1 Improvement of Operational Amplifier DC-gain Using Positive Feedback Technique in 180-nm CMOS Process Reza Ghoushchian Moghaddam, Seyed Mahmoud Anisheh* Hadaf Institute of Higher Education, Sari, Iran *Corresponding Author’s e-mail: s.m.anisheh@gmail.com Abstract Operational amplifiers (Op-amps) are one of the main elements of integrated systems. Operational amplifiers with high gain and high speed can be used in various applications such as analog-to-digital converters with high precision, digital-to-analog converters, band gap voltage references and sample-and-hold circuits. The purpose of this research is to present the structure of an operational trans-conductance amplifier (OTA) with high DC- gain. For this purpose, the positive feedback technique is used to increase the output resistance and thus the DC-gain. By using the proposed structure, other parameters of the operational amplifier such as unity gain bandwidth and stability are not damaged. To assess the performance of the proposed structures, some simulations are performed in a 0.18 μm CMOS process with the supply voltage of 1.8 V. The simulation results show the better performance of the proposed operational amplifier compared to the existing structures. For this purpose, merit coefficients are also used for comparison. Keywords: Operational amplifier, positive feedback, output resistance, DC-gain. 1. Introduction Operational amplifiers (op-amps) are the basic element of many analog and mixed (analog- digital) circuits. Op-amps with varying degrees of complexity are widely used to accomplish various tasks, from DC bias generation to high-speed amplification or filtering. An op-amp is defined as a differential amplifier with high gain [1-4]. One of the most important aims in the op-amp designs is to increase the DC-gain. The gain increase is done with the help of various techniques that have their own characteristics. It is clear that it is not possible to improve all the parameters in an op-amp and there is always a compromise between its parameters [5-8]. In order to have both high gain and high swing, a two-stage op-amp is used, which provides the first stage with high gain and the second stage with high swing. Each stage can contain different structures, but the second stage is usually a common source stage with an active load to provide maximum swing in the output [9-12]. The concept of applying positive feedback has been reviewed in various articles. The general feature of the introduced structures is based on creating a negative resistance in the output node. Here, a large resistance is created at the output node, which leads to an increase in the DC-gain [13-16]. In [17], positive feedback is created by connecting the outputs to the bulk terminal. Here, voltage divider resistors are used to control the amount of voltage given to the bulk terminal