King Fahd University of Petroleum and Minerals

Karama M Al-Tamimi

Munir A Al-Absi

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 I. 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 Ican be varied to provide a variable gain in the circuit.

CMOS logarithmic current generator and method for generating a logarithmic current

The high dynamic range exponential current generator produces an output waveform (current/voltage) which is an exponential function of the input waveform (current/voltage). The exponential characteristics are obtained in BiCMOS or Bipolar technologies using the intrinsic characteristics (I/V) of the bipolar transistors. The high dynamic range exponential current generator is biased in weak inversion region. MOSFETs biased in weak inversion region are used not to utilize the inherent exponential (I/V) relationship but to simply implement xand xterms using translinear loops. The term xis realized by two cascaded squaring units. The approximation equation used is 

High dynamic range exponential current generator with MOSFETs

The current-mode CMOS logarithmic function circuit provides an ultra-low power circuit that produces an output current proportional to the logarithm of the input current. An OTA (operational transconductance amplifier) constructed from CMOS transistors, in combination with two PMOS transistors configured in weak inversion mode for providing a reference voltage input and a voltage input from the input current to the OTA, provides the circuit with a high dynamic range, controllable amplitude, high accuracy, and insensitivity to temperature variation.

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Dhahran, Saudi Arabia

Karama M Al-Tamimi