Design of OTA-C oscillators and current-mode filters using network transposition

OTA-C filters have received a lot of attention. for high frequency continuous-time applications. The design of OTA-C filters has traditionally been considered as voltage mode signal processing. Over the last decade, current mode signal processing has emerged as an important class of analogue circuitry because of the simplicity associated with implementing basic operations such as signal summing and replication, and the potential to operate at higher signal bandwidths and lower voltage than in the voltage mode approach. A number of researchers have investigated as to how to design current mode filters. For instance, Sun and Fidler have proposed current-mode two-integratorloop dual-output OTA-C filters and current-mode multiple-loop feedback filters. In principle, various current-mode filter structures can be generated by these systematic methods. However, complicated voltage/current calculation is needed for loop analysis or approaches requiring solution of nonlinear equations are required to solve coefficient matching equations associated with the feedback matrix. However, the principle of network transposition, proposed by Bhattacharyya and Swamy as early as 1971, can be used to advantage to derive, in a straightforward manner, current-mode OTA-C filter structures from well-known OTA-C voltage-made filter structures. Application of network transposition is extended to oscillators, and several new oscillator circuits derived from existing oscillator circuits are given. Furthermore, it is shown that the application of network transposition to differential-input, differential-output networks leads to the realization of fully differential current mode filters and oscillators. As illustrations, several new current-mode fully differential OTA-C filters and oscillators are derived using network transposition. Experimental results on several voltage-mode and current-mode filters and oscillators, built using commercial OTA (LMI 13700) devices, capacitors and resistors will be presented.