An audio amplifier circuit for providing an output signal to an audio transducer may include a power amplifier and a control circuit. The power amplifier may include an audio input for receiving an audio input signal, an audio output for generating the output signal based on the audio input signal, and a power supply input for receiving a power supply voltage, wherein the power supply voltage is variable among at least a first supply voltage and a second supply voltage greater than the first supply voltage. The control circuit may be configured to predict, based on one or more characteristics of a signal indicative of the output signal, an occurrence of a condition for changing the power supply voltage, and responsive to predicting the occurrence of the condition, change, at an approximate zero crossing of the signal indicative of the output signal, the power supply voltage.

An audio amplifier circuit for providing an output signal to an audio transducer may include a power amplifier and a control circuit. The power amplifier may include an audio input for receiving an audio input signal, an audio output for generating the output signal based on the audio input signal, and a power supply input for receiving a power supply voltage, wherein the power supply voltage is variable among at least a first supply voltage and a second supply voltage greater than the first supply voltage. The control circuit may be configured to predict, based on one or more characteristics of a signal indicative of the output signal, an occurrence of a condition for changing the power supply voltage, and responsive to predicting the occurrence of the condition, change, at an approximate zero crossing of the signal indicative of the output signal, the power supply voltage.

Methods, systems, and devices for controlling a variable capacitor. One aspect features a variable capacitance device that includes a capacitor, a first transistor, a second transistor, and control circuitry. The control circuitry is configured to adjust an effective capacitance of the capacitor by performing operations including detecting a zero-crossing of an input current at a first time. Switching off the first transistor. Estimating a first delay period for switching the first transistor on when a voltage across the capacitor is zero. Switching on the first transistor after the first delay period from the first time. Detecting a zero-crossing of the input current at a second time. Switching off the second transistor. Estimating a second delay period for switching the second transistor on when a voltage across the capacitor is zero. Switching on the second transistor after the second delay period from the second time.

Methods, systems, and devices for controlling a variable capacitor. One aspect features a variable capacitance device that includes a capacitor, a first transistor, a second transistor, and control circuitry. The control circuitry is configured to adjust an effective capacitance of the capacitor by performing operations including detecting a zero-crossing of an input current at a first time. Switching off the first transistor. Estimating a first delay period for switching the first transistor on when a voltage across the capacitor is zero. Switching on the first transistor after the first delay period from the first time. Detecting a zero-crossing of the input current at a second time. Switching off the second transistor. Estimating a second delay period for switching the second transistor on when a voltage across the capacitor is zero. Switching on the second transistor after the second delay period from the second time.

Methods and apparatus for detecting zero-volt crossing in a field-effect transistor. A comparator compares a drain-to source voltage of the transistor to a threshold voltage. A gate voltage signal of the transistor is provided to a clock input of the comparator such that said gate voltage signal is used to latch a result of said comparison to an output of the comparator. A control function with respect to the transistor is performed based on the value of the comparator output.

Methods and apparatus for detecting zero-volt crossing in a field-effect transistor. A comparator compares a drain-to source voltage of the transistor to a threshold voltage. A gate voltage signal of the transistor is provided to a clock input of the comparator such that said gate voltage signal is used to latch a result of said comparison to an output of the comparator. A control function with respect to the transistor is performed based on the value of the comparator output.

Methods and apparatus for detecting zero-volt crossing in a field-effect transistor. A comparator compares a drain-to source voltage of the transistor to a threshold voltage. A gate voltage signal of the transistor is provided to a clock input of the comparator such that said gate voltage signal is used to latch a result of said comparison to an output of the comparator. A control function with respect to the transistor is performed based on the value of the comparator output.

Methods and apparatus for detecting zero-volt crossing in a field-effect transistor. A comparator compares a drain-to source voltage of the transistor to a threshold voltage. A gate voltage signal of the transistor is provided to a clock input of the comparator such that said gate voltage signal is used to latch a result of said comparison to an output of the comparator. A control function with respect to the transistor is performed based on the value of the comparator output.

A power amplifier device includes first and second pairs of semiconductor switches, transformers, and a zero-crossing detection circuit for detecting a zero voltage crossing of an analog input signal. The switches of the first pair receive a respective positive and negative component of the input signal. The transformers store energy from the positive and negative components, respectively. Each transformer releases accumulated energy when the respective switch of the first pair turns off The switches of the second pair have opposite switching states and are connected between a respective transformer and a load, e.g., a transducer, speak, or motor. Each switch receives released energy from the respective transformer. A switching state of each switch of the second pair changes in response to a detected zero voltage crossing of the input signal to transfer the released energy to the load. A system includes the device and the load.

A power amplifier device includes first and second pairs of semiconductor switches, transformers, and a zero-crossing detection circuit for detecting a zero voltage crossing of an analog input signal. The switches of the first pair receive a respective positive and negative component of the input signal. The transformers store energy from the positive and negative components, respectively. Each transformer releases accumulated energy when the respective switch of the first pair turns off The switches of the second pair have opposite switching states and are connected between a respective transformer and a load, e.g., a transducer, speak, or motor. Each switch receives released energy from the respective transformer. A switching state of each switch of the second pair changes in response to a detected zero voltage crossing of the input signal to transfer the released energy to the load. A system includes the device and the load.