Operational Amplifier Circuits with Oxide Thin Film Transistors

Abstract

Amorphous silicon thin film transistors (TFTs) have been used widely due to good uniformity and low process cost. However, They are not suitable for a circuit due to the low carrier mobility. Amorphous Indium-Gallium-Zinc-Oxide Thin Film Transistor (a-IGZO TFT) exhibits higher mobility and lower leakage current than amorphous silicon TFT and it is common to use only n-type a-IGZO TFT due to a poor electrical property of p-type TFT. In this paper, we developed operational amplifier (op-amp) circuit using only the n-type a-IGZO TFTs. Recently, medical device circuits for measuring bio-signals are being implemented on a wearable devices. Oxide TFTs are suitable for the integrated circuit for such applications. The analog signal processing such as amplification, and filtering of much noise are necessary for analog electrical signals introduced from bio-sensors. The op-amp is most widely used amplifiers due to ideal linear performance characteristics. An op-amp is composed of two input voltages (+Vin, -Vin), two power supply voltages (+VDD, -VDD) and one output voltage (Vout). It amplifies the voltage difference between the two input voltages of the non-inverting and inverting inputs (+Vin, -Vin). The op-amp with a-IGZO TFTs has lower gain than that of the op-amp by CMOS process and need increase of the gain by cascaded amplifier stages. We developed a single power supply (+VDD = +15V) OP amp with improved gain. The circuit is composed of three stages: the first is differential amplifier(T1~T8) of input stage, the second is common-source amplifier(T9~T17), and the third is output buffer(T18~T19) of the output stage. The differential amplifier of the first stage amplified the differential voltage for given input signals (+Vin, -Vin). The output voltage was amplified at the common-source amplifier stage of the second stage. Common-source amplifiers were used for a large voltage gain. After optimization of the circuits, the input voltage of 0.05V was amplified to about 13V.