In this paper, an ultra low-power delta-sigma (ΔΣ) modulator applying charge-transfer amplifier (CTA) technique for voice-band applications is presented. Both the fully-differential charge-transfer amplifier and integrator are developed for higher dynamic range of the modulator. A 67 dB of the peak SNR within a 4 kHz of bandwidth is reached in the presented modulator under a 2.5 MHz of sampling rate. The prototype circuit has been implemented in a 0.18 μm 1P6M CMOS technology. The chip area excluding PADs is 0.50 × 0.28 mm2. Due to its zero static current of CTA, only dynamic power is consumed in the circuit. The power consumption of the analog part of the presented second-order ΔΣ modulator is only 3.4 μW. The total power consumption of the whole modulator is 36 μW at a 1.8 V of supply voltage.