215anotes_1 - EE215A Fall 08 B. Razavi HO #2 Why Analog? -

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EE215A B. Razavi Fall 08 HO #2 1 Why Analog? - Naturally-occurring signals, e.g., voice and video, are analog. - System and medium non-idealities often make it necessary to treat digital signals as analog: o Data Retrieved from Disk o Digital Wireless Communications Why is analog design such a big deal? - Entails more trade-offs than digital design: Digital Analog speed power speed power - More sensitive to noise and cross-talk. - More sensitive to second-order effects in devices. - More difficult to automate. - More difficult to model and simulate. - We want to design analog circuits in mainstream VLSI technologies, e.g., CMOS, with no additional processing steps, trimming, factory calibration, etc.
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EE215A B. Razavi Fall 08 HO #2 2 REVIEW OF MOS DEVICES MOS Structure (NMOS) - A piece of polysilicon with a width of W and length of L on top of a thin layer of oxide defines the gate area. - Source and drain areas are heavily doped. - Substrate usually tied to the most negative voltage. - Le ff = L – 2L D , where L D is the side diffusion of source and drain. MOS Symbols (Enhancement Type) PMOS NMOS - MOS structure is symmetric. - MOS devices have a very high input impedance.
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EE215A B. Razavi Fall 08 HO #2 3 MOS characteristics o How does the device turn on and off? o What is the drain-source current when the device is on? Threshold Voltage For V GS < V TH , holes in substrate are repelled from gate area, leaving negative ions behind. (No current flows because no carriers are
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This note was uploaded on 06/18/2010 for the course CE 01 taught by Professor Bazgei during the Spring '09 term at University of California, Santa Cruz.

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215anotes_1 - EE215A Fall 08 B. Razavi HO #2 Why Analog? -

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