應用於無線與硬碟讀取系統之高速線性轉導運算放大器

Abstract

在現今的通訊系統中，類比濾波器已經被廣泛的使用在高頻的應用。而與主種動式RC濾波器、切換式電容濾波器等其他種類的類比濾波器做比較，轉導電容式濾波器更適合被運用在高頻濾波器。而轉導式運算放大器則是轉導電容式濾波器中最重要的基本元件。但是轉導式運算放大器往往受限於它較差的線性度。此外隨著CMOS製程技術的進步，類比電路往往被要求工作在更低的工作電壓與效耗更少的功率。而製程上的誤差與溫度的改變也會影響到類比電路的效能。因此如何在新製程下，設計出一個高線性度的轉導式運算放大器去了提升轉導電容式濾波器的效能就是一個值得研究的課題。

本論文實現兩種不同提升線性度的架構。第一種是利用雙差動輸入對，來消除第三諧波失真，以增進轉導式運算放大器的線性度。並利用一個源極退化電流鏡來調整轉導值，這個電路能使在調整轉導值的過程中，轉導式運算放大器的線性度較不受影響。而此轉導式運算放大器的工作電壓為1.8V並消耗功率1.21mW。當輸入信號為頻率15MHz且振幅為0.4Vpp時，可達到第三次諧波失真為-68.94dB。另外在本論文中，也介紹並實現了一個轉導電容式四階低通濾波器。而第二種在是利用操作在飽和區且固定Vds之偽差動輸入對來取代傳統操作在三極區之架構，此種改良架構能更有利於高頻的應用。而為了更進一步提升線性度，在此架構加入移動補償的電路。而此轉導式運算放大器的工作電壓為1.5V並消耗功率2.34mW。當輸入信號為頻率90MHz且振幅為0.4Vpp時，可達到第三次諧波失真為-59.2dB。而這些電路是以TSMC 0.18um CMOS製程所實現的。

In present communication systems, analog filters have been widely adopted for high frequency applications. Compared with the other types of analog filters such as active-RC or switched-capacitor filters, Gm-C filters are more suitable for high-frequency applications. The operational transconductance amplifier (OTA) is the most important building block in the Gm-C filters. Linearity is a critical concern in the design of the OTA. In addition, with the progress of the CMOS technology, the analog circuit is required to operate under low supply voltage and low power. The process tolerances and temperature variation will also degrade the entire circuit performance. Therefore, how to increase the linearity for high performance Gm-C filters is a main topic in the design of the OTA under new CMOS process. In this thesis, two linearity improved structures have been proposed. The first one improves linearity by using double differential pair to cancel the third harmonic distortion. In order to avoid reducing the linearity while tuning Gm values, source-degeneration current mirrors are utilized for Gm tuning. This OTA works under a 1.8V supply voltage with 1.21mW power consumption. The measurement results show the HD3 of -68.94dB with 0.4-Vpp 15MHz input signal. Moreover, a 4-order Gm-C lowpass filter based on the first OTA was also designed in this thesis. The second one uses a linearization technique where MOFETs operates with constant Vds in the saturation region, instead of operating in the triode region. Such an improved structure will better facilitate high-frequency application. In order to further engance linearity, a mobility compensation circuit is added to this structure. This OTA works under a 1.5V supply voltage with 2.34mW power consumption. The measurement results show the HD3 of -59.2dB with 0.4-Vpp 90MHz input signal. These circuits were all fabricated by TSMC 0.18um CMOS technology.