Logarithmic Amplifier

The circuit is a modification to the logarithmic amplifier that uses a diode in the feedback path of the op-amp. The diode is now replaced by a PNP BJT. This variant is for processing negative input voltage and the output is a positive voltage. Notes: The magnitude of Vin was determined so that it is about at the same scale as the expected maximum value of the magnitude of the output voltage. This was done because of an effort to find suitable scaling of the input and output voltages so that their simultaneous graphs can be conveniently viewed. Moreover, the graph should convey insight about the logarithmic relation between the output and input voltages. The value of Rin can be determined so that the current through the BJT will not exceed its maximum rating when the magnitude of the input voltage goes to its maximum. Since the BJT used is not a model of a real device, Rin's value of 10 Ω is just to match the value used in the circuit "Logarithmic Amplifier (Diode Feedback, Negative Input - Positive Output)". This makes the currents in the two circuits identical and might be used for comparative purpose.