Abstract: Methods and apparatus for Class-D amplifier circuits with improved power efficiency. The circuit has an output stage with at least first and second switches and a modulator that receives an input signal to be amplified, SIN, and a first clock signal fSW. The modulator controls the duty cycles of the first and second switches, within a switching cycle based on the input signal, wherein the switching cycle has a switching frequency based on the first clock signal. A frequency controller controls the frequency of the first clock signal in response to an indication of the amplitude of the input signal so as to provide a first switching frequency at a first input signal amplitude and a second, lower, switching frequency at a second, lower, input signal amplitude. A lower switching frequency can be tolerated at low signal amplitudes and varying the switching frequency in this way thus maintains stability whilst reducing switching power losses.

Abstract: Amplifier arrangements for read-out of MEMS capacitive transducers, such as low-noise amplifiers. An amplifier circuit has first and second MOS transistors, with the gate of the first transistor driven by the input signal, and the gate of the second transistor driven by a reference. The sources of the first and second transistors are connected via an impedance. Modulation circuitry is arranged to monitor a signal with a value that varies with the input signal and to modulate the back-bias voltage between the bulk and source terminals of the first and second transistors with the applied modulation being equal for each transistor and based on said monitored signal. The back-bias of the first transistor can be increase to extend the input range of the transistor in situations where the input signal may otherwise result in signal clipping, while avoiding noise and power issues for other input signal levels.

Abstract: Methods and apparatus for detection and tracking of a signal envelope. The circuit comprises absolute value circuitry configured to receive data samples and output a first value corresponding to the magnitude of said data samples. An envelope tracker maintains an envelope output value and compares the first value to the current envelope output value and modifies the envelope output value based on said comparison to provide the envelope output value with predetermined attack and decay characteristics. The absolute value circuitry has a first input for receiving a first digital signal at a first sample rate and a second input for receiving an interpolated version of the first digital signal at a second sample rate which is higher than the first sample rate and outputs the first value based on the magnitudes of the samples received at the first input and the samples received at the second input.

Abstract: A host device for use with a removable peripheral apparatus having a microphone, and to the biasing circuitry for said microphone. The host device may have a device connector for forming a mating connection with a respective peripheral connector. A source of bias is arranged to supply an electrical bias to a device microphone contact of the device connector via a biasing path. A capacitor is connected between a reference voltage node and a capacitor node of the biasing path. A first switch is located between the capacitor node and the device microphone contact. Detection circuitry detects disconnection of the peripheral connector and device connector; and control circuitry controls the switch to disable the biasing path.

Abstract: Class D amplifier circuits for amplifying an input signal. The amplifier has an H-bridge output stage and thus has switches for switchably connecting a first output to a first voltage, e.g. Vdd, or a second voltage (e.g. ground) and for switchably connecting a second output to the first or second voltages. A switch controller is configured to control the H-bridge stage so as to vary between a plurality of states including at least a first state in which the outputs are both connected to the first voltage and a second state in which the outputs are both connected to said second voltage. The switch controller is configured to vary the proportion of time spent in the first state relative to the second state based on an indication of the amplitude of the input signal.

Abstract: A wireless cellular telephone with an audio codec for converting digital audio signals to analog audio signals. The audio codec comprises two digital audio bus interfaces for coupling to respective digital audio buses, and a digital-only signal path between the two digital audio bus interfaces, such that no analog processing of the audio signals occurs in the digital-only signal path.

Abstract: A MEMS comprises a back-plate (7) having an inner portion (7a) and an outer portion (7b), the inner portion (7a) connected to the outer portion (7b) by a sidewall (7c). A raised section or anchor ring (60) is formed in the outer portion (7b) of the back-plate, in a region of the back-plate near the inner perimeter of the outer portion. The anchor ring may comprise angled sidewalls. The thickness of the back-plate may be greater than the thickness of the material supporting the anchor ring. Embodiments are also disclosed in which a membrane comprises a raised portion and an outer portion connected by an angled sidewall.

Abstract: A method of controlling a noise cancellation system, for use in an audio device, comprises: generating an ambient noise signal representative of ambient noise; filtering and applying gain to the ambient noise signal to generate a noise cancellation signal; passing the noise cancellation signal to a speaker; and generating an error signal from an error microphone, wherein the gain applied to the ambient noise signal is controlled based on the error signal, and the method further comprises: determining from the error signal whether the audio device is in an off-ear position, and controlling the noise cancellation system based on said determination as to whether the audio device is in the off-ear position.

Abstract: There is provided an audio amplifier circuit (200), comprising: an input, for receiving an input signal; an amplifier (205) for receiving a modified input signal and outputting an amplified signal; and a feedback loop (245) comprising digital circuitry for feeding back the amplified signal and combining a feedback signal with the input signal to generate the modified input signal. The feedback loop (245) comprises: a low-pass filter (220) for filtering the amplified signal; a comparator (225) for comparing the filtered signal with a threshold value and outputting a comparison signal; and an integrator (230) for integrating the comparison signal and outputting the feedback signal. According to another embodiment, the feedback loop comprises an integrator and a sigma-delta modulator.

Abstract: A method of fabricating a micro-electrical-mechanical system (MEMS) transducer comprises the steps of forming a membrane on a substrate, and forming a back-volume in the substrate. The step of forming a back-volume in the substrate comprises the steps of forming a first back-volume portion and a second back-volume portion, the first back-volume portion being separated from the second back-volume portion by a step in a sidewall of the back-volume. The cross-sectional area of the second back-volume portion can be made greater than the cross-sectional area of the membrane, thereby enabling the back-volume to be increased without being constrained by the cross-sectional area of the membrane. The back-volume may comprise a third back-volume portion. The third back-volume portion enables the effective diameter of the membrane to be formed more accurately.

Abstract: A method of fabricating a micro-electrical-mechanical system (MEMS) transducer comprises the steps of forming a membrane on a substrate, and forming a back-volume in the substrate. The step of forming a back-volume in the substrate comprises the steps of forming a first back-volume portion and a second back-volume portion, the first back-volume portion being separated from the second back-volume portion by a step in a sidewall of the back-volume. The cross-sectional area of the second back-volume portion can be made greater than the cross-sectional area of the membrane, thereby enabling the back-volume to be increased without being constrained by the cross-sectional area of the membrane . The back-volume may comprise a third back-volume portion. The third back-volume portion enables the effective diameter of the membrane to be formed more accurately.

Abstract: A headphone comprises a speaker. The rear volume of the speaker is coupled to a mixing volume, the front volume of the speaker is coupled to the mixing volume, and the mixing volume is coupled to the exterior. The acoustic impedances resulting from the rear volume, the front volume, the mixing volume, and the passages between them can be adjusted, in order to achieve the desired sound egress properties. Acoustic damping material can be included in the various leakage paths in order to achieve the desired properties, depending on the type of speaker to be used, the acoustic design of the headphone, the mechanical properties of the headphone body, and the desired frequency response characteristics of the headphone.

Abstract: An amplifier circuit comprises an input, for receiving an input signal to be amplified; a preamplifier, for amplifying the input signal based on a variable gain; a power amplifier for amplifying the signal output from the preamplifier; and a variable voltage power supply for supplying one or more supply voltages to the power amplifier. The supply voltages are adjusted based on the variable gain or the input digital signal. According to other aspects of the invention, a power supply of an amplifier circuit is clocked using a clock signal, whereby the clock signal has a frequency that varies in accordance with a volume signal or an input signal.

Abstract: A MEMS capacitive transducer with increased robustness and resilience to acoustic shock. The transducer structure includes a flexible membrane supported between a first volume and a second volume, and at least one variable vent structure in communication with at least one of the first and second volumes. The variable vent structure includes at least one moveable portion which is moveable in response to a pressure differential across the moveable portion so as to vary the size of a flow path through the vent structure. The variable vent may be formed through the membrane and the moveable portion may be a part of the membrane, defined by one or more channels, that is deflectable away from the surface of the membrane. The variable vent is preferably closed in the normal range of pressure differentials but opens at high pressure differentials to provide more rapid equalisation of the air volumes above and below the membrane.

Abstract: Charge pump circuits having circuit components such as transistors which may be damaged by voltage transients greater than the normal operating voltage levels of the charge pump circuit, such as may be experienced during powering down. The circuit components to be protected are connected in parallel with a leakage element arranged to have a leakage current that is small enough during normal operation to allow the charge pump to operate effectively but which is large enough, during development of a voltage transient, to prevent excess voltage levels being achieved. The leakage element may have a significant leakage current at a voltage less than the breakdown voltage of the circuit component. Suitable leakage elements are poly diodes.

Abstract: An audio codec (100) is provided with a control sequencer (110) interposed between control interface logic (120) and control registers (130) of the codec. A memory (114, 116) associated with the control sequencer (110) enables a burst of operations to be preloaded into the audio codec. In response to a single command from an external control processor (20), the sequencer (110) can then execute the burst with precise delays between each operation and without further direction from the processor. This frees the processor (20) to concentrate on other tasks within the system, and/or to enter a low-power mode to conserve energy. The system may be a personal media player.

Abstract: An earphone has a housing, with a speaker mounted within the housing. A cable inlet contains a cable that includes a wire connected to the speaker. The cable and the cable inlet have different cross-sectional shapes, such that the cable is in contact with the inner surface of the cable inlet over a substantial portion of their length, while a rear volume of the speaker is vented through the cable inlet. This ensures that the cross-sectional area through which the rear volume is vented through the cable inlet remains relatively constant. The earphone may further comprise a microphone, positioned to detect ambient noise approaching the ear of a wearer of the earphone, and the cable may then further include a wire connected to the microphone.

Abstract: A signal amplifying circuit and associated methods and apparatuses, the circuit comprising: a signal path extending from an input terminal to an output terminal, a gain controller arranged to control the gain applied along the signal path in response to a control signal; an output stage within the signal path for generating the output signal, the output stage having a gain that is substantially independent of its supply voltage, and a variable voltage power supply comprising a charge pump for providing positive and negative output voltages, the charge pump comprising a network of switches that is operable in a number of different states and a controller for operating the switches in a sequence of the states so as to generate positive and negative output voltages together spanning a voltage approximately equal to the input voltage.

Abstract: An apparatus and method for regulating analogue-to-digital converters. First and second input signals are received at controlled oscillator circuitry which generates respective first and second pulse streams with pulse rates based on the relevant input signal. Difference circuitry determines the difference in number of pulses of the first and second pulse streams and outputs a first digital signal. Circuitry also determines a signal independent value based on the number of pulses of the first and/or second pulse streams. In one embodiment this value is the sum or average of the number of pulses of the first and second pulse streams. This value can be used to calibrate for any variation in transfer characteristic of the oscillator circuitry. In one embodiment this value is compared to a reference value and a regulation signal passed to control circuitry to regulate the operation of the oscillation circuitry.

Abstract: A MEMS capacitive transducer with increased robustness and resilience to acoustic shock. The transducer structure includes a flexible membrane supported between a first volume and a second volume, and at least one variable vent structure in communication with at least one of the first and second volumes. The variable vent structure includes at least one moveable portion which is moveable in response to a pressure differential across the moveable portion so as to vary the size of a flow path through the vent structure. The variable vent may be formed through the membrane and the moveable portion may be a part of the membrane, defined by one or more channels, that is deflectable away from the surface of the membrane. The variable vent is preferably closed in the normal range of pressure differentials but opens at high pressure differentials to provide more rapid equalization of the air volumes above and below the membrane.