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Single-ended design with Digital Amplifiers
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By
Kevin Belnap, Marketing Manager, Texas Instruments
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Page 1 of 2
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Audio DesignLine
(03/21/2007 3:16 PM EDT)
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One of the biggest advantages of digital amplifiers is flexibility of the digital data path for design reuse. Because the signal is kept in the digital domain until it is practically at the speakers, there is much more flexibility in signal routing. This flexibility also applies to processing both on-the-fly and on the production line with stuffing options and/or firmware changes. One common mode where the digital amp is used is called single-ended operation. This article discusses basics of single-ended design and the engineering tradeoffs involved.
Typically, the digital amplifier has a two-stage architecture " a pulse-width modulation (PWM) processor, followed by the power stage as shown in Figure 1. The logic-level PWM processor accepts the audio data, usually in IIS format. It performs audio processing and converts the pulse-code modulated (PCM) data into PWM data. Typically, the PWM processor is controlled through IIC to change volume, tone controls, or other audio processing functions like equalization. Most PWM processors have another key feature which is the ability to change signal routing, even "on-the-fly." This allows the designer flexibility in PCB routing, or the ability for the user to route content to different speakers. The power stage takes the 3.3V PWM signal, translates it to a higher voltage, and applies it to the speaker through the MOSFET H-bridge and second-order LC filter.
Figure 1: Digital amplifier datapath showing the power stage H-bridge
The power stage contains a MOSFET H-bridge shown in Figure 1. This is where the MOSFETs are used as switches to apply the +V voltage across the speaker, either in a positive or negative direction. Bridge-tied load (BTL) is the normal configuration for most stereo power stages where the speaker is connected between the two MOSFET half-bridges. This is shown in Figure 1. Single-ended (SE) refers to when each speaker is driven by a single MOSFET half-bridge. The channel count is doubled in SE mode when compared to BTL mode, but the power per channel is cut to about 25 percent for a given output load. In single-ended mode, the +V voltage is applied to the speaker in a positive direction when the PWM signal is "high." When the PWM signal is "low," the speaker is connected to ground.
Single-ended operation for a digital amplifier shown in Figure 2 and is not much different than single-ended operation for a linear audio amp. The main difference is that the reconstruction filter (second order LC filter) sorts out the higher frequency components from the PWM signal, leaving the baseband audio signal. Applying the audio signal directly across the speaker results in a large DC voltage across the speaker, which is equal to PVDD/2. Because the speaker impedance has a predominant inductive component, this is equivalent to placing a large DC voltage across an inductor and causes the current to increase linearly to a very large value, possibly damaging the speaker. A large capacitor (DC blocking cap) is placed between the amplifier and the speaker to filter out this DC component. However, this cap also attenuates the lower audio frequencies and has a "3dB point of approximately 1/(2 RspC) where Rsp is the impedance of the speaker. To pass more frequency bandwidth through the speaker, a larger capacitance can be used. However, this comes at a tradeoff of cost and PCB area.
Figure 2: Single-ended digital amplifier with DC blocking cap configuration
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