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The real story about closed-, open-loop Class D amps

Posted: 04 Sep 2009 ?? ?Print Version ?Bookmark and Share

Keywords:Class D amplifier? PSRR? rail supplies?

Historically, power supply rejection ratio (PSRR) has been an excellent measurement of an amplifier's ability to reject supply noise from its output. With the ever-increasing presence of Class D amplifiers and the efficiency advantages they provide, however, it is no longer sufficient to rely only on PSRR as an indicator of power supply noise rejection.

This is especially evident when comparing PSRR specs of open- and closed-loop digital-input I?S amplifiers. Many times the PSRR specs are identical, but when listening to the amplifiers with less than ideal power supplies, clearly there is a difference in audio performance. This article provides an overview of the traditional PSRR measurement, explains why it does not adequately capture the supply rejection performance in Class D amplifiers in a bridge-tied load (BTL) configuration, and describes an alternate way to measure the effects of power supply noise in Class D amplifiers.

To understand why the PSRR measurement no longer adequately captures the supply rejection performance, we need to look back in time when Class AB amplifiers dominated consumer audio electronics. Class AB amplifiers were commonly configured in either a single-ended (SE) or BTL output configuration, as they are today. In fact, it was fairly common for SE Class AB amplifiers to have split rail supplies (i.e. +/- 12V) as power supplies were predominantly transformer-based and adding a second rail was not cost-prohibitive. The BTL configuration was more commonly used in audio systems that did not have a split rail supply. Whether talking about SE or BTL configurations, Class AB amplifiers inherently have good PSRR, given their fundamental architecture and output levels that are typically well below the supply rail voltage.

For Class AB amplifiers, the PSRR measurement provides a relatively good indication of the amplifier's ability to reject supply noise, and is especially accurate for the SE configuration. Fast forward a bit in time and we start to see Class D amplifiers hitting the market. Their extremely efficient operation changed the market dynamics, enabling considerable innovation in industrial design, especially in smaller form factors. However, their architecture was fundamentally different than Class AB amplifiers, and their output configuration of choice was almost exclusively BTL.

In the BTL configuration, the Class D amplifier has two output stages consisting of four FETS (also known as a full-bridge). While the SE Class D amplifier has just a single output stage consisting of two FETS (also known as a half-bridge). The BTL output configuration has a number of advantages over SE configurations, including four times the output power for a given supply rail, better bass response, and superior turn on/off click and pop performance. A disadvantage of the BTL architecture is that you need twice the number of FET transistors. This means a larger silicon die size and associated cost, and double the reconstruction filter (LC filter) costs. While in today's market you can see both SE and BTL Class D amplifiers, the majority are BTL.

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