隨著科技的進步與人類對於健康管理的需求，工程科學與生物醫學的結合發展將帶給人們便利與許多新的契機。人體植入式晶片的應用，即是半導體科技與生物醫學的結合產物。關於植入式裝置的研究有相當多，包括了矽耳蝸、人工視網膜、以及帕金森氏症等等。這些裝置植入人體後，人們會想要知道這些部位的神經活動或是刺激後的神經所產生的訊號反應，並且加以記錄。同時，觀測並記錄神經細胞的活動電位對於閉迴路控制的深層腦刺激是相當中要的，例如像是在癲癇或是帕金森氏症的治療中。
植入式晶片的信號讀取最前端是陣列式電極，此種電極所能夠讀取到的信號是既微小(10μV-200μV)又低頻(0.1Hz-10kHz)的胞外信號。不幸的是，MOSFET製程在低頻帶天生就有很大的低頻雜訊，這會造成胞外信號讀取的SNR相當差。
當植入式裝置讀取神經訊號時，微電極與人體體液接觸面所產生的偏差電壓將可以很容易得使得放大器飽和。為了解決這個問題，在前端放大器加上了負回授電阻以及電容形成高通濾波以濾除值流偏差電壓。然而，此高通濾波的轉角頻率必須低於0.1Hz的神經訊號，所以通常需要很大的電容或電阻去實現，這會浪費很多面積。於是，在本研究論文採用MOS電阻取代了巨大面積的電阻。
在HPF後方的前端放大器採用了全差動OTA的型式。筆者認為，操作在適度反轉區的電晶體比弱反轉區要來得好。將前端放大器的輸入差動對操作於適度反轉區，同時將其他電晶體操作於高度反轉區並且設計成長通道元件；這可以使得輸入差動對主宰前端放大器的熱雜訊、閃爍雜訊以及偏差電壓，並且避開弱反轉區中閃爍雜訊以及偏差電壓上升的現象。此外，由於所使用的製程中PMOS的閃爍雜訊能量隨通道反轉程度上升的幅度相當可觀，因此在前端放大器中，使用了操作在深歐姆區的電晶體當作PMOS的源極鈍化以降低PMOS閃爍雜訊的貢獻。在前端放大器之後，則是以一個一階的gm-C濾波器過濾不要的高頻雜訊。
測量的結果顯示了，這個以TSMC 1P6M 0.18μm製程製作的放大器在0.1Hz-10 kHz的頻寬範圍內等效輸入雜訊為5.62μVrms，前端放大器的功率消耗為14.2μW，NEF為7.45，增益、CMRR、PSRR則分別為49.5dB、70dB以及57dB。

Much research is being done on implantable devices, such as cochlear implants, retinal prostheses, motor prostheses, etc. Acquiring information regarding the stimulated neurons and recording the neural activities in these neural prosthetic devices is essential. It is also crucial to monitor and process the neural action potential signals in real time for closed-loop controlled deep brain stimulation (for example, in epilepsy and Parkinson’s disease).
The front end of the implantable device is an array of stimulating/recording electrodes. These electrodes read extracellular neural signals (ENG), which are very small (10μV-200μV) and have a low frequency (0.1Hz-10kHz), requiring a low-noise amplifier (LNA) for signal amplification to acquire the neural signal. Unfortunately, MOSFET process has inherent 1/f noise that dominates at low frequencies, while ENG is in the same frequency band as 1/f noise, resulting in a very poor SNR.
When reading the neural signals in an implantable device, the DC offset of the tissue-electrode interface can easily saturate the amplifier. To solve this problem, feedback capacitors and resistors are connected to the preamp to form a high pass function to filter the DC offset. The corner frequency of this HPF has to be lower than 0.1Hz, which requires a very huge capacitor/resister to implement. A MOSFET resistor is used to provide an area-efficient means of creating a large resistance.
Behind the HPF, a fully differential OTA is used to implement the preamp. EKV model is used for analysis because the transistors are operated in different inversion levels. The input transistors of the preamp are operated in modereate inversion, while non-input transistors in higher inversion level and long channel, so that the input stage will dominate the thermal noise, 1/f noise and offset contribution. Transistors operated in deep trode region are used as source-degenerated resistors to lower down the trans-conductance of PMOS operated in strong inversion, while the PSD of 1/f noise get higher with inversion level of PMOS in the process we used. Behind the preamp, a gm-C first order filter removes the high-frequency noise carried in the unwanted band.
Measurement results shows the input-referred noise of the system is 5.62μVrms from 0.1Hz to 10 kHz, power consumption of preamp is 14.2μW, NEF is 7.45, the gain is 49.5dB, CMRR is 70dB and PSRR is 57dB. The amplifier was fabricated using a TSMC 0.18μm 1P6M CMOS process.

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