CMOS:Logic Design of CMOS Networks.

Logic Design of CMOS Networks

The logic design of CMOS networks can be done in the same manner as that of nMOS logic networks, because the nMOS subcircuit in each CMOS logic gate, with the pMOS subcircuit regarded as a variable load, essentially performs the logic operation, as seen from Figure 39.2. The design procedures discussed for nMOS networks in Chapter 33, Section 33.3 can be used more effectively than in the case of nMOS

networks, because more than four MOSFETs can be in series inside each logic gate, unless we are concerned about the transfer-curve shift problem or high-speed operation. Also, an appropriate use of transmission gates, discussed in the next paragraph, often simplifies networks.

The transmission gate shown in Figure 39.4 is a counterpart of the pass transistor (i.e., transfer gate) of nMOS, and is often used in CMOS network design. It consists of a pair of p-channel and n-channel MOSFETs. The control voltage d is applied to the gate of the n-channel MOSFET, and its complement d is applied to the gate of the p-channel MOSFET. If d is a high voltage, both MOSFETs become

conductive, and the input is connected to the output. (Unlike a pass transistor with nMOS whose output voltage is somewhat lower than the input voltage, the output voltage of the transmission gate in CMOS is the same as the input voltage after the transition period.) If d is a low voltage, both MOSFETs become non-conductive, and the output is disconnected from the input, keeping the output voltage (which gradually becomes low because of current leakage) at its parasitic capacitance, as it was before the disconnection. Since the input and output are interchangeable, the transmission gate is bidirectional. A D-type flip-flop is shown in Figure 39.5, as an example of CMOS circuits designed with transmission gates.

A full adder in CMOS with transmission gates is shown in Chapter 41. Also, pass transistors realized in nMOS can been used mixed with CMOS logic gates to reduce area or power consumption, as will be discussed in Chapter 40.