Design of a Preamplifier for Capacitive Sensors With Wide Low-Frequency Range and Low Drift Noise

This paper describes a novel DC-coupled single input preamplifier which is robust to drift noise and variation due to the circuit operation conditions. The preamplifier is intended for capacitive sensors such as the capacitive electret film type sensor, which is one of the most sensitive and accurate sensors presently available. There are both DC-coupled and AC-coupled conventional preamplifiers available for such sensors. However, DC-coupled preamplifiers do not always suppress drift noise and variation, while AC-coupled preamplifiers require a large capacitor to expand the frequency range, thus making the large-scale integration (LSI) very large and inducing problems associated with the large capacitor. We describe a DC-coupled preamplifier with wider low-frequency range as well as less drift noise and variation. The design combines a time constant amplifier and rail-to-rail transconductance amplifier, and is implemented in an LSI with 6-GHz f_T bipolar 11-V process with 5-V supply voltage. For an LSI with this preamplifier, the output voltage variation due to changes in load current is within ± 35 mV, whereas the variation for a conventional preamplifier is proportional to the load current up to 2.5 V. The output voltage drift due to changes in temperature and load current is restricted to within ± 35 mV for load resistance of 1-5 kΩ , whereas the drift for a conventional preamplifier is 800 mV.