CMOS logarithmic current generator and method for generating a logarithmic current

摘要

A CMOS logarithmic current generator includes current mode circuitry having a design principle based on a Taylor's series expansion that approximates an exponential function. A MOSFET circuit provides a function generator core cell having a biasing current Ib. The FETs of the circuit are matched and are biased in the weak inversion region. Additional transistors are used to convert a pair of input currents to a pair of voltages to provide an output current based upon a current mode logarithmic function. The biasing current Ib can be varied to provide a variable gain in the circuit.

主权项

1. A complimentary metal-oxide semiconductor (CMOS) logarithmic current generator circuit, comprising:
a first metal-oxide semiconductor field-effect transistor (MOSFET) and a second MOSFET matched with each other and configured in the (CMOS) logarithmic current generator circuit as a first MOSFET pair biased in a weak inversion region to provide a first function generator core cell; a third MOSFET and a fourth MOSFET matched with each other and configured in the (CMOS) logarithmic current generator circuit as a second MOSFET pair biased in the weak inversion region to provide a second function generator core cell; a fifth MOSFET connected to a source voltage and the first function generator core cell, the fifth MOSFET contributing to an output current Iout of the CMOS logarithmic current generator circuit; a sixth MOSFET connected to the source voltage and the second function generator core cell, the sixth MOSFET contributing to the output current Iout of the CMOS logarithmic current generator circuit; a seventh MOSFET in operable communication with the first function generator core cell, the seventh MOSFET providing an input current IX to the CMOS logarithmic current generator circuit to produce a corresponding first voltage VA; and an eighth MOSFET in operable communication with the second function generator core cell, the eighth MOSFET providing an input current IY to the CMOS logarithmic current generator circuit to produce a corresponding second voltage VB, wherein the first function generator core cell and the second function generator core cell are each biased by a biasing current Ib that varies based upon the first voltage VA and the second voltage VB applied to the first function generator core cell and the second function generator core cell, the first voltage VA being determined by the input current IX and the second voltage VB being determined by the input current IY, wherein the weak inversion region is defined by a current, I2, whereI2=Ib·exp⁡[(VA-VB)nUT],  where n is a weak inversion slope factor and UT=KbT/q, where Kb is Boltzmann's constant, q is electron charge, and T represents temperature, and wherein the CMOS logarithmic current generator circuit provides the output current Iout based upon a current mode logarithmic function defined by the relation:Iout=2⁢⁢Ib·ln(IYIX).