Oxide TFT Frontend Amplifiers for Flexible Sensing Systems

This article presents a frontend amplifier based on the 50-
$\mu \text{m}$
channel length n-type only indium–tin-oxide (ITO)-stabilized ZnO thin-film transistors (TFTs) on glass substrate. The amplifier has an external-bias ac-coupled, open-loop capacitor bootstrap structure. The area and power consumption are 41 mm
²
and 0.1 mW (8-V supply), respectively. Electrical measurement results show a gain of 20 dB, a bandwidth of 3 Hz–6 kHz, a common-mode rejection ratio (CMRR) up to 42 dB, an input impedance of 100
$\text{M}\Omega $
, and an input-referred noise of
$106 ~\mu \text{V}_{rms}$
(integrated from 1 to 200 Hz). The noise efficiency factor (NEF) and power efficiency factor (PEF) of the amplifier are, respectively, 248 and
$4.9\times10$
⁵
, which is comparable among the state of the arts. The amplifier is successfully used to measure the heart-rate signal, which demonstrates its practicality.