Exponential conversion circuit

Abstract

A first voltage conversion circuit converts first and second reference input voltages into first and second differential output voltage. A second voltage conversion circuit converts the first reference input voltage and a control input voltage into a third differential output voltage. The third differential output voltage is inputted to an exponential conversion element. The first and second differential output voltages are inputted to an active impedance bridge. The active impedance bridge outputs a gain control voltage of the first and second voltage conversion circuits. A balanced condition of the active impedance bridge determines the exponential conversion characteristic of the output current to the control input voltage of the exponential conversion element.

Description

CROSS REFERENCE TO RELATED APPLICATION This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No.2003-336192, filed Sep. 26, 2003, the entire contents of which are incorporated herein by reference. FIELD OF THE INVENTION This invention relates to an exponential conversion circuit to output a current which varies exponentially according to input voltage. DESCRIPTION OF THE BACKGROUND In recent years, as demand of mobile communications represented by a portable telephone is expanded, a communication system having high efficiency in use of frequency is needed. A Code Division Multiple Access (CDMA) system is one of the communication systems which meet the demand. In such a CDMA system, fine control of the transmitting electric power in a mobile station is required. Thus, it is necessary that a variable gain amplifier which is used in an IF (intermediate frequency) section operates to control a wide range gain of 70 dB or more. I...

AI Technical Summary

Benefits of technology

One aspect of the present invention is to provide an exponential conversion circuit that comprises a first voltage conversion circuit converting first and second reference input voltages to first and second differential output voltages on the basis of a gain control signal, a second voltage conversion circuit converting the first reference input voltage and a first control input voltage to a third differential output voltage on the basis of the gain control signal, an exponential conversion element producing an output current which varies exponentially according to the third differential output voltage, and an impedance bridge including a first, a second, a third and a fourth transconductance elements

Problems solved by technology

Also, it is required that a portable te

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