note08 - Introduction to Microelectronics Chapter 8 CMOS...

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Introduction to Microelectronics Chapter 8 CMOS D IGITAL C IRCUITS 8.1 The digital circuit family It is probably correct to say that the integrated digital circuits had played a flagship role in the technology revolution in the last 50 years: computers, controllers, telecommunication and Internet. Today a computer processor contains up to 1 billion transistors, and more than 98% of them are implemented as digital circuits. Although one can argue that the boundary between digital and analog circuits is diminishing with aggressively scaled V DD , the operating region for digital circuits is still meant to be different from analogy circuits. While analog circuits mainly use the high-gain region to create amplification and impedance match, digital circuits take mainly the low-gain (or gain saturation) region to stabilize the digitization with the high-gain region as the unstable part for signal regeneration. There are two basic families of digital circuits: logic and memory. They are also named as combinatorial and sequential circuits, respectively. The CMOS family is shown in Fig. 8.1. In this chapter, we will walk through the CMOS digital circuits one by one. The goal is not to train you to become a competent digital circuit designer, which needs a dedicated course and in- depth introduction of the top-down design approach from the hardware description language (HDL). Instead, we will emphasize the circuit principles behind each family in this bottom-up circuit introduction. Fig. 8.1. The digital circuit family with CMOS emphasis. Edwin C. Kan Page 8-1 10/25/2009 Logic Circuit Families CMOS Bipolar BiCMOS GaAs Full Complementary CMOS Pseudo NMOS Pass Trans. Logic Dynamic Logic TTL ECL Fully Compl. Memory Circuit Families Static Pseudo Static Dynamic Nonvolatile
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Introduction to Microelectronics 8.2 The CMOS inverter Following the discussions of inverters at the end of Chapter 4, the digital circuits often use simpler switch models: a digital transistor can be thought as an ideal switch in series with an I ON current source or a R ON resistor. It is further denoted that the NMOS passes strong “0”, but weak “1”, while the PMOS passes strong “1”, but weak “0”. Also, when the switch is off, the current is not zero, but a subthreshold leakage current exists. An important digital circuit benchmark is the ON-OFF current ratio : I ON /I OFF , which needs to be as large as possible so that the circuit can be fast due to large I ON filling the unavoidable capacitance on the circuit nodes, while the circuit can also be low-powered due to small I OFF leakage current. Fig. 8.2. The switch model for NMOS and PMOS. To implement digital logic, the signal is fed into the gates of MOSFET, which is mostly capacitive (so the load in digital circuits is often just C L ). The PMOS is often efficient only as a “pull-up network” (PUN) , while NMOS as a “pull-down” network (PDN) , as shown in Fig.
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note08 - Introduction to Microelectronics Chapter 8 CMOS...

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