A power supply circuit according to an embodiment includes an driver, a control circuit, and a protection circuit. The driver includes a first transistor connected between a high-potential-side power supply and a node and a second transistor connected between a low-potential-side power supply and the node. The control circuit generates, according to an output voltage to a load connected to the node via a first low-pass filter circuit, first and second switching pulses for alternately switching the first and second transistors. The protection circuit outputs, when a voltage of the node via a second low-pass filter circuit exceeds a first reference voltage, an interruption signal for making at least the first transistor nonconductive.

Methods and system for using a multifunctional filter to minimize insertion loss in a multi-mode communications system are described. Specifically described is a multifunctional filter that is configurable to operate in a band-pass mode when a first type of signal is propagated through the multifunctional filter, and to operate in a low-pass mode when a second type of signal is propagated through the multifunctional filter. The multifunctional filter presents a lower insertion loss to the second type of signal when operating in the low-pass mode than in the band-pass mode.

A vacuum tube subwoofer extraction circuit system includes: a front-end circuit; and a vacuum tube subwoofer extraction circuit connected to the front-end circuit, the vacuum tube subwoofer extraction circuit including: a small signal amplification vacuum tube for receiving an input signal from the front-end circuit and outputting an audio signal; and a passive filtering circuit connected to the small signal amplification vacuum tube and adapted to perform a filtering process on the audio signal so as to output a filtered signal, wherein the front-end circuit includes a switch power circuit for providing low voltage to the vacuum tube subwoofer extraction circuit, wherein the small signal amplification vacuum tube does not have gain effect on the input signal.

A Bulk Acoustic Wave (BAW) resonator filter can include a BAW resonator pass-band filter ladder, where the BAW resonator pass-band filter ladder can be configured to pass frequency components of an input signal in a pass-band of frequencies received at an input node of the BAW resonator pass-band filter ladder to an output node of the BAW resonator pass-band filter ladder. A first rejection-band series resonator can be coupled in series between an input port of the BAW resonator pass-band filter ladder and the input node, where the first rejection-band series resonator can have a first anti-resonant frequency peak in a rejection-band of frequencies that is less than the pass-band of frequencies. A second rejection-band series resonator can be coupled in series between an output port of the BAW resonator filter and the output node, where the second rejection-band series resonator can have a second anti-resonant frequency peak in the rejection-band of frequencies.

Embodiments of radio frequency (RF) filtering circuitry are disclosed. In one embodiment, the RF filtering circuitry includes a first port, a second port, a first RF filter path, and a second RF filter path. The first RF filter path is connected between the first port and the second port and includes at least a pair of weakly coupled resonators. The weakly coupled resonators are configured such that a first transfer response between the first port and the second port defines a first passband. The second RF filter path is coupled to the first RF filter path and is configured such that the first transfer response between the first port and the second port defines a stopband adjacent to the first passband without substantially increasing ripple variation of the first passband defined by the first transfer response.

An acoustic wave filter device includes a ground connection terminal connected to an external ground electrode, a serial arm resonator, a first parallel arm circuit connected to a first node of the serial arm resonator, the first node being positioned closer to an input/output terminal, and to the ground connection terminal, and a second parallel arm circuit connected to a second node of the serial arm resonator, the second node being positioned closer to an input/output terminal, and to the ground connection terminal. The first parallel arm circuit includes a parallel arm resonator having a resonant frequency higher than a center frequency of a filter pass band, and a frequency varying circuit connected to the parallel arm resonator and the ground connection terminal. The second parallel arm circuit includes a parallel arm resonator having a resonant frequency higher than the center frequency of the filter pass band.

A filter includes: a series part disposed between a signal input terminal and a signal output terminal, and including at least one first bulk-acoustic resonator; an inductor portion including a first inductor disposed between ends of the series part and a second inductor having a first end connected to a connection node of the series part and the first inductor; and a shunt part disposed between a second end of the second inductor and a ground, and including at least one second bulk-acoustic resonator, wherein a resonant frequency of the at least one second bulk-acoustic resonator is higher than a resonant frequency of the at least one first bulk-acoustic resonator.

An acoustic wave device includes a piezoelectric substrate made of LiNbO.sub.3, and a dielectric film provided on the piezoelectric substrate to cover first and second IDT electrodes on the piezoelectric substrate. The first and second IDT electrodes include main electrode layers. When wave lengths determined by electrode finger pitches of the first and second IDT electrodes are .sub.1 and .sub.2, respectively, the average value thereof is .sub.0, .sub.1/.sub.0=1+X, and .sub.2/.sub.0=1X, a relationship of 0.05X0.65 is satisfied. The wavelength .sub.1 is the longest, and the wavelength .sub.2 is the shortest. In Euler angles (, , ) of the piezoelectric substrate, is 05, is 010, and satisfies Expression 1, wherein a relationship of B.sub.1-<Tr0.10.sub.0 and B.sub.2<Tr0.10.sub.0 are satisfied.

A switch component includes a common terminal, at least two selection terminals, a switching circuit that selectively connects the common terminal to each of the at least two selection terminals, and an inductor. One end of the inductor is connected to one of the at least two selection terminals. The switching circuit is integrated with the inductor.

A circuit includes a bandpass filter and a self-tracking circuit. The bandpass filter has a first input node configured to receive an input square wave signal and an output node configured to provide an output sine wave signal. The bandpass filter includes a first binary-weighted programmable resistor array. The self-tracking circuit includes a second input node coupled to the output node. The self-tracking circuit includes a counter, and the counter includes an output node coupled to the first binary weighted programmable resistor array.