VLSI Design
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CMOS LOGIC
A CMOS transistor (or device) has four terminals: gate , source , drain , and a fourth terminal that we shall ignore until the next section. A CMOS transistor is a switch. The switch must be conducting or on to allow current to flow between the source and drain terminals (using open and closed for switches is confusing—for the same reason we say a tap is on and not that it is closed ). The transistor source and drain terminals are equivalent as far as digital signals are concerned—we do not worry about labeling an electrical switch with two terminals.
• V AB is the potential difference, or voltage, between nodes A and B in a circuit; V AB is positive if node A is more positive than node B.
• Italics denote variables; constants are set in roman (upright) type. Uppercase letters denote DC, large-signal, or steady-state voltages.
• For TTL the positive power supply is called VCC (V CC or V CC ). The 'C' denotes that the supply is connected indirectly to the collectors of the npn bipolar transistors (a bipolar transistor has a collector, base, and emitter—corresponding roughly to the drain, gate, and source of an MOS transistor).
• Following the example of TTL we used VDD (V DD or V DD ) to denote the positive supply in an NMOS chip where the devices are all n -channel transistors and the drains of these devices are connected indirectly to the positive supply...