We propose a CMOS circuit that can be used as an equivalent to resistors. This circuit uses a simple differential pair with diode-connected MOSFETs and operates as a high-resistance resistor when driven in the subthreshold region of MOSFETs. Its resistance can be controlled in a range of 1-1000 MΩ by adjusting a tail current for the differential pair. The results of device fabrication with a 0.35-µm 2P-4M CMOS process technology is described. The resistance was 13 MΩ for a tail current of 10 nA and 135 MΩ for 1 nA. The chip area was 105 µm110 µm. Our resistor circuit is useful to construct many high-resistance resistors in a small chip area.

This paper presents an automated design of linear and non-linear differential analog circuits accelerated by reuse of genetic operations. The system first synthesizes circuits using pairs of simplified MOSFET model. During the evolutionary process, genetic operations that improve circuit characteristics are stored in a database and reused to effectively obtain a better circuit. Simplified elements in a generated circuit are replaced by MOSFETs and optimization of the transistor size is performed using an optimizer available in market if necessary. The capability of this method is demonstrated through experiments of synthesis of a differential voltage amplifier, a circuit having cube-law characteristic in differential mode and square-law characteristic in common-mode, and a dB-linear VGA (Variable Gain Amplifier). The results show the reuse of genetic operations accelerates the synthesis and success rate becomes 100%.