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U. Glaeser

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FIGURE 11.6 Current-mode CMOS quaternary threshold logic full adder I/O transfer characteristic.<br />

FIGURE 11.7 Current-mode CMOS quaternary threshold logic full adder schematic.<br />

The sum-of-logical-inputs must lie between ZERO and SEVEN times the reference current. This sumof-currents,<br />

Iin, is received and mirrored by input transistor M1 to replicate the input current seven times<br />

by identical NMOS transistors M2–M8. These seven identical copies of the input current are the inputs<br />

to seven current comparators [15] that compare the input weighted sum to the seven thresholds. The<br />

other halves of these comparators are PMOS transistors M9–M15. The comparators generate seven binary<br />

voltage swings, A–G, that are capable of driving standard CMOS logic gates. Comparator output signal<br />

D controls the CMOS transmission gate that connects a unit value of logical current to the CARRY<br />

output line. The seven logical comparator output signals are combinationally reduced in groups of three<br />

variables with three standard CMOS logic gates to a set of control signals, X, Y, and Z [ X = (A + D)E,<br />

Y = (B + D)F, Z = (C + D)G],<br />

that connect three current sources of unit reference value to the SUM<br />

output line through three CMOS transmission gates. Logical currents of 10, 20, and 30 µA are used in the<br />

QFA presented in this paper, requiring threshold currents of 5, 15, 25, 35, 45, 55, and 65 µA. Gains of<br />

© 2002 by CRC Press LLC

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