An Improved Voltage Reference Based On Subthreshold Mosfets

Along with the development of modern science and technology, the society is getting bigger and bigger to integrated circuit's demand, from the aerospace to mobile phone. Can be said that people's lives are increasingly depend on the semiconductor products. It is also for these reasons, the people had more and more high request to the chip performance and the type. Voltage reference's performance of chip determines the chip's performance, which is a extremely important in the circuit unit.By analyzing the characteristics of MOSFETs devices in the subthreshold, and combine with the principle of bandgap reference, then putting forward another structure of the reference voltage source based on subthreshold CMOS with analysis of G. Ciustolisi in [3] proposed the reference structure. The reference with a simple structure of CMOS had defeated the complex of bipolar transistor technology. The circuit of PTAT. can simplify the integrate circuit's structure, simultaneously provides the positive temperature coefficient voltage. The reference is simulated in TSMC 0.18um CMOS process and the HSPICE simulation results show that the voltage reference can work nomally; in the condition of 1.8V power supply voltage and room temperature, the low-frequency power supply rejection ratio is -25dB. The PSRR will reduce to 0 dB later start in the frequency range from 1M. When the temperature ranges from -40℃to 100℃, the temperature coefficient which the output voltage of the reference with the improvement structure is 36.5ppm/0C. The output voltage is 529 mV in room temperature. The results show that the circuit has good performance and meets the design requirement. The PSRR is low in the low-frequency range, is zero in the high-frequency range. So this is the place which needs to improve.The voltage reference based on subthreshold MOSFETs had a problem, about-25dB, which it was't enough high of the PSRR in low-frequency, which it was zore in high-frequency. We proposed two methods of the circuit's improvement, the first method is increase the voltage follower module behind power source's out-port, and realizes to enhances PSRR in the frequency range performance. The second kind is increases a constant voltage module in front end, causes the secondary circuit work under a stabler voltage, realizes through this method to enhances PSRR the goal. The reference is simulated in TSMC 0.18um CMOS process and the HSPICE simulation results show that the voltage follower module circuit's temperature coefficient had't the obvious changes. And the PSRR had the obvious changes in high-frequency, from OdB to -25dB, but it had't the obvious changes in low-frequency. About the second method, the temperature ranges from -40℃to 100℃, the temperature coefficient which the output voltage of the reference is 43.5ppm/0 C. The output voltage is 590.6mV in room temperature. The PSRR of improvement circuit is -51dB in low-frequency, enhance about -25dB than the before type, this kind of structure's PSRR is quite ideal in high-frequency. Only when the frequency in 1M the PSRR is minimum of -9dB. Therefore we had discovered the effect obvious that the second method enhanced the PSRR performance.