A hybrid modulation scheme for digital Class D amplifier modulators

Abstract

In this paper, we adopt a hybrid modulation scheme for digital modulators of Class D amplifiers of power-critical digital hearing aids. The hybrid modulation scheme comprises the Delta-Sigma modulation (ΔΣM) scheme and the Pulse Width Modulation (PWM) scheme. The ΔΣM of the hybrid modulation scheme quantizes down the input data while the PWM generates 3-state output pulses corresponding to the quantized data. For this hybrid modulation scheme, we propose to quantize down the input to an output with 5 quantization levels. By using this amount of quantization levels, it is possible to obtain higher output linearity than that obtained by using the ΔΣM alone (same 3-state, but only 3 quantization levels). It is also possible to generate the output pulses without using a higher clock frequency than the input sampling frequency - unlike the usual PWM pulse generation that typically requires higher clock frequency. We also propose a simple pulse randomizer to improve the output linearity. We simulate a digital modulator design embodying the hybrid modulation scheme and the pulse randomizer, and compare its performance with that of a similar design employing a 3-state ΔΣM scheme alone. The THD+N of the output from the hybrid scheme modulator is −85 dB and is 10 dB better than that of the ΔΣM modulator.