Design of a railt-to-rail CMOS operational amplifier

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Dr. Bognár György
Department of Electron Devices

In this dissertation, I will present an operational amplifier with rail-to-rail type input stage. The basic design objects are low supply voltage (maximum 3V), relative low bias input current (~25µA on the „BIAS” pin) and approximately 10MHz border frequency. The main object is to design an operational amplifier with these electrical parameters, which can work as a unity gain buffer circuit on the specified frequency.

First of all, I will present the structural parts of an operational amplifier, such as current mirror, differential input stages, different types of output stages or the basic part, the MOS transistor itself. We will see some basic circuits and typical characteristics.

After that we will see the designed amplifiers schematic. Than next the description of the parts, characteristics, design why’s and how-to’s. In this chapter we must speak about the question of frequency stability and frequency compensation, more specific I will present the Bode-type frequency compensation method.

I take some simulations on the final circuit, such as the transfer characteristics temperature and technology parameter dependence, supply voltage and bias current dependence, design parameter (channel length and width) dependence. We will see the results all of these simulations and I will explain the reasons to.

In the final chapter, the circuit’s layout plan will be presented. Full layout with explanation, design difficulties and there solutions.


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