An Amorphous-Silicon Operational Amplifier and Its Application to a 4-Bit Digital-to-Analog Converter

A circuit topology suitable for the implementation of operational amplifiers (OPAMPs) in an amorphous-silicon (a-Si) thin-film-transistor (TFT) technology is presented in this paper. Due to the use of a positive feedback in the input differential pair, the gain of the OPAMP is effectively boosted, facilitating analog circuit designs for system-on-panel (SoP) applications. Based on the proposed technique, a 4-bit digital-to-analog converter (DAC) prototype, which consists of a resistor string and a unity-gain buffer, is developed. The DAC is implemented by using an 8-¿m a-Si NTFT process. Operated at a supply voltage of 25 V, the fabricated circuit consumes a dc power of 3.68 mW. For an output voltage range from 9.5 to 12.5 V, the measurement results indicate a differential non-linearity (DNL) of ±0.216 LSB and an integral non-linearity (INL) of ±0.667 LSB. With a settling time less than 1.1 ms, the DAC circuit can operate at a maximum conversion rate of 0.9 kS/s.