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Pulse width modulated amplifier


Pulse width modulated (PWM) amplifier generate a current that switches between high and low output levels. The analog signal input controls the duty cycle of an output pulse train that switches on and off once during each cycle. When a high output is required, the pulse train remains on for most of the cycle. When a low output is required, the pulse train remains off during most of the cycle. For both high and low outputs, PWM amplifier chips modulate the pulse train in the time domain and use LC filtering to extract an analog signal output. These low-pass filters consist of an inductor (L) connected in series and a capacitor (C) connected in parallel. Typically, pulse width modulated amplifiers are used to emulate linear constant voltage amplifiers or linear constant current amplifiers. PWM amplifier chips consume less power than these linear devices and are less expensive. Some PWM amplifier chips can be operated with a single power supply or include an embedded current limiter. Others include thermal shutdown protection circuitry or on-chip electrostatic discharge (ESD) protection.

Selecting PWM amplifier chips requires an analysis of performance specifications. Output current is the maximum continuous current that can be delivered in the output. Input offset voltage is the amount of DC voltage that amplifiers produce even when 0 V is applied to the input. The supply voltage range includes minimum and maximum amounts. Internal power dissipation is the maximum amount of power that can be safely supported. Quiescent current is produced during normal operation. The power bandwidth or large-signal bandwidth describes an amplifier’s ability to provide a maximum output voltage swing with increasing frequency. The peak output swing is the output voltage at the frequency which represents the upper limit of the power bandwidth. A high switching frequency allows smaller output filters to be built into the amplifier enclosure. Typically, suppliers list the switching frequency for PWM amplifier chips as a maximum amount.





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