An audio amplifier with duty ratio control is provided. The audio amplifier comprises a pulse width modulation modulator, a power stage, and a voltage converter. The pulse width modulation modulator is configured to receive an input signal for generating a pulse width modulation signal. The power stage is configured to output an output signal according to a supply voltage and the pulse width modulation signal. The voltage converter is configured to adjust voltage level of the supply voltage according to the pulse width modulation signal. The audio amplifier is configured to adjust the voltage level of the supply voltage when duty ratio of the pulse width modulation signal is greater than a duty ratio threshold.

There is disclosed an envelope tracking amplification stage comprising an input path and an envelope path and a delay stage in one of the input path and the envelope path, the delay of the delay stage being determined in dependence on the slope polarity of a signal representing the input and whether the signal in the input path leads or lags the signal in the envelope path.

An audio amplifier system is described comprising: a variable gain audio processor for processing digital audio signal, a digital to analog converter coupled to the audio processor, and configured to receive the processed digital audio signal, a variable gain amplifier having an input coupled to the output of the digital to analog converter and operably connected to a power supply, a controller coupled to the variable gain audio processor and the variable gain amplifier and configured to switch the audio amplifier system between a first operating mode having a first power supply voltage value and a second operating mode having a second higher power supply voltage value; wherein the controller is operable in the first operating mode to set the audio amplifier system gain to a desired gain value and in the second operating mode to maintain the desired gain value.

A radiofrequency (RF) amplifier includes an input terminal, an output terminal, and a power supply and biasing stage having an output coupled to the input terminal. An amplification stage of the RF amplifier includes a first transistor having a control terminal coupled to the input terminal and a first conduction terminal coupled to the output terminal. The power supply and biasing stage is configured to generate a bias voltage at the control terminal of the first transistor to simultaneously regulate a power supply voltage of the amplification stage to a first voltage and a bias current of the amplification stage to a first current.

A differential amplifier includes a positive leg, a negative leg, and biasing circuitry. The positive leg includes at least one positive leg transistor, a first positive leg degeneration capacitor, and positive leg degeneration capacitor biasing circuitry configured to bias the first degeneration capacitor during a reset period. The negative leg includes at least one negative leg transistor, a negative leg degeneration capacitor, and negative leg degeneration capacitor biasing circuitry configured to bias the negative leg degeneration capacitor during the reset period. The biasing circuitry biases current of both the at least one positive leg transistor and the at least one negative leg transistor based on capacitance of the first positive leg degeneration capacitor, capacitance of the first negative leg degeneration capacitor, and a sampling time during an amplification period. The differential amplifier may be a stage amplifier in an Analog to Digital Converter (ADC).

A compensation method for a characteristic difference of a photoelectric element is disclosed. The method includes (S1) providing a test substrate with a connector, a photoelectric element and a plurality of gain units, wherein the plurality of gain units are connected in parallel; (S2) connecting the connector to a test fixture which includes a test resistor and a test control unit, wherein when the test fixture is connected with the connector, the test resistor is electrically connected between the second pin and the third pin; (S3) providing input power to the connector so as to generate a test voltage on the photoelectric element; (S4) selecting the corresponding gain unit according to the test voltage and a classification data table; (S5) driving a production line to connect the first contact and the second contact of the selected gain unit.