Question 16.6: Objective: Determine the low output voltage of an NMOS NAND ...
Objective: Determine the low output voltage of an NMOS NAND circuit. Consider the NAND logic circuit shown in Figure 16.13 biased at V_{DD} = 2.5 V. Assume transistor parameters of k´_{n} = 100 µA/V² , V_{T N D} = 0.4 V, V_{T N L} = −0.6 V, (W/L)_{D} = 8, and (W/L)_{L} = 1. Neglect the body effect.
If either A or B is a logic 0, then v_{O} = logic 1 = 2.5 V.
If A = B = logic 1 = 2.5 V, then both driver transistors are driven in the nonsaturation region and the output goes low. As a good approximation, we will assume the effective length of the driver transistor doubles. Then, using Equation (16.27(b)), we have
\frac{K_{D}}{K_{L}} [2(v_{I} − V_{T N D})v_{O} − v^{2}_{O} ] = (−V_{T N L})^{2} (16.27(b))
\frac{\frac{1}{2} \cdot \left(\frac{W}{L} \right)_{D}}{\left(\frac{W}{L} \right)_{L}} [2(v_{I} − V_{T N D})v_{O} − v^{2}_{O} ] = (−V_{T N L})^{2}
or
\frac{8}{(2)(1)} [2(2.5 − 0.4)v_{O} − v^{2}_{O} ] = [−(−0.6)]^{2}
The output voltage is found to be v_{O} = 21.5 mV.
This output voltage is the same value that would be obtained for a simple inverter with (W/L)_{D} = 4, and (W/L)_{L} = 1.
Comment: If an N-input NMOS NAND logic gate were to be fabricated then the width-to-length ratio of the drivers would need to be N times that of a single driver in an NMOS inverter to achieve a given value of V_{O L} . The increase in the required area of the driver transistors in a NAND logic gate means that logic gates with more than three or four inputs are not attractive.