A Differential Sallen-Key Low-Pass Filter in Amorphous-Silicon Technology

In this paper, the design and implementation of a second-order low-pass filter (LPF) utilizing an amorphous-silicon (a-Si) thin-film-transistor (TFT) process technology is presented. In order to boost the gain of the amplifiers, a positive-feedback technique is developed for the design of a differential operational amplifier (OPAMP) and a differential-difference amplifier (DDA). Based on the Sallen-Key filter structure, DDAs with the proposed gain-enhancement technique are employed for the implementation of the LPF such that the chip area and power consumption can be minimized. Using an 8-mu m a-Si technology, the DDA and the LPF are realized for demonstration. Consuming a dc power of 9.5 mW from a 25-V supply, the fabricated LPF exhibits a 3-dB bandwidth of 0.3 Hz. For an input frequency of 0.3 Hz, the measured THD and SFDR are -21.66 and 23.76 dB, respectively.