Non-ideal Effect and Improvement of The Current-Mode First-Order High-frequency OTA-C Filter Structure
| Autor: | Yi-Zhen Liao, 廖宜楨 |
|---|---|
| Rok vydání: | 2005 |
| Druh dokumentu: | 學位論文 ; thesis |
| Popis: | 93 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 capacirances from the given capacitors to obtain a precise high-frequency circuit. The non-ideal effect in an OTA is resulted from the parasitic capacitances spreaded among the MOS transistors which are called the frequency dependent transconduce, namely, the ratio between the output current phasor and the input voltage phasor, of an OTA. The non-ideal effect out of the OTA includes the input and output parasitic capacitance and the output parasitic conductance of an OTA and the nodal parasitic capacitance at each internal node. In this thesis, a current-mode first-order OTA-C universal filter structure is used for example to demonstrate this new improvement for a high-frequency circuit. When 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 absorptions of capacitances can enter the very precise range with the error lower than 1% for the simulation resonance frequency. In addition, the number of pole and zero in OTA increase by increasing frequency. So that equation of the non-ideal transconductance needs to correct in the simulation of the realistic saturation. Finally, the above propos improvement was verified by UMC05 H-spice simulation with supply voltages ±2.5V. |
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