Earlier today I posted a voltage-versus-current curve for a 1N5817 Schottky diode, to confirm the theoretical formula , where IS is the saturation current of the diode:
I also said that I should characterize the base-emitter junction of a PNP and an NPN transistor this way also, for setting the appropriate resistances for the log amplifier in the loudness circuit. I did that this evening for the S9012 PNP and S9013 NPN transistors:
For both transistors, the region where the logarithmic fit is good is from about 0.5µA to about 50mA (maybe only 35mA for the NPN transistor). That gives about a 100dB working range for a log amplifier, if the largest current corresponds to 50mA. Of course, the op amps that are driving the input of the log amplifier don’t have that much drive capability, and we are probably limited to about 20mA—only a 90dB dynamic range on the log amplifier.