A Class D amplifier is described herein that includes an outer loop, an inner loop, and a notch filter. The notch filter can be located between an output of the outer loop and an input of the inner loop. Alternatively or in addition, the notch filter can be located within the outer loop of the Class D amplifier. Ripple content can initially be present at an input to the inner loop of the Class D amplifier, causing nonlinearity in the inner loop and distortion in the audio output signal. The notch filter can filter the ripple content at the input to the inner loop, thereby reducing the nonlinearity present in the inner loop and the distortion in the audio output signal.

A circuit for receiving an input signal is described. The receiver comprises a first receiver input configured to receive a first input of a differential input signal; a second receiver input configured to receive a second input of a differential input signal; a differential pair having an inverting input and a non-inverting input; a first impedance matching element coupled to the differential pair, wherein the first impedance matching element provides DC impedance matching from the inverting input and non-inverting input of the differential pair; and a second impedance matching element coupled to the differential pair, wherein the second impedance matching element provides AC impedance matching from the inverting input and non-inverting input of the differential pair.

An electrical circuit comprising a modulating chopper configured to receive a differential input signal at a first frequency and modulate the differential input signal to a second frequency to form a modulated differential signal, a null amplifier coupled to the modulating chopper and configured to amplify the modulated differential signal to form an amplifier output, wherein amplifying the modulated differential signal causes a ripple in the amplifier output, a demodulating chopper coupled to the null amplifier and configured to demodulate the amplifier output to form a demodulated differential signal having a first portion at the first frequency and a second portion at a third frequency, an integrator coupled to the demodulating chopper and configured to integrate the demodulated differential signal to form an integrated differential signal, and an attenuator coupled to the integrator and configured to attenuate the integrated differential signal to compensate for at least part of the ripple.

An amplifier circuits inductive/magnetic sensor interface can include a main signal path including one or more amplifiers configured to receive an input signal and to produce an output signal based on the input signal. The input signal may include a square-wave demodulated signal having an associated modulation frequency and an undesired frequency component at twice the modulation frequency of the square-wave demodulated signal. The amplifier circuit may include a gain feedback loop configured to set a gain of the amplifier circuit. The amplifier circuit may include a ripple reduction feedback loop configured to receive an intermediate signal on the main signal path and extract the undesired frequency component of the intermediate signal to produce a filtered version of the intermediate signal and provide the filtered version of the intermediate signal to the main signal path.

A computer sound card includes a power supply noise isolation circuit for reducing ground plane noise generated during application of heavy electrical loads to the power supply, such as loads presented by computer GPUs. The power supply isolation circuit isolates the output ground from the ground presented from the power supply to the isolation circuit input. The isolation circuit in one embodiment includes switching circuitry and a transformer to reduce the power supply ripple noise that might otherwise be introduced by the power supply into an amplifier such as a microphone preamp. In some embodiments a differential amplifier stage is added to the output of the microphone preamp stage to further reduce noise, such as common mode noise.

Described examples include multistage amplifier circuits having first and second forward circuits, a comparator or sensor circuit coupled to sense a signal in the second forward circuit to identify nonlinear operation or slewing conditions in the multistage amplifier circuit, and one or more sample hold circuits operative according to a sensor circuit output signal to selectively maintain the amplitude of an amplifier input signal in the second forward circuit and/or in a feedback circuit in response to the sensor circuit output signal indicating nonlinear operation or slewing conditions in the multistage amplifier circuit. Certain examples further include a clamping circuit operative to selectively maintain a voltage at a terminal of a Miller compensation capacitance responsive to the comparator output signal indicating nonlinear operation or slewing conditions.

A low power amplifier architecture that employs a single-ended (single triode) push-pull (SEPP) vacuum tube and output transformer arrangement, and that cancels unwanted amplifier signal components such as hum and noise. The SEPP amplifier operates to cancel power supply ripple and local EMI induced noise in the output transformer by providing reverse polarity of the primary coils of the output transformer.

An example apparatus includes: a low-pass filter and a high-pass filter coupled to a first current terminal of the first transistor; an input of a first amplifier coupled to the output of the high-pass filter; an input of a negative gain amplifier coupled to an output of the first amplifier; inputs of an adder coupled to an output of the negative gain amplifier and an output of the low-pass filter; a first input of a second amplifier coupled to the output of the adder; a control terminal of a second transistor coupled to the output of the second amplifier, a second current terminal of the second transistor coupled to the first input of the second amplifier; and a control terminal of a third transistor coupled to the output of the second amplifier, a second current terminal of the third transistor coupled to an output terminal.

Described examples include multistage amplifier circuits having first and second forward circuits, a comparator or sensor circuit coupled to sense a signal in the second forward circuit to identify nonlinear operation or slewing conditions in the multistage amplifier circuit, and one or more sample hold circuits operative according to a sensor circuit output signal to selectively maintain the amplitude of an amplifier input signal in the second forward circuit and/or in a feedback circuit in response to the sensor circuit output signal indicating nonlinear operation or slewing conditions in the multistage amplifier circuit. Certain examples further include a clamping circuit operative to selectively maintain a voltage at a terminal of a Miller compensation capacitance responsive to the comparator output signal indicating nonlinear operation or slewing conditions.

A single-end amplifier includes: a noise cancelling circuit, coupled to a power supply, configured to receive a power signal and to cancel a part of ripples and noises in the power signal to generate an initial signal; an amplifying circuit, configure to receive the initial signal at a first end of the amplifying signal, and to amplify the initial signal to generate a first signal at a second end; and a first transmitting circuit, configured to receive the power signal and to generate a second signal at the second end of the amplifying circuit. The first signal and the second signal are superimposed and outputted to cancel most part of the ripples and noises in the power signal. The noise cancelling circuit includes a first capacitor and a first choke coil.