A high accurate logarithmic amplifier system with wide input range and extreme low temperature coefficient

A high accurate current amplifier with very precise logarithmic transfer characteristic and extreme high temperature stability is presented. The application of such an amplifier lies on the field of optoelectronic signal processing. With a photo diode connected to the input of the amplifier it is possible to measure any light levels very precisely. The presented circuitry consists of a low leakage current input stage with fixed amplification, a high performance low noise logarithmic amplifier and a low voltage drop current output stage. High temperature stable reference current is generated on chip by a bandgap reference. Minimum input signal level goes down to some 100pA, maximum input signal level up to some 10μA. The high precision logarithmic transfer characteristic exceeds four decades, the precision is within 1% over the complete range. The output current is in the order of some 100μA, its temperature variation is lower than 3μA over the wide temperature range from -40°C to 100°C. Current consumption of the sensor circuitry is as low as 670μA which makes it applicable for low power systems. The circuitry is realized in a standard 0.6μm BiCMOS technology on a total chip area of only 1.46mm².