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U. Glaeser

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FIGURE 2.12 Multifunction circuitry using pass transistor logic and the function table.<br />

FIGURE 2.13 The transmission gate: (a) schematic, (b) logic symbol, and (c) simplified logic symbol.<br />

FIGURE 2.14 Transmission gate implementation of<br />

XOR gate.<br />

pass transistors [3,7]. In this circuit the output is a function of the inputs A and B and the functional<br />

inputs P1, P2, P3, and P4. Depending on the values of P1, P2, P3, and P4, the F output is either the<br />

NOR, XOR, NAND, AND, or OR of inputs A and B. This is summarized in the table in Fig. 2.12.<br />

The simple pass transistor only passes one logic level well, but if we put NMOS and PMOS in parallel<br />

we get a simple circuit that passes both logic levels well. This simple circuit is called the transmission gate<br />

(TG). The schematic and logic symbol of the transmission gate are shown in Fig. 2.13(a,b). Figure 2.13(c)<br />

shows the simplified logic symbol. The CMOS transmission gate operates as a bi-directional switch between<br />

nodes A and B, which is controlled by S. The transmission gate requires two control signals. The control<br />

signal S is applied to the NMOS and the complement of the control signal S′ to the PMOS. If the control<br />

signal S is high, both transistors are turned ON providing a low resistance path between A and B. If the<br />

control signal S is low, both transistors will be OFF and the path between the nodes A and B will be an<br />

open circuit.<br />

The transmission gate can be used to realize logic gates and functions. Consider the exclusive-OR<br />

(XOR) gate shown in Fig. 2.14 [5]. When both inputs A and B are low, the top TG is ON (and the bottom<br />

TG is OFF) and its output is connected to A, which is low (logic 0). If both the inputs are high, the bottom<br />

TG is ON (and the top TG is OFF), and its output is connected to A′, which is also a low (logic 0). If A<br />

is high and B is low, the top TG is on and the output is connected to A, which is a high (logic 1).<br />

Similarly, if A is low and B is high, the bottom TG is on and the output gets connected to A′, which is<br />

a high (logic 1), which is the expected result of a XOR gate. In Fig. 2.14, if we change B to B′ and B′ to<br />

B, the circuit will realize the exclusive-NOR (XNOR) function. In the next section we will use transmission<br />

gates to realize latches and flip-flops.<br />

© 2002 by CRC Press LLC

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