Logarithmic current measurement circuit with improved accuracy and temperature stability and associated method

The circuit for logarithmically measuring an input electric current I.sub.SIGNAL comprising:

• first circuit means for logarithmically converting input electric current I.sub.SIGNAL to provide a first output signal V.sub.OUT1 wherein the first circuit means includes a first pair of transistors, a first operational amplifier, a divider including first and second resistors and a first current regulator and wherein the first output signal V.sub.OUT1 corresponds to the equation:
• V.sub.OUT1 =N.sub.1 (1+R.sub.2 /R.sub.1)(k/q)T.sub.1 ln(10)log(I.sub.REF /I.sub.SIGNAL)
• wherein N.sub.1 is the emission coefficient of the first two transistors, R.sub.1 is the first resistor, R.sub.2 is the second resistor, k is Boltzman's constant, q is the electronic unit charge, T.sub.1 is the absolute temperature of the first transistor pair in degrees Kelvin, and I.sub.REF is the reference current transmitted through the current regulator;
• second circuit means for providing a temperature-dependant second output signal V.sub.OUT2 wherein the second circuit means includes a second pair of transistors which is well-matched to the first transistor pair and which is biased at a fixed collector current ratio, a second operational amplifier, a second divider including third and fourth resistors which cooperate with the second transistor pair to provide a second output signal V.sub.OUT2 in accordance with the equation:
• V.sub.OUT2 =N.sub.2 (1+R.sub.4 /R.sub.3)(k/q)T.sub.2 ln(10)log(K.sub.i)
• wherein N.sub.2 is the emission coefficient of the second two transistors, R.sub.3 is the third resistor, R.sub.4 is the fourth resistor, k is Boltzman's constant, q is the electronic unit charge, T.sub.2 is the absolute temperature of the second transistor pair in the degrees Kelvin, and K is the ratio of the fixed collector currents of the second transistor pair;
• and
• means connected to the first and second circuit means for ratioing the first and second output signals V.sub.OUT1 and V.sub.OUT2 to provide a resultant output signal V.sub.OUT which corresponds to the equation:
• V.sub.OUT =V.sub.OUT1 /V.sub.OUT2 =[(1+R.sub.2 /R.sub.1)/(1+R.sub.4 /R.sub.3)]log(I.sub.REF /I.sub.SIGNAL)/log(K.sub.i)
• so that the resultant output signal V.sub.OUT is temperature independent.

A method for logarithmically operating upon an input electric signal I.sub.SIGNAL for measurement purposes comprising the steps of:

• providing a quad array of dielectrically isolated, well-matched bipolar transistors and utilizing a first two of the transistors in the quad array to convert the input signal I.sub.SIGNAL logarithmically to provide a first output signal V.sub.OUT1 which is dependent upon the temperature of the first two transistors in the array;
• biasing the second two of the transistors of the quad array at a fixed collector current ratio and utilizing the second two transistors to provide a second output signal V.sub.OUT2 which is dependent upon the temperature of the second two transistors in the array;
• and
• ratioing the first and second output signals V.sub.OUT1 and V.sub.OUT2 to provide a resultant output signal V.sub.OUT which is temperature independent.