# analog design projects- DESIGN OF A 120dB PSEUDO LOGARITHMIC AMPLIFIER IMPLEMENTED IN THE AMI 0.5μm PROCESS

This paper discusses several design methodologies for achieving logarithmic amplification and focuses on the design, simulation, and layout of a 120dB pseudo-logarithmic amplifier. The overall design consists of two majors parts—a cascade of limiting current amplifiers and a difference amplifier. The pseudo-logarithmic amplifier accepts a single-ended current input and produces a single-ended output voltage that is linear-in-dB with the input. The circuit runs on a 5V rail, accepts input currents in the range of 1nA to 1mA, and generates an output voltage in the range of roughly 0V to 4V There are several different methods through which logarithmic amplification can be achieved. Two such methods include feedback through a non-linear device, and approximation by summing the outputs of limiting amplifiers. These two topologies will be investigated in sections 2.1 and 2.2 respectively.

2.1: Non-linear Feedback

Non-linear feedback is perhaps the most common method used to achieve logarithmic

amplification as it is the simplest. This approach places a non-linear device, almost always a

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