Current Drive Can Overcome Pitfalls Of Class D Audio Amplifiers

Focus:

MRI gradient amplifiers perform similarly to class D audio amplifiers except that their high power and absolute accuracy is well beyond what commercial audio amplifiers can achieve. In the MRI, the drive employs a controlled current to create an accurate magnetic field gradient across the patient without direct feedback of the magnetic field. The driven gradient coils are complex loads and are typically modeled as third- or fifth-order impedances. Driving these complex impedances with a controlled current rather than voltage, results in greater accuracy. As this article explains, the same benefit can be obtained when driving a loudspeaker with a current and this approach also eliminates errors associated with using class D amplifiers to drive speakers, as well as the need to adopt GaN power transistors. This article explains operation of class D amplifiers conceptually, identifies their sources of error, compares current sensing vs voltage sensing feedback, presents simple electrical models of loudspeakers, and describes a circuit alternative to class D, a current-controlled amplifier employing hysteretic control.

What you’ll learn:

• How to drive audio amplifiers more accurately using current rather than voltage drive
• How to understand the operation of class D amplifiers
• How to understand the sources of error in class D amplifiers

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Author & Publication:

Anthony Esposito, Avatar Engineering, Fountain Hills, Ariz., How2Power Today, Mar 13 2020

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