Operational Transconductance Amplifiers For Gigahertz Applications

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Zheng, You
operational transconductance amplifiers , gigahertz
A novel CMOS operational transconductance amplifier (OTA) is proposed and demonstrated in this thesis. Due to its feedforward-regulated cascode topology, it breaks the previous OTA frequency limit of several hundred MHz and operates at frequencies up to 10 GHz with a large transconductance. This is confirmed by an in-depth high-frequency analysis, simulations, and experimental demonstrations using purpose-built circuits. Experimental results also show that the proposed OTA has high linearity and low intermodulation distortion, which is of particular interest in microwave circuits. The OTA’s noise behavior and the effects of process variations, device mismatch, and power supply noise on the transconductance are also studied. To the best of our knowledge, the noise analysis here is the first of its kind on regulated cascode circuits, which can be applied to other regulated cascodes with minor changes. Three microwave applications of this OTA are explored in this thesis: 1) an active bandpass filter with a wide tuning range, 2) a 2.4-GHz ISM-band variable phase shifter, and 3) a microwave active quasi-circulator, which are all in CMOS MMIC form. These three circuits can be easily integrated with other chip components for System-on-Chip (SoC) realizations. The use of the OTA makes these three applications super compact: the active filter is at least 5 times smaller than previous circuits with a similar topology, and the phase shifter and quasi-circulator are at least 3 times smaller than previous works in that frequency range. Furthermore, the tunability of the developed OTA on its transconductance gives its applications extra freedom in tuning their frequencies and gains/losses electronically. In the first application, the active bandpass filter has a novel narrowband-filtering topology and has a wide tuning-range of 28% around 1.8 GHz, which makes it very suited for reconfigurable multi-band wireless systems. In the second and third applications, the active variable phase shifter has a comparable variable phase shift range of 120º in the 2.4-GHz ISM band and the active quasi-circulator has transmissions close to 0 dB and directivities over 24 dB from 1.5 GHz to 2.7 GHz.
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