doi:10.1016/S0026-2692(98)00084-6 
Copyright © 1998 Elsevier Science Ltd All rights reserved.
On the design of low-frequency filters using CMOS OTAs operating in the subthreshold region
Gursel Duzenlia, 
, 
, Yavuz Kilicb, Hakan Kuntmanc and Atilla Atamanb
aSakarya University, Department of Electrical and Electronics Engineering, Engineering Faculty, 54040 Esentepe, Adapazari;, Turkey
bYildiz Technical University, Department of Electronics and Communication Engineering, Faculty of Electrical and Electronics Engineering, 80626 Maslak, Istanbul, Turkey
cIstanbul Technical University, Department of Electronics and Communication Engineering, Faculty of Electrical and Electronics Engineering, 80626 Maslak, Istanbul, Turkey
Available online 11 January 1999.
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Abstract
Design considerations on a circuit technique based on subthreshold operation of MOS transistors is described for the realization of low-frequency OTA-C active filters with small capacitance values of the order of 25–400 pF. The circuit technique described is applied to the α (8–12 Hz), β (13–40 Hz), θ (4–8 Hz) and δ (1–4 Hz) band filters for EEG signals. Because of small capacitance values the filter circuit is suitable for realization on a single VLSI chip using the CMOS technology, and enables the user to implement the circuit on implantable biotelemetric applications.
Keywords: CMOS OTA; Low power; Subthreshold region; Filter
Fig. 1. Realized CMOS symmetrical cascode OTA.
Fig. 2. Basic OTA-C filters.
fig. 3. (a) Enlarged top view of the three OTAs IC chip. (b) Enlarged top view of the four band-pass filters IC chip.
fig. 4. Dependence of OTA output current on input differential voltage for several biasing current values in the subthreshold region, obtained by measurements with parameter analyzer HP 4145A.
fig. 5. Measured variation of OTA transconductance Gm with control voltage Vcon and biasing current IB.
Fig. 6. Measured frequency response of OTA transconductance in the subthreshold region.
Fig. 7. Realized band-pass filter structure.
Fig. 8. Measured frequency response of band-pass OTA-C filter operating in the subthreshold region.
Fig. 9. Experimental output waveforms: (a) for Vin=Vin max=80 mV; (b) for Vin=90 mV>Vin max.
Fig. 10. Simulated frequency responses of realized EEG band filters.
Table 1.
Transfer functions and the relationships between component values and filter transfer functions of the basic OTA-C filters
Table 2.
Dimensions of MOS transistors
Table 3.
Capacitance values for the α, β, θ and δ bands of the EEG signal
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