1. Field of the Invention
The present invention relates to function generators, and particularly to an arbitrary power law function generator using semiconductors operating in a current-mode, and wherein the power law functions generated are current-controlled.
2. Description of the Related Art
Power law function generators are very attractive circuits in analog signal processing. Such circuits have many applications as basic blocks in communication electronic circuits, measurement systems and modeling of the non-linear current-voltage characteristics of many devices. Power-law circuits implemented in voltage mode techniques are usually built around operational amplifiers and diodes, analog multipliers, operational transconductance amplifiers (OTAs), the current differencing transconductance amplifier (CDTA), bipolar transistors, or MOSFETs working in the weak inversion region where the exponential relationship between the drain current and the gate-to-source voltage is exploited to advantage. Among these techniques, the OTA-based circuits are preferred due to their programmability and modularity. However, such realizations either depend on approximations of the power law function, or are temperature dependent and silicon intensive, as it requires a large number of OTAs or else one can realize only one power law function, e.g., the cube-law. Power-law circuits implemented in the transconductance mode, that is, input voltage and output current, have also been reported using a bipolar junction transistor (BJT). In both circuits, the power-law function is a function of the thermal voltage, and hence is temperature sensitive.
Due to the many benefits it has, current-mode implementation of power law circuits have also been reported. These circuits are true power-law realizations with temperature independent characteristics. The problem with existing current-mirror power law implementations is the staking nature of the BJTs used as diodes to get the required power law, restricting such circuits to operation with relatively high voltage power supplies. Moreover, the power factor can be adjusted either by controlling the gain of an operational amplifier-based voltage amplifier, the ratio of a resistors-based potentiometer, or the number of p-n junction diodes. Current-mode power law function generator circuits based on a transconductor, a square-root function generator, a cube-root function generator and a weighting transimpedance amplifier can provide power factors between ½ and ⅓ only. Other existing circuits can realize a function of the form io=iy(ix/iz)m and use a logarithmic function generation, an exponential function generator, and a voltage amplifier and can realize power factor values over a continuous range. However, the power factor m is controlled by adjusting the gain of a voltage amplifier.
Yet other circuits are built around two logarithmic circuits and a single exponential circuit, and can realize the function io=iy(ix/iz)m. However, the power factor m is controlled by connecting an external resistance to control the gain of an operational-amplifier-based voltage amplifier or by connecting an external voltage divider. Such circuits do not enjoy the attractive property of current-controlled power factor, and therefore cannot be described as current-controlled current-mode power-law function generators.
Thus, an arbitrary power law function generator solving the aforementioned problems is desired.