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Tuesday, May 10, 2011

IC Design Philosophy


Integrated-circuit fabrication technology poses constraints on-and provides opportunities to-the circuit designer. Thus, while chip-area considerations dictate that large- and even moderate-value resistors are to be avoided, constant-current sources are readily available Very small capacitors, in the Pico farad and fraction of a Pico farad range, however, are easy to fabricate in IC MOS technology and can be combined with MOS amplifiers and MOS switches to realize a wide range of signal processing functions. As a general rule, in designing IC MOS circuits, one should strive to realize as many of the functions required as possible using MOS transistors only and, when needed, small MOS capacitors. MOS transistors can be sized; that is, their W and L values can be selected, to fit a wide range of design requirements.

At this juncture, it is useful to mention that to pack a larger number of devices on the same IC chip, the trend has been to reduce the device dimensions. At the time of this writing (2006), CMOS process technologies capable of producing devices with a 0.06-┬Ám minimum channel length are in use. Such small devices need to operate with dc voltage supplies close to 1 V. While low-voltage operation can help to reduce power dissipation, it poses a host of challenges to the circuit designer. For instance, such MOS transistors must be operated with overdrive voltages of only 0.2 V or so. In our discussion of MOS amplifiers, we will make frequent comments on such issues.
The MOS-amplifier circuits that we shall study will be designed almost entirely using MOSFETs of both polarities-that is, NMOS and PMOS-as are readily available in CMOS technology. As mentioned earlier, CMOS is currently the most widely used IC technology for analog and digital as well as combined analog and digital (or mixed-signal) applications. Nevertheless, bipolar integrated circuits still offer many exciting opportunities to the analog design engineer. This is especially the case for general-propose circuit packages, such as high-quality op amps that are intended for assembly on printed-circuit (pc) boards (as opposed to being part of a system-on-chip). As well, bipolar circuits can provide much higher output currents and are favored for certain applications, such as in the automotive industry, for their high reliability under severe environmental conditions. Finally, bipolar circuits can be combined with CMOS in innovative and exciting ways.

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