Design of a 3rd Order 1.5-Bit Continuous-Time Fully Differential Sigma-Delta (ΣΔ) Modulator Optimized for a Class D Audio Amplifier Using Differential Pairs

This paper presents a 3 order 1.5-bit Continuous-Time Fully Differential Ʃ∆ modulator with distributed feedback for a class D audio amplifier, using BJT differential pairs to implement the integrator stages. By relying on simple gain blocks instead of operational amplifiers to build the loop filter, a simpler overall circuit is obtained, where the non-ideal effects are embedded in the loop filter transfer function. This leads to a more difficult design process for the loop filter circuit, solved through the use of an optimization procedure based on genetic algorithms. Simulations of the electrical circuit show that it is capable of achieving a SNDR value of 73.4 dB and THD+N of about -80 dB for a signal bandwidth of 20 kHz and a sampling frequency of 1.28 MHz.