PWM Audio Amplifiers

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PWM Audio Amplifiers
Zhiming Deng Chinwuba Ezekwe Dimitrios
Katsis
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Outline

• PWM Basics
• Digital Modulator Signal Flow
• Pulse Edge Delay Error Correction
• Volume Control

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PWM Basics

• Efficient modulation method when only two states
  (ON OFF) are available
• Linear modulation Average pulse ON time
  proportional to signal
• Extract modulating signal with low pass filter
• Modulation depth (Mlt1) signal peak/triangle peak

www.powerdesigners.com
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Natural PWM (NPWM)

• Straightforward analog implementation

• What about PCM input?

www.powerdesigners.com
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Uniformly Sampled PWM (UPWM)

• Sample modulating signal at the edges of the
  sawtooth
• Use this to estimate the pulse width
• Problem Introduces an error!

Jorge Varona, http//www.eecg.toronto.edu/kphang/
ece1371/pwrdac.pdf
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Weighted PWM (WPWM)

• Estimates pulse width using the information from
  two consecutive samples

• NPWM approximation in the digital domain
• Yields improved THD compared to UPWM

Morten Kjaer Johansen et al, A review and
comparison of digital PWM methods for digital PMA
systems, Proceedings of the 107th AES
Convention, 1999.
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Digital Modulator Signal Flow

• Oversampling gives better dynamic range
• WPWM is used to simulate NPWM in order to
  decrease THD
• Noise shaping also helps lower the bit-rate while
  keeping a high resolution

Steen Munk et al, State of the Art Digital Pulse
Modulated Amplifier System, AES 23rd
International Conference, 2003.
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Sigma-Delta Noise shaper

• Use noise shaping to preserve in-band SNR

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Amplification Errors in Analog Power Stage

• Non-stable supply causes amplitude errors.
• Dead time in switches causes delay of rising edge
  of output PWM signal.
• Variation of load changes the frequency
  characteristics of the demodulation filter.
• Finite On resistance of semiconductor switch and
  output filter resistance lead to output
  resistance.
• Non-linearity of the demodulation filter leads to
  errors in the demodulated output signal.

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Error Correction

• Analog signal referenced control system
• NOT POSSIBLE in a digital PMA because of the
  absence of analog reference signal.
• Pulse Edge Delay Error Correction (PEDEC)
• PEDEC is a pulse referenced control system
  that eliminates all types of error by re-timing
  the edges of the PWM inputs.

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Pulse Edge Delay Error Correction

• Pulse referenced control system
• Eliminates error by retiming pulse edges

Karstem Nielsen, Digital Pulse Modulation
Amplifier (PMA) topologies based on PEDEC
Control, Proceedings of the 106th AES
Convention, 1999.
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Edge Delay (ED) Unit

• Makes absolute correction
• No quantization error
• Limited correction range? saturates if correction
  range is exceeded
• Increasing phase lag of feedback signal w.r.t.
  reference signal leads to saturation for very
  short or long pulses

for
Steen Munk et al, State of the Art Digital Pulse
Modulated Amplifier System, AES 23rd
International Conference, 2003.
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Voltage Feedback before Demodulation Filter

• Feedback from switching power stage output Vp.
• Feedback path compensator A(s) is a simple
  attenuation.
• No reference signal shaping

Karstem Nielsen, Digital Pulse Modulation
Amplifier (PMA) topologies based on PEDEC
Control, Proceedings of the 106th AES
Convention, 1999.
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Voltage Feedback before Demodulation Filter

• 1st order reference shaping with matched 1st
  order output feedback shaping

Karstem Nielsen, Digital Pulse Modulation
Amplifier (PMA) topologies based on PEDEC
Control, Proceedings of the 106th AES
Convention, 1999.
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Voltage Feedback after Demodulation Filter

• Using global feedback from demodulator output Vo.
• 2nd order reference shaping
• This is the ONLY topology that eliminates the
  errors in the demodulation filter

Karstem Nielsen, Digital Pulse Modulation
Amplifier (PMA) topologies based on PEDEC
Control, Proceedings of the 106th AES
Convention, 1999.
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Volume Control

• A digital volume control is simple to implement
  but will decrease the dynamic range as the signal
  is attenuated.
• An analog volume control can retain the dynamic
  range, but it is not applicable to a digital PMA.
• GOAL a volume control system that will not
  decrease dynamic range.
• Intelligent Volume Control (IVC)

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Intelligent Volume Control (IVC)

• Use multiple supply voltages in power stage for
  coarse control
• Use digital attenuation through modulation depth
  modification for fine adjustment

Steen Munk et al, State of the Art Digital Pulse
Modulated Amplifier System, AES 23rd
International Conference, 2003.
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IVC (two-level example)
Steen Munk et al, State of the Art Digital Pulse
Modulated Amplifier System, AES 23rd
International Conference, 2003.
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IVC Advantages

• Improved dynamic range
• Improved edge related noise
• Improved efficiency
• Improved electromagnetic interference (EMI)
  characteristics

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Summary

• Improve THD with WPWM
• Reduce noise by noise shaping
• Correct various errors through PEDEC
• Improve SNR of volume control by using multiple
  supply voltages