Analog filters are the main units in the structures of analog integrated circuits. Due to extensive applications of these filters and also due to their relatively large sizes, their suitable design will help to design the integrated circuits more effectively. Different approaches have been presented for the implementation of the filters. One method is the use of OTA-C structure which has the advantages of operational capability in high frequencies and also the controllability of the characteristics of the filters. The method is used vastly in the communication circuits. Two typical applications are wireless equipment and complex (multi-phase) filters. In present work, different structures and implementations of OTA-C filters are studied and the design of OTA circuit as the main unit of the OTA-C structure is presented. Also, the design and analyze of an OTA-C low pass filter with a continuously controllable cutoff frequency is presented. In the presented structure, to have an adjustable OTA circuit a current controller is added to a fixed OTA circuit. In this work, also, the gm/I D method is employed in designing of an OTA circuit. In this method the diagram of gm/I D vs. I D / (w/l) is used to obtain the sizes of transistors. The diagram includes all the working regions of the transistors and it is fixed for the used technology.
