Design of an nFET-Only Fully-Differential Op Amp with Output Stage Bootstrapping for Flexible IC Technology

Flexible IC technologies are useful in low-cost applications such as biomedical, wearable sensors and product tracking. This paper presents the design of a fully-differential high gain op amp based on the Pragmatic 600 nm FlexIC thin-film transistor (TFT) technology. This technology allows solely for the implementation of n-type FETs, resistors and capacitors. The proposed op amp implements novel gate-bootstrapping technique at the output stage pull-up and shunt-shunt feedback n-channel FETs resulting in an increased output voltage swing even when sourcing significant output current. The op amp is intended for applications processing periodic signals such as in fast cyclic voltammetry, thus ensuring the repeated charging of the bootstrapping capacitor. It has an open loop gain of 94 dB and a differential output swing of ±3.5 V when operated at a supply voltage of 5 V. Although here a fully-differential op amp is described, the design can be adapted for single-ended applications by removing one of the output stages and the common mode feedback correction circuitry.