REMODELING LIGHT-EMITTING DIODE IN LOW CURRENT REGION

A general expression is proposed for current-voltage (I-V) characteris tics of junction diode by which it is possible to determine forward vo ltage across diode at actual current and temperature. Expressions for the band gap and temperature coefficient of junction voltage based on the experimental measurements are also presented. The expressions incl ude only experimentally obtained and derived values. These expressions have been used to check the performance of logarithmic amplifier usin g light emitting diode (LED) as a feedback element, at different tempe ratures. Further, it has been shown that it is possible to predict the behavior of a logarithmic amplifier without temperature compensation techniques within 5% accuracy for the current range 10(-12)-10(-5) A a nd for the temperature range -5 degrees-60 degrees C. The alternative could be the use of temperature compensation circuitry which may make electrometer circuit more complex. The proposed scheme is useful in ap plications where it is difficult to incorporate temperature compensati on technique and space and power are at premium.