具有最少元件的電流式可調 OTA-C 萬用二階濾波電路於高頻操作時非理想效應之研究與改進

Abstract

There are several kinds of parasitic capacitances, including input and output parasitic capacitances of an OTA and the nodal parasitic capacitance at the internal node in an OTA-C (Operational Transconductance Amplifier and Capacitor) circuit. This leads to the difficulty to have the same places for both given capacitors and all the parasitic capacitances. When the differential-input OTA and floating capacitances are employed in the circuit structure, and the impossibility to obtain a high–frequency circuit with precise output responses by giving a proper capacitor value after absorbing the parasitic capacitance. Recently, the “Analytical Synthesis Method” has been proposed to realize the high-order OTA-C circuits which achieving the following three important criteria simultaneously for the design of OTA-C filters: (i) using single-ended-input OTAs (overcoming the feedthrough effect due to the use of differential-input OTAs), (ii) using grounded capacitors (absorbing the shunt parasitic capacitance), and (iii) using the least number of component counts (reducing the total parasitic effects). Note that all the parasitic capacitances have the same places as those of all the given capacitors in the realized circuits achieving the above three important criteria. An improvement approach is then proposed by the absorption of parasitic capacitances from the given capacitors to obtain a precise high-frequency circuit. The thesis is focused on the design of biquads using the tunable OTA, leading to very precise output responses such as lowpass, bandpass, band-reject, all-pass,and highpass filtering signals. When the simulation resonance frequency is higher than the theoretical value, this implies that the given transconductance is also higher than the exact value. The reduction of the given transconductance leads to approach the theoretical prediction. On the contrary, if the simulation resonance frequency is lower than the theoretical value, it means that the additional parasitic capacitance makes a total capacitance larger than the exact value. The absorption of parasitic capacitance from the given capacitor leads to close the ideal requirement. After several reductions of transconductances or absorptions of capacitances can enter the very precise range with the error lower than 1% for the simulation resonance frequency. Furthermore, as the operational frequency increases, the numbers of pole and zero of the frequency dependent transconductance of an OTA three-OTA circuit increases, too Based upon different simulation results, different frequency dependent transconductance are given to consist with the practical variation. Finally, The above proposed improvement was verified by TSMC035 H-spice simulation with supply voltages ±1.65V.