Log Domain Integrator using Quasi-Floating Gate MOSFET

Background: A MOS transistor operating in weak inversion exhibits logarithmic behavior and could serve as replacement of bipolar transistors which have often been employed in implementing log domain circuits. However, threshold voltage restricts the operation of MOS based log domain circuits at low supply voltage which could be eliminated by floating gate transistors (FGMOS) at the cost of low gain, low output impedance and degraded bandwidth. These limitations can be further overcome by using quasi-floating gate MOSFET (QFGMOS).The log domain technique has been employed to enhance the dynamic range of analog circuits suitable of operation at low supply voltage. Methods: In this paper, we have designed QFGMOS based integrator and its performance has been compared with CMOS and FGMOS versions. The behavior of these circuits has been verified by using OrCAD PSpice with level 7 parameters for 0.13 μm technology obtained from TSMC with supply voltage of 1V. Results: It has been observed that QFGMOS based compressor and expander exhibits better performance as compared to their CMOS and FGMOS counterparts. Further QFGMOS based log domain integrator has been designed and its performance compared with that of CMOS and FGMOS based log domain integrators and found that QFGMOS based log domain integrator exhibits larger bandwidth (4.26 MHz) as compared to FGMOS (3.2 MHz) and it dissipates less power (88.7 μW) then FGMOS (0.114 mW) and CMOS (0.129 mW) based log domain integrators and could be useful for low voltage applications. Conclusion: In this paper functional blocks of log domain integrator like compressor and expander have been designed using CMOS, FGMOS, QFGMOS transistors and their performance studied through PSpice simulations. It has been observed that QFGMOS based compressor and expander exhibits better performance as compared to their CMOS and FGMOS counterparts. Further QFGMOS based log domain integrator has been designed and its performance compared with that of CMOS and FGMOS based log domain integrators.