Hi, I need to measure currents from 1nA to 1 mA, the current comes from collector of PNP transistor in base-comon configuration. I tried to use transimpedance amplifier to measure the current, but the problem is that after 1uA the output of amplifier shows saturation and it is not possible to measure the complete current range. I would like to know if you can recommend me an amplifier or one correct configuration to measuring with precision the current. Below I show the basic circuit used.
Diego,
I have received your schematic by email and have attached it here:
For all practical purposes you can consider the input impedance of your transimpedance amplifier or a LOG amplifier to be zero. Both types of amplifiers drive the summing junction of an op amp and the input voltage is held at zero volts by the feedback of the amplifier. The collector voltage of your transistor being tested will only vary from 0V only according to the offset voltage of the op amp (or the internal op amp in the case of the LOG amp). The very small variation of the offset voltage on the collector voltage will not affect your measurement.
Regards, Bruce.
