An amplifier includes a differential amplifier stage, a voltage amplification stage and a power output stage. The bias level of the output stage is proportional to current through the voltage amplification stage. The voltage amplification stage includes a current mirror whereby controlled current through a first leg of the current mirror controls current through the remaining, second leg, which, in turn, determines the bias level of the power output stage. Control circuitry senses a parameter of the amplifier, such as DC bias or input signal level to generate a control signal which is applied to the first leg of the current mirror to thereby control bias level of the power output stage.

Adaptively controlled circuits are provided that can include an output buffer circuit, a sensing circuit, and a current conversion circuit. The output buffer circuit can include, for example, inputs, outputs and a bias control input. The sensing circuit can also include inputs and outputs. Two or more sensing circuit outputs can simultaneously turn to lower or higher level than a predetermined threshold level. The current conversion circuit can be composed of one or more set of cascaded transistors. Inputs of the cascaded transistors can be connected to outputs of the sensing circuit. The output of the cascaded transistors can be connected to foresaid bias control input of the output buffer. An output current of the current conversion circuit increases when signal level of the inputs of the sensing circuit become equal or close to each other.

An operational amplifier including a differential input section generating a first signal as a differential voltage between two input signals; an amplifying section amplifying the first signal into second and third complementary signals; a first MOS transistor between a first supply voltage and an output node, a conduction state of the first MOS transistor controlled responsive to the second signal; a second MOS transistor between a second supply voltage and the output node, a conduction state of the second MOS transistor controlled responsive to the third signal; and a step-up section stepping up the first and second supply voltages to generate a step-up voltage higher than the first and second supply voltages, the amplifying section driven by the step-up voltage so that an absolute value of the maximum level of the second or third signal becomes larger than the absolute value of the first or second supply voltage.

An amplifier comprises an input circuit that receives an input to the amplifier. A start-up circuit communicates with the input circuit, generates a start-up signal, and turns off the start-up signal when an output of the amplifier reaches a threshold voltage. The start-up circuit includes a first transistor having first and second terminals and a base terminal and a second transistor having first and second terminals and a base terminal. The base terminals of the first and second transistors receive a bias input, the first terminals of the first and second transistors communicate with each other and with a first current source, and the second terminals of the first and second transistors communicate with the input circuit.