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U. Glaeser

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FIGURE 11.5 Current-mode CMOS quaternary-to-binary decoder logic.<br />

two-quaternary-digit output, CS, that is the base-four value of this sum of the inputs. Logical currents<br />

have been used in QFA circuits realized with integrated injection logic (I 2 L) (see, for example, [22]), currentmode<br />

logic (CML), and current-mode CMOS [15]. The well-known QFA function is summarized below.<br />

The QFA circuit to be described implements threshold functions. The two basic operations of a<br />

threshold function are the formation of a weighted-sum-of-inputs and the comparison of that weightedinput-sum<br />

to the multiple thresholds. The QFA adds two quaternary inputs A and B and a binary input<br />

carry C i to produce a weighted-input-sum within the range 0–7. Representing this weighted sum in basefour<br />

with the two-digit output CS requires the CARRY, C, output to assume only binary values ZERO<br />

and ONE, while the SUM, S, output will assume values ZERO, ONE, TWO, and THREE. The DC inputoutput<br />

transfer function for the ideal QFA is shown in Fig. 11.6. Several organizations of threshold<br />

detectors can be used to generate this two-digit output from the eight-valued weighted-sum-of-inputs.<br />

We will summarize the operation of two approaches to realizing this function with combinations of<br />

complementary MOS transistors serving as current sources and threshold detectors, and standard CMOS<br />

logic gates.<br />

The first QFA discussed, shown in Fig. 11.7, is a direct implementation of the QFA definition. The<br />

input summing-node combines logical current inputs that are integer multiples of a reference current.<br />

© 2002 by CRC Press LLC

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