a transducer signal comprising a semiconductor substrate. The semiconductor substrate comprises a signal limiting network comprising first and second parallel legs coupled between an input of a preamplifier and a first predetermined electric potential of the integrated amplification circuit. The first leg comprises a plurality of cascaded semiconductor diodes coupled to conduct current in a first direction through the limiting network and the second leg comprises a plurality of cascaded semiconductor diodes coupled to conduct current in a second direction through the limiting network. A current blocking member is configured to break a parasitic current path between an anode or a cathode of a semiconductor diode of the first leg or the second leg and the semiconductor substrate.
Acceptable handling of transducer signals with large signal amplitude presents significant challenges to inputs of integrated amplification circuits, in particular for amplification circuits handling signals from a capacitive electret or condenser transducer element. A miniature ECM may comprise such a miniature capacitive electret or condenser transducer element to perform sound pick-up or recording in portable devices like mobile terminals, hearing instruments, headsets, sound recording cameras etc.
The large signal amplitude supplied by the transducer must be handled in a substantially linear manner by an input stage of a preamplifier up till a desired maximum amplitude. Above the desired maximum amplitude, the transducer signal must be limited to avoid irreversible damage to input devices of the preamplifier and/or other active or passive circuit components coupled to the input of the integrated amplification circuit. A known method of limiting the maximum amplitude of the input signal has been to place an input signal limiting network comprising a pair of anti-parallel diodes between the input of the preamplifier and ground, or any other suitable electric potential between the supply voltage and ground, such that input signal amplitudes exceeding about +/−one diode drop, typically +/−0.5-0.6 V, were limited to the latter peak level. However, this maximum amplitude level lies far below a maximum undistorted amplitude level delivered by contemporary miniature ECMs and other types of capacitive microphones. The low maximum amplitude handling capability of the input signal limiting network leads to premature distortion and audio signal clipping when utilized for microphone preamplifiers for capacitive microphones such as miniature ECMs. The premature distortion and audio signal clipping mean that an upper portion of a dynamic range of the capacitive microphones, e.g. sound pressure levels above 110, 120 or 130 dB SPL etc. depending on microphone sensitivity, is rendered useless.
It would accordingly be highly desirable to provide an input signal limiting network capable of handling maximum amplitudes well above the above-mentioned peak-peak level of about +/−0.5-0.6 V. The input signal limiting network should preferably be implementable in existing integrated semiconductor processes such as sub-micron CMOS and occupy a small semiconductor die area to keep costs low.
One possible methodology to increase the maximum input signal capability at the input of the preamplifier would be placing more than one diode, or other non-linear components, in series in each leg of the input signal limiting network. However, this is not feasible in standard semiconductor processes because any diffusion diode manufactured in a well suffers from a parasitic diode coupled between the well and the semiconductor substrate. Since the semiconductor substrate is coupled to ground, the parasitic diode creates a parasitic current path from the anode or cathode to ground when forward biased and spoils the intended operation of the semiconductor diode of the input signal limiting network for large input signal amplitudes e.g. above the previously mentioned peak-peak level of about +/−0.5-0.6 V. This problem has, however, been circumvented in accordance with the present invention where a current blocking member is configured to break the parasitic current path from an anode or cathode of the semiconductor diode to the semiconductor substrate.
One embodiment of the invention is based on the provision of a diode topology or design that comprises a current blocking member which creates a second parasitic diode in addition to the first parasitic diode towards the semiconductor substrate. This second, additional, parasitic diode is coupled in series with the first parasitic diodes and arranged to conduct forward in opposite direction to the first parasitic diode hence blocking any flow of parasitic current to or from the semiconductor substrate. This semiconductor diode topology is preferably based on a semiconductor process which offers an additional well diffusion compared to standard semiconductor processes.
Another embodiment of the invention is based on the provision of a diode topology or design that comprises a first poly-silicon member of a first polarity and second poly-silicon member of a second polarity arranged in electrical contact on top of a thick oxide layer opposite to the semiconductor substrate. The thick oxide layer functions as a current blocking member insulating both the cathode and anode terminals of the diode topology from the semiconductor substrate and accordingly prevents the creation of any parasitic diode current path to ground.
US 2011/0026739 A1 describes a dual-path microphone preamplifier structure comprising signal limiting network coupled to an input terminal of the amplification circuit. The signal limiting network comprises a pair of legs which each comprises between 2 and 5 cascaded diodes coupled in the same direction of forward conduction.
A first aspect of invention relates to an integrated amplification circuit for a transducer signal comprising a semiconductor substrate. The semiconductor substrate comprises a first polarity semiconductor material and the semiconductor substrate further comprises:
The integrated amplification circuit may be designed as an ASIC, or included as a sub-circuit of an ASIC, fabricated in CMOS semiconductor processes such as analog, digital or mixed-signal sub-micron CMOS. The ASIC may comprise a plurality of additional circuits and components in addition to the integrated amplification circuit such as a charge pump, a controller or sequencer, a clock generator, an A/D converter etc. The integrated amplification circuit is preferably shaped and sized for placement inside a housing structure of transducer such as a miniature capacitive or condenser microphone. In this embodiment, the transducer signal may be generated by a capacitive transducer element electrically coupled to the input of the preamplifier.
The plurality of cascaded semiconductor diodes of the first leg are oriented or configured with the same direction of forward conduction and the plurality of cascaded semiconductor diodes of the second leg are configured with the same direction of forward conduction. However, the first direction of forward conduction through the first leg is opposite to the second direction of forward conduction through the second leg. Hence, each leg exclusively conducts current in one direction either for positive input signal amplitudes or for negative input signal amplitudes exceeding the sum of forward diode voltage drops of the plurality of cascaded semiconductor diodes of the leg in question. The skilled person will understand that the first and second legs may comprise the same number of cascaded semiconductor diodes or a different number of cascaded semiconductor diodes. The cascaded semiconductor diodes of each leg are preferably of same type e.g. simple diodes, poly-silicon diodes, diode-coupled MOS transistors or diode-coupled bipolar transistors etc.
Furthermore, if the first and second legs comprise the same number of cascaded semiconductor diodes of the same type, the maximum positive input signal amplitude may be substantially equal to the maximum negative input signal amplitude when signal limiting is activated to provide substantially symmetrical limiting of the input signal. The skilled person will understand that maximum positive and negative input signal amplitude before limiting can be adjusted or controlled by suitable selection of the number and type of cascaded semiconductor diodes of each of the first and second leg. The number of cascaded semiconductor diodes of each of the first and second legs is two or more such as between 2 and 20. In some embodiments, the maximum positive and negative input signal amplitude lies between +/−1.2 V-10 V. In this context, the maximum positive and negative input signal amplitude can be defined as the peak amplitude of the input signal at 1 kHz when the signal limiting network induces a total harmonic distortion of 10% into the input signal.
The signal limiting network is preferably DC coupled to the input of the preamplifier to set a DC bias point of the preamplifier. In this embodiment, the signal limiting network also functions as a bias network for the preamplifier. The signal limiting network may be connected to ground potential or to another suitable electric potential of the integrated amplification circuit such as a positive or negative DC reference voltage. Alternatively, the signal limiting network may be coupled to an input pad of the integrated amplification circuit and connected to the input of the preamplifier through a coupling capacitor such that the maximum peak input voltage is limited directly at the input of the integrated amplification circuit.
The current blocking member ensures that flow of current through a parasitic current path between the anode or cathode of at least one of the plurality of semiconductor diodes and the semiconductor substrate is suppressed or substantially eliminated. The parasitic current path often comprises one or more parasitic diodes coupled to an anode diffusion or a cathode diffusion of a semiconductor diode arranged in the semiconductor substrate. These parasitic diodes are coupled from the anode or cathode diffusion to the semiconductor substrate which typically is held at ground potential. Hence, the plurality of cascaded semiconductor diodes of the first or the second leg is coupled to ground via one or more parasitic diodes which therefore short circuit the plurality of cascaded semiconductor diodes. Consequently, the cascade of semiconductor diodes fails to provide the desired increase of peak input voltage handling capability unless the parasitic current path(s) are blocked or eliminated. Preferably, each semiconductor diode of at least one of the first and second legs comprises a current blocking member configured to break a parasitic current path between an anode or a cathode of the semiconductor diode and the semiconductor substrate. This embodiment maximizes the peak input voltage handling capability of the leg or legs in question by blocking all parasitic current paths.
One embodiment of the invention is based on a semiconductor process which offers an additional well diffusion compared to standard semiconductor processes. This embodiment comprises a semiconductor diode topology or design which comprises a second parasitic diode in addition to the above-discussed first parasitic diode formed between the anode or cathode of the semiconductor diode and the semiconductor substrate. This second, additional, parasitic diode is coupled in series with the first parasitic diode and arranged to conduct forward in opposite direction to the first parasitic diode. Hence, flow of parasitic current between the anode or cathode and the semiconductor substrate is blocked or disrupted. According to one such embodiment, a semiconductor diode of the first leg or the second leg comprises:
As mentioned above, each semiconductor diode of the first leg and/or each semiconductor diode of the second leg may comprise a current blocking member.
The skilled person will understand that the first and second polarities will depend on the specific semiconductor process. Consequently, the first polarity may be p-type and the second polarity may be n-type such that the semiconductor substrate comprises a p-type semiconductor material, e.g. P−. Alternatively, the first polarity may be n-type and the second polarity may be p-type such that the semiconductor substrate comprises n-type semiconductor material, e.g. N−. A doping of the second diffusion of the second polarity is preferably higher than a doping of the second well diffusion of the second polarity. Likewise, a doping of the first diffusion of the first polarity is preferably higher than a doping of the first well diffusion of the first polarity.
Each semiconductor diode preferably comprises a second diffusion of the first polarity arranged inside the first well diffusion. Since the first well diffusion of the first polarity forms part of the anode or cathode terminal, the second diffusion of the first polarity may be used as electrical coupling point for a contact member establishing electrical connection from the anode or cathode terminal to other components (for example adjacent semiconductor diodes) of the signal limiting network by metal wire routing. A doping of the second diffusion of the first polarity is preferably higher than a doping of the first well diffusion of the first polarity.
The second diffusion of the second polarity may be electrically decoupled, i.e. floating, from the integrated amplification circuit which prevents that any parasitic current path is formed from the midpoint of the oppositely oriented first and second parasitic diodes to the semiconductor substrate. The second diffusion of the second polarity may alternatively be electrically connected to a third predetermined electrical potential that prevents latch-up through the first and second parasitic diodes.
A preferred embodiment of the invention provides a compact design or layout of the cascaded semiconductor diodes of the first and second legs in the semiconductor substrate. This embodiment preferably utilizes an adjacently arranged pair of semiconductor diodes where the semiconductor diodes of the pair circuit wise are arranged in different ones of the first and second legs of the signal limiting network. In this embodiment, each semiconductor diode of the first leg comprises:
The first diffusion of the first polarity and the second diffusion of the second polarity are preferably connected to a ground potential of the integrated amplification circuit. The first diffusion of the first polarity may comprise a P+ type of diffusion if the polarity of the semiconductor material of the semiconductor substrate is P−. In this embodiment, the second diffusion of the second polarity preferably comprises N+ doped material such that N− doped material of the second N-well of the second polarity is electrically coupled to ground through the second diffusion of the second polarity. In the alternative, the second diffusion of the first polarity and the second diffusion of the second polarity may each be coupled to another predetermined electric potential of the integrated amplification circuit that prevents latch-up through the first and second parasitic diodes.
In a preferred embodiment, the second diffusion of the first polarity (GND) is arranged in-between the second well diffusion of the second polarity and the third well diffusion of the second polarity to provide a compact layout of the adjacently arranged pair of semiconductor diodes. This embodiment can be utilized for each pair of semiconductor diodes of the plurality of cascaded semiconductor diodes of the first and second legs to minimize die area consumption of the entire signal limiting network.
The first well diffusion of the first polarity, which forms part of the anode or cathode terminal of the semiconductor diode of the first leg, preferably comprises a second diffusion of the first polarity arranged inside the first well diffusion. The second diffusion of the first polarity may be used as electrical coupling interface for a contact member establishing electrical connection from the anode or cathode terminal of the semiconductor diode of the first leg to other components (for example adjacent semiconductor diodes) of the signal limiting network by metal wire routing. A doping of the second diffusion of the first polarity is preferably higher than a doping of the first well diffusion of the first polarity. Likewise, the third well diffusion of the second polarity, which forms part of the anode or cathode terminal of the semiconductor diode of the second leg, preferably comprises a third diffusion of the second polarity such that electrical coupling can be established to the anode or cathode terminal of the semiconductor diode in a similar manner.
According to yet another preferred embodiment of the invention, a semiconductor switch, preferably comprising a MOS transistor, is coupled across one or more of the plurality of cascaded semiconductor diodes of the first leg or a MOS transistor is coupled across one or more of the plurality of cascaded semiconductor diodes of the second leg. In this manner, drain and source terminals of the MOS transistor are coupled across the anode and cathode terminals of at least one semiconductor diode. Due to the low on-state impedance of the NMOS transistor it can essentially short circuit the semiconductor diode or diodes in question. The ability to selectively short circuit one or more of the semiconductor diodes of at least one of the first and second legs is highly useful in certain situations. The NMOS transistor may provide rapid settling of a DC input bias voltage at the microphone preamplifier input during powering-up of the integrated amplification circuit. This is useful because the signal limiting network may function as an extremely high impedance DC bias point setting network of the preamplifier input where the extremely high impedance ensures small or insignificant loading of the transducer. The reverse biased chains or cascades of semiconductor diodes may present an impedance larger than 1 GΩ, or even larger than 10 GΩ, under quiescent conditions at the input of the preamplifier. This impedance may lead to an unacceptable slow settling of DC operating points of the preamplifier in connection with start-up of the powering-up of the integrated amplification circuit as described in further detail below in connection with the appended drawings. In the present embodiment such start-up problems can be eliminated, or at least reduced, by switching the NMOS transistor to an on-state or conducting state during start-up of the microphone amplification circuit. Thereby, one or more of the plurality of cascaded semiconductor diodes are essentially bypassed by the low on-resistance of the cross-coupled MOS transistor such that the input of the preamplifier can be clamped to a desired electric potential through a low impedance path during start-up. Once, start-up of the microphone amplification circuit has been completed, the cross-coupled MOS transistor is switched to a non-conducting state or off-state such that the DC operating point is controlled by the impedance of one or more cascaded semiconductor diodes of the first or second legs.
In one embodiment, a plurality of semiconductor switches, preferably implemented as respective MOS transistors, is coupled across respective semiconductor diodes of the first leg and/or a plurality of semiconductor switches is coupled across respective semiconductor diodes of the second leg. In this embodiment, a cross-coupled MOS transistor may be arranged across each semiconductor diode of either the first leg or the second leg. In another embodiment a single semiconductor switch, e.g. a MOS transistor, may be cross-coupled across all semiconductor diodes of the first leg or across all semiconductor diodes of the second leg. The latter embodiment will require a higher drain source break-down voltage of the MOS transistor for a given maximum amplitude of the input signal at the preamplifier than the earlier mentioned embodiment using a plurality of MOS transistors.
In an advantageous embodiment of the above-mentioned MOS transistor or MOS transistors, the MOS transistor has been tightly integrated with one of the previously discussed semiconductor diodes by placing a source and drain diffusion of the MOS transistor inside the first well diffusion. The first well diffusion comprises the anode and cathode diffusions of the semiconductor diode. In this embodiment, the MOS transistor comprises:
In one embodiment, the third diffusion of the second polarity, forming the drain or source terminal of the MOS transistor, is electrically connected to the anode terminal of the first semiconductor diode while the third diffusion of the second polarity is connected to an anode or cathode terminal of another semiconductor diode of the plurality of cascaded semiconductor diodes of the same leg as the first semiconductor diode. In the latter embodiment, the MOS transistor may be cross-coupled across two or more of the plurality of cascaded semiconductor diodes of the same leg.
The skilled person will understand that each of the {first diffusion of the second polarity, second diffusion of the second polarity, third diffusion of the second polarity, first diffusion of the first polarity, second diffusion of the first polarity, third diffusion of the first polarity} preferably comprises an electrical contact member such as a metal contact connected thereto. In this manner, each of these diffusions can be electrically coupled to other components and electrical potentials of the integrated amplification circuit through appropriate wiring routing deposited on the semiconductor substrate.
As mentioned above, the semiconductor diode of the first leg or the second leg preferably comprises a diode selected from a group of {a diode-coupled bipolar transistor, a diode-coupled MOS transistor, a diode}.
Another embodiment of the invention is based on the provision of a diode topology or design that comprises a first poly-silicon member of a first polarity and second poly-silicon member of a second polarity arranged in electrical contact on a thick oxide layer. According to this embodiment, one or more of semiconductor diode(s) of the first leg and/or one or more of semiconductor diode(s) of the second leg comprises:
Preferably, each semiconductor diode of the plurality of cascaded semiconductor diodes of at least one of the first and second legs comprise the above-mentioned diode topology based on poly-silicon members. The first poly-silicon member comprises an anode structure or cathode structure of the semiconductor diode and the second poly-silicon member forms an opposite structure depending on the choice of the first and second polarities, i.e. either N+ or P+. In both instances, the thick oxide layer functions as a current blocking member insulating both the cathode and anode structure of the present diode topology from the semiconductor substrate. Accordingly, the thick oxide layer prevents the creation of any parasitic current path from the cathode and anode structures to the semiconductor substrate and therefore effectively shields the cascade of semiconductor diodes from being short-circuited to the semiconductor substrate at the large input signal amplitudes.
In one embodiment, the first poly-silicon member and the second poly-silicon member are adjacently arranged on the top of the thick oxide layer and abutted to each other to create the electrical contact. Hence, a diode junction is formed at the interface edges between the first and second poly-silicon members. In another embodiment, a third poly-silicon member is arranged in-between the first and second poly-silicon members. The third poly-silicon member may be of the first polarity or of the second polarity or without polarity. In the latter case, the third poly-silicon member comprises intrinsic poly-silicon or poly silicon without any doping. The third poly-silicon member is arranged in-between, and in abutment with, the first poly-silicon member and the second poly-silicon member to create the electrical contact. The third poly-silicon member preferably has a lower doping than a doping of the first or the second poly-silicon member for example without doping at all. A saturation current of the semiconductor diode can be adjusted by controlling a width of the third poly-silicon member allowing control and adaptation of the diode resistance to fit a particular application or semiconductor process.
The skilled person will appreciate that the semiconductor diode of the first leg and the semiconductor diodes of the second leg may be of different types such that at least one, and preferably each, semiconductor diode of the first or second leg comprises
In one embodiment, the integrated amplification circuit comprises a DC bias voltage generator for supplying a DC bias voltage at a first externally accessible terminal. The DC bias voltage generator may comprise a charge pump to provide a higher bias voltage for a plate of a capacitive transducer element of a condenser microphone coupled to the first externally accessible terminal. A second externally accessible terminal of the integrated amplification circuit may be coupled to the input of the preamplifier such that the second externally accessible terminal could be coupled to another plate of the plate of a capacitive transducer element.
Another aspect of the invention relates to a capacitive or condenser microphone assembly comprising a capacitive transducer element having a back plate and a diaphragm arranged in a spaced relationship thereto. The diaphragm is displaceable relative to the back plate in response to sound. An integrated amplification circuit according to any of the above described embodiments thereof is electrically coupled to the capacitive transducer element such that at least one of the diaphragm and the back plate is electrically coupled to the input of the preamplifier to supply a microphone signal to the preamplifier. The capacitive or condenser microphone may comprise a miniature ECM (Electret Condenser Microphone) which is a particular kind of condenser microphone which comprises a permanently electrically charged electret layer. The charged electret layer provides DC bias voltage for a capacitive transducer element. Hence, the integrated amplification circuit is not required to deliver a DC bias voltage to the back plate or diaphragm of the miniature ECM and the microphone audio signal can be routed to the input of the preamplifier through a single pad of the integrated amplification circuit. In another embodiment, the capacitive transducer element may require the DC bias voltage to be supplied to the back plate or diaphragm to provide an electrical field between these structures. According to one embodiment, the condenser microphone assembly comprises a miniature MEMS condenser capsule enclosing the back plate and diaphragm. The miniature MEMS condenser capsule is preferably mounted on a carrier or substrate such as a printed circuit board or ceramics board for assembly for example by surface mount techniques. In the latter situation, the integrated amplification circuit may comprise the above mentioned first and second externally accessible terminals providing the DC bias voltage and the preamplifier input, respectively, for the back plate and diaphragm of the miniature MEMS condenser capsule.
Embodiments of the invention will be described in more detail in connection with the append drawing in which:
a) is a simplified schematic circuit diagram of a prior art microphone amplification circuit, comprising a DC bias network, coupled to a miniature ECM,
b) is a simplified schematic circuit diagram of a prior art microphone amplification circuit, comprising a DC bias network, coupled to a miniature MEMS condenser microphone,
a) is a simplified schematic circuit diagram of a prior art microphone amplification circuit and its associated input signal limiting network,
b) is a substrate vertical cross-sectional view through a single diode of the cascaded semiconductor diodes of the input signal limiting network of the prior art microphone amplification circuit,
a) is a simplified schematic circuit diagram of a microphone amplification circuit and its associated input signal limiting network in accordance with a first embodiment of the present invention,
b) is a substrate vertical cross-sectional view through a single diode of the cascaded semiconductor diodes of the input signal limiting network of the microphone amplification circuit in accordance with the first embodiment of the invention,
a) is a simplified schematic circuit diagram of a microphone amplification circuit and its associated input signal limiting network in accordance with a second embodiment of the present invention,
b) is a substrate vertical cross-sectional view through a single diode of the cascaded semiconductor diodes of the input signal limiting network of the microphone amplification circuit of the second embodiment of the invention,
a) is a simplified schematic circuit diagram of a microphone amplification circuit and its associated input signal limiting network in accordance with a third embodiment of the present invention,
b) is a substrate vertical cross-sectional view through a single combined diode and MOS transistor structure of the cascaded semiconductor diodes of the input signal limiting network of the microphone amplification circuit in accordance with the third embodiment of the invention,
a) is a simplified schematic circuit diagram of a microphone amplification circuit and its associated input signal limiting network in accordance with a fourth embodiment of the present invention,
b) is a substrate vertical cross-sectional view through a single combined diode and MOS transistor structure of the cascaded semiconductor diodes of the input signal limiting network in accordance with the fourth embodiment of the invention,
a) is a simplified schematic circuit diagram of a microphone amplification circuit and its associated input signal limiting network in accordance with a fifth embodiment of the present invention; and
b) is a substrate vertical cross-sectional view through a single combined diode and MOS transistor structure of the cascaded semiconductor diodes of the input signal limiting network in accordance with the fifth embodiment of the invention.
a) is a simplified schematic circuit diagram of a prior art microphone amplification circuit 100, comprising a DC bias network 107, coupled to a miniature ECM schematically illustrated as a variable capacitance, CECM, illustrating its change of capacitance in response to impinging sound. The miniature ECM (electret condenser microphone) is a particular kind of condenser microphone which comprises a permanently electrically charged electret layer which provides DC bias voltage for a capacitive transducer element (not shown) of the miniature ECM.
The amplification circuit comprises a preamplifier 109 having an input electrically coupled to the miniature ECM and to the DC bias network 107. The input impedance of the preamplifier 109 is often extremely large, for example tens of GΩ or higher, to avoid loading the miniature ECM and attenuate generated audio signals. The DC bias network 107 comprises a pair of anti-parallel diodes coupled between the input, IN, of the preamplifier 109 and ground such that the DC input bias voltage of the microphone preamplifier 109 is set close to a ground potential or any other suitable potential. The skilled person will understand that DC bias network 107 also operates as a signal limiting network such that input or audio signal amplitudes delivered by the miniature ECM exceeding about +/−one diode drop, typically +/−0.5-0.6 V, are limited to the latter peak level. As previously mentioned, this maximum amplitude level often lies far below the maximum undistorted amplitude level capability of the miniature ECM and therefore leads to premature distortion or audio signal clipping.
b) is a simplified schematic circuit diagram of another prior art microphone amplification circuit 110, comprising a DC bias network 117, coupled to a miniature Micro Electro Mechanical System (MEMS) fabricated condenser microphone. The MEMS condenser microphone is once again schematically illustrated as a variable capacitance, CMEMS, illustrating its change of capacitance in response to impinging sound. The MEMS condenser microphone is a particular kind of condenser microphone which operates without the permanently charged electret layer of the above discussed ECM. Hence, the microphone amplification circuit 110 comprises a DC bias voltage circuit which may comprise a charge pump 115 or other type of DC-DC converter to generate the required DC bias voltage VBIAS for the capacitive transducer element. The charge pump 115 may be supplied with power from an external DC voltage source 113 such as a power supply rail or a battery voltage of an associated piece of electronic circuitry or equipment. The microphone amplification circuit 110 comprises an optional low-pass filter coupled between an output of the charge pump VPUMP and the DC bias voltage VBIAS. The low-pass filter comprises a series resistor R1 and a capacitor C1 to ground. The microphone amplification circuit 110 comprises a microphone preamplifier 119 having a first externally accessible pad, IN, electrically coupled to one terminal of the MEMS condenser microphone CMEMS. The first terminal is furthermore electrically coupled to a DC bias network 117 with characteristics similar to the above-discussed DC bias network 107.
a) is a simplified schematic circuit diagram of a prior art microphone amplification circuit 200 and its associated signal limiting network 207. The signal limiting network 207 comprises first and second parallel legs coupled between the input IN of the preamplifier 209 and ground potential of the integrated amplification circuit. The first leg comprises a plurality of cascaded semiconductor diodes D11-D1m coupled to conduct current in a first direction from IN to ground through the limiting network 207. The second leg also comprises a plurality of cascaded semiconductor diodes D21-D2n but coupled to conduct current in a second direction, opposite to the first direction, from ground to IN. The intended purpose of this proposed design of the signal limiting network 207 is to increase its maximum amplitude handling capability from the previously mentioned level +/−0.5-0.6 V peak. However, each of the semiconductor diodes fabricated in the standard semiconductor process utilized for the design of the signal limiting network 207 and suffers from one of parasitic diode Dp11-Dp13 and Dp21-Dp2n coupled to the semiconductor substrate which may be at ground potential (refer to layer P− in
b) is a vertical cross-sectional view through a single semiconductor diode, such as D13, of the cascades of semiconductor diodes of the signal limiting network 207. The semiconductor diode such as D13 comprises a N-well, N−, which is surrounded by a P minus or P− substrate. A field oxide layer 211 is deposited on top of an upper surface of the P− substrate in which the semiconductor diode D13 is formed by conventional semiconductor process steps. The semiconductor diode D13 is schematically indicated by the black diode symbol interconnecting a P+ anode diffusion 221 arranged adjacently to a cathode N+ diffusion 223 inside N− well. The P+ anode diffusion 221 is electrically connected to a contact 213 and the cathode N− well with N+ diffusion 223 is electrically connected to another contact 215 such that the anode and cathode can be electrically coupled to other components of the limiting network 207 by metal wire routing. The semiconductor diode D13 further comprises another P+ diffusion 225 with an associated electrical contact member 217 which in combination provides a substrate ground connection for the P− substrate. However, in addition to the desired or designed semiconductor diode D13 a parasitic diode Dp13 is formed between the cathode N+ diffusion 223 and the P− substrate with P+ diffusion 225 as illustrated by the hatched diode symbol. Likewise, another parasitic diode Dp21 is formed in addition to the intended or designed semiconductor diode D21 in the second leg of the limiting network 207. Once this parasitic diode Dp21 becomes forward biased the input signal is clamped at one diode voltage drop below ground, i.e. typically minus 0.5-0.6 V, which spoils the desired increase of negative peak voltage handling capability of the second leg of the limiting network 207 where the plurality of cascaded semiconductor diodes D21-D2n can sustain a much larger negative input voltage amplitude. A corresponding mechanism does not exist in the first leg wherein the top-most parasitic diode Dp11 is only coupled to a midpoint between the desired semiconductor diodes D11-D12. Hence, the parasitic diode Dp11 becomes backwards biased for positive input signal amplitudes above plus 0.5-0.6 V. However, the asymmetry in maximum amplitude handling of the input signal of the entire signal limiting network 207 leads to an asymmetric clipping or limiting of the audio or input signal of the preamplifier 209. This creates highly unpleasant audio signal distortion and tends to raise a DC voltage at the input, IN, of the preamplifier 209 from the intended DC voltage leading to an erroneous DC bias voltage across the capacitive transducer element (please refer to item CMEMS of
a) is a simplified schematic circuit diagram of a microphone amplification circuit 300 with a signal limiting network 307 in accordance with a first embodiment of the present invention.
The first leg of the signal limiting network 307 comprises a plurality of cascaded semiconductor diodes D11-D1m coupled to conduct current in a first direction from the input of the preamplifier 309 to ground. The second leg also comprises a plurality of cascaded semiconductor diodes D21-D2n coupled to conduct current in a second, opposite, direction from ground to the input of the preamplifier 309. The skilled person will understand that the number of cascaded semiconductor diodes of the first leg may equal to, or different from, the number of cascaded semiconductor diodes of the second leg. The number of cascaded semiconductor diodes of each of the first and second legs is preferably equal or larger than two such as between 2 and 20 semiconductor diodes. By comparing the vertical cross-sectional views through the semiconductor diode topologies of
The present embodiment of the invention comprises current blocking members coupled to respective ones of the cascaded semiconductor diodes diodes D11-D1m of the first leg and current blocking members coupled to respective ones of the semiconductor diodes D21-D2n of the second leg. A first current blocking member is illustrated as the parasitic diode Dp11r of the first leg and a second current blocking member is illustrated as another parasitic diode Dp21r of the second leg of the signal limiting network 307 on
b) is a vertical cross-sectional view through a single semiconductor diode, such as D11, of the plurality of semiconductor diodes of the signal limiting network 307. The semiconductor diode D11 is arranged inside a P-well diffusion 328 (P−) which is surrounded by a N-well diffusion, N−. The N-well diffusion is arranged within a P-type semiconductor substrate, P−. A field oxide layer 311 is deposited on top of an upper surface of the P− substrate in which the semiconductor diode D11 is embedded by appropriate semiconductor process steps. The semiconductor diode D11 is schematically indicated by a black diode symbol interconnecting a P+ diffusion 321 functioning as a well-contact for an anode of the semiconductor diode D11 comprising the P− well diffusion 328. Inside the P− well diffusion a diode cathode N+ diffusion 323 is arranged adjacently to the P+ diffusion 321. The anode of the semiconductor diode D11 accordingly comprises the P− well 328 together with the P+ diffusion 321 and is electrically connected to a contact 313. The cathode of the semiconductor diode D11 formed by the N+ diffusion 323 is electrically connected to another contact 315 such that the anode and cathode can be electrically coupled to other components of the limiting network 307 through appropriate wiring routing deposited on the upper surface of the semiconductor die or substrate P−. The semiconductor diode D11 further comprises another P+ diffusion 325 with an associated electrical contact member 317 which in combination provides a substrate ground connection for the P− substrate similar to the design depicted in
a) is a simplified schematic circuit diagram of a microphone amplification circuit 400 with a signal limiting network 407 in accordance with a second embodiment of the present invention.
b) is a substrate vertical cross-sectional view through the adjacently arranged pair of semiconductor diodes D13 and D23 of the first and second legs, respectively, of the signal limiting network 407. The semiconductor diode D23 is arranged inside a P-well diffusion 428 (P−) which is surrounded by a N-well diffusion 426 (N−). The N-well 426 is arranged in a P− type semiconductor substrate, P−. A field oxide layer 411 is deposited on top of an upper surface of the P− substrate in which the semiconductor diode D23 is embedded by appropriate semiconductor process steps. The semiconductor diode D23 of the second leg is schematically indicated by a black diode symbol interconnecting a P+ diffusion 421 of the semiconductor diode D21 and an adjacently arranged cathode N+ diffusion 423 inside P− well 428. The P− well 428 together with P+ diffusion 421 form an anode of the semiconductor diode D23 and is electrically connected to a contact 413. The cathode of the semiconductor diode D23, formed by the N+ diffusion 423, is electrically connected to another contact 415 such that the anode and cathode can be electrically coupled to other components of the limiting network 407 through appropriate wiring routing deposited on the upper surface of the semiconductor die or substrate P−. The semiconductor diode D23 further comprises another P+ diffusion 425 with an associated electrical contact member 417 which in combination provides a substrate ground connection for the P− substrate similar to the design depicted in
The semiconductor diode D13 of the first leg is schematically indicated by another black diode symbol interconnecting a P+ diffusion 431 and an adjacently arranged cathode N+ diffusion 430 inside N− well 432 of the semiconductor diode D13. The P+ diffusion 431 and cathode N+ diffusion 430 are surrounded by a second N-well 432 embedded in the P-type semiconductor substrate P− adjacent to the P+ diffusion 425. The parasitic diode Dp13 is formed between the cathode N+ diffusion 430 inside the N− well 432 and the P− substrate with the forward current conduction orientation indicated by the hatched diode symbol. The P+ diffusion 431 is electrically connected to a contact 433 and the N+ diffusion 430 together with N− well 432 is electrically connected to another contact 429 such that the anode and cathode can be electrically coupled to other components of the limiting network 407 in accordance with the schematic diagram of
a) is a simplified schematic circuit diagram of a microphone amplification circuit 500 with a signal limiting network 507 in accordance with a third embodiment of the present invention. The signal limiting network 507 comprises first and second parallel legs coupled between an input IN of the preamplifier 509 (A1) and a ground electric potential of the integrated amplification circuit 500. The first leg comprises a plurality of cascaded semiconductor diodes D11-D1m and the second leg comprises a plurality of cascaded semiconductor diodes D21-D2n. The skilled person will understand that electrical characteristics of the preamplifier 509 may be similar to those of the preamplifier 209 discussed above. The microphone amplification circuit 500 is preferably fabricated in a sub-micron analog, digital or mixed-signal CMOS process. The plurality of cascaded semiconductor diodes diodes D11-D1m of the first leg are coupled to conduct current in a first direction from the input of the preamplifier 509 to ground. The plurality of cascaded semiconductor diodes D21-D2n are coupled to conduct current in a second, opposite, direction from ground to the input of the preamplifier 509. The skilled person will understand that the number of cascaded semiconductor diodes of the first leg may equal to, or different from, the number of cascaded semiconductor diodes of the second leg. The number of cascaded semiconductor diodes of each of the first and second legs is preferably between 2 and 20 such as between 5 and 10.
b) shows three different embodiments of a single semiconductor diode, such as D21, in substrate vertical cross-sectional view in drawings b1, b2, b3 and b4. The skilled person will understand that all semiconductor diodes D11-D1m and diodes D21-D2n may be of the same embodiment or may be of different embodiments. The four different embodiments of the semiconductor diodes D11-D1m and diodes D21-D2n depicted on drawings b1, b2, b3 and b4 share a common feature in that the semiconductor diodes are implemented in a poly-silicon layer of the semiconductor process. The semiconductor diode, such as D21, embodiment illustrated on drawing b1 lies above a semiconductor substrate P−. A field or thick oxide layer 511 is deposited on an upper surface of the semiconductor substrate P−. A first poly-silicon member 523 of N+ polarity and second poly-silicon member 521 of P+ polarity are arranged in abutment on the top of the thick oxide layer 511 on an opposing surface relative to the semiconductor substrate. The junction where the first poly-silicon member 523 and the second poly-silicon member 521 are abutted creates an electrical contact between these members such that a diode junction or effect is created. The first poly-silicon member 523 of N+ polarity forms a cathode of the semiconductor diode and the second poly-silicon member 521 of P+ polarity forms an anode of the semiconductor diode. The P+ anode member 521 is electrically connected to a contact 513 through an opening in a second oxide layer 529 and the N+ cathode member 523 is similarly electrically connected to another contact 515 through a second opening in the second oxide layer 529 such that the anode and cathode can be electrically coupled to other components of the limiting network 507 in accordance with the schematic diagram of
The semiconductor diode, such as D21, embodiments illustrated on respective ones of drawings b2, b3 and b4 are also arranged on top of the semiconductor substrate P−. The characteristics discussed above in connection with the semiconductor diode embodiment on drawing b1 also applies to the present second and third embodiments and corresponding features have been supplied with identical reference numerals to ease comparison. The semiconductor diode, such as D21, embodiment illustrated on drawings b2, b3 and b4 comprises a third poly-silicon member 522 arranged in-between, and in abutment with, the P+ anode member 521 and N+ cathode member 523 of the semiconductor diode D21. The third poly-silicon member 522 may have a N-type of doping which is lower than the N-type doping of the N+ cathode member 523 as illustrated in the embodiment on drawing b2. Alternatively, the third poly-silicon member 522 may have a P-type of doping which is lower than the P-type doping of the P+ anode member 521 as illustrated in the embodiment on drawing b3. In yet another alternative, the third poly-silicon member 522 may have no doping; i.e. formed by intrinsic poly-silicon, as illustrated in the embodiment on drawing b4. The depicted variants of the third poly-silicon member 522 may lead to different saturation currents of the semiconductor diode D21. This will result in different equivalent resistances, noise, etc. such that the choice of diode variant can be tailored to particular process technology.
a) is a simplified schematic circuit diagram of a microphone amplification circuit 600 with a signal limiting network 607 in accordance with a fourth embodiment of the present invention. The signal limiting network 607 comprises first and second parallel legs coupled between an input IN of the preamplifier 609 (A1) and a ground electric potential of the integrated amplification circuit 600. The first leg comprises a plurality of cascaded semiconductor diodes D11-D1m and the second leg comprises a plurality of cascaded semiconductor diodes D21-D2n. The skilled person will understand that electrical characteristic of the preamplifier 609 may be similar to those of the preamplifier 209 discussed above. The characteristics of the signal limiting network 307 discussed above in connection with the microphone amplification circuit 300 of the first embodiment of the invention also applies to the present embodiment. Corresponding features of the first embodiment of the invention and the present embodiment have been supplied with identical reference numerals to ease comparison. The present microphone amplification circuit 600 comprises a plurality of semiconductor switches comprising respective NMOS transistors M12-M1m that are coupled across respective ones of the plurality of cascaded semiconductor diodes D12-D1m of the first leg of the signal limiting network 607. The skilled person will understand that the second leg of the signal limiting network 607 may comprise a corresponding arrangement of semiconductor switches in other embodiments of the invention. The purpose of the NMOS transistors M12-M1m is to facilitate rapid settling of the DC input bias voltage at the microphone preamplifier input IN during powering-up of the microphone amplification circuit 600. As previously mentioned, the signal limiting network 607 also functions as a DC bias point setting network of the microphone amplification circuit 600 where the extremely high impedance of the signal limiting network 607 under quiescent conditions, or conditions where the input signal level lies far below the limiting voltage of the network, ensures that loading of the capacitive microphone element is small. However, when abrupt voltage changes are required at the input of the preamplifier 609, such as during pre-charging of the capacitive microphone element or during start-up of the microphone amplification circuit 600, the extremely high impedance of the signal limiting network 607 results in drift of the DC operating points of the preamplifier 609. The extremely high impedance of the signal limiting network 607 causes an undesired slow subsequent settling of the input of the preamplifier 609 to a nominal DC operating point. Hence, the preamplifier 609 may cease to function as intended, i.e. according to prescribed specifications, for a prolonged period of time unless precautions as described below are taken. In the present embodiment, NMOS transistors M12-M1m are set to their conducting state, i.e. on-state, by a controller (not shown) during start-up of the microphone amplification circuit 600. Thereby, each of the cascaded semiconductor diodes D12-D1m becomes essentially bypassed by a low on-resistance of the associated cross-coupled NMOS transistor except for diode D11. The latter diode cannot be bypassed because a parasitic diode of such NMOS transistor would short circuit the cascaded semiconductor diodes D21-D2m. Nevertheless, a substantial temporary decrease of impedance of the signal limiting network 607 is obtained when the NMOS transistors M12-M1m are set to their respective conducting states. These conducting states create a lower impedance path from the input IN of the preamplifier 609 to ground such that the preamplifier input is fixed at, or close to, ground level, or another well-defined electric potential, during start-up of the microphone amplification circuit 600. The controller can control on-states and off-states of the NMOS transistors M12-M1m by adjusting a gate control voltage on the joint control terminal R. The controller preferably detects when start-up is finalized and turns the NMOS transistors M12-M1m to their respective non-conducting states or off-states in response thereto. In this manner, the temporary bypassing of the cascaded semiconductor diodes D12-D1m is terminated and normal operation of the preamplifier 609 is resumed because the NMOS transistors M12-M1m are switched to respective high-impedance off-states. The skilled person will understand that switches M12-M1m may be controlled with a shared control signal R as well as an independent control signal for each switch M12-M1m, respectively.
b) is a vertical cross-sectional view through a semiconductor design of a combined semiconductor diode, such as D13, and its corresponding NMOS based bypass transistor switch such as NMOS transistors M13 utilized in the present microphone amplification circuit 600 in accordance with the schematic circuit diagram in
a) is a simplified schematic circuit diagram of a microphone amplification circuit 700 with a signal limiting network 707 in accordance with a fifth embodiment of the present invention. The skilled person will understand that electrical characteristic of the preamplifier 709 may be similar to those of the preamplifier 209 discussed above. The characteristics and functionality of the signal limiting network 607 discussed above in connection with the microphone amplification circuit 600 of the fourth embodiment of the invention also applies to the present embodiment. However, the plurality of semiconductor switches coupled across respective ones of the plurality of cascaded semiconductor diodes D12-D1m of the first leg in the fourth embodiment have been replaced by a single semiconductor switch comprising NMOS transistor M1 in the present embodiment of the invention. The NMOS transistor M1 is cross-coupled across the chain of semiconductor diodes D12-D1m to bypass or short circuit these as described above in connection with the fourth embodiment. The use of a single cross-coupled single semiconductor switch in accordance with the present embodiment can save die area and simplify the circuit layout of the signal limiting network 707 compared to the fourth embodiment discussed above. However, the single NMOS transistor M1 must now be capable of withstanding a larger drain to source voltage than each of the NMOS transistors M1-Mn of the fourth embodiment. This may however be feasible if a break down drain-source voltage of the single NMOS transistor M1 is larger than the peak input signal voltage at the preamplifier input.
The semiconductor layout or design of the combined semiconductor diode D1n and the associated NMOS cross-coupled transistor M1 is illustrated in a vertical cross-sectional view in
| Number | Name | Date | Kind |
|---|---|---|---|
| 5341114 | Calviello et al. | Aug 1994 | A |
| 6900698 | Ikeda | May 2005 | B2 |
| 20110026739 | Thomsen et al. | Feb 2011 | A1 |
| Number | Date | Country | |
|---|---|---|---|
| 20130293297 A1 | Nov 2013 | US |
