Question 16.5: Objective: Determine the low output voltage of an NMOS NOR c...
Objective: Determine the low output voltage of an NMOS NOR circuit.
Consider the NOR logic circuit in Figure 16.12 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} = 4, and (W/L)_{L} = 1. Neglect the body effect
If, for example, A = logic 1 = 2.5 V and B = logic 0, then M_{D A} is biased in the nonsaturation region and M_{DB} is cut off. The output voltage is determined from Equation (16.27(b)), which is
\frac{K_{D}}{K_{L}} [2(v_{I} − V_{T N D})v_{O} − v^{2}_{O}] = (−V_{T N L})^{2}
or
\frac{4}{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.
If both inputs go high, then A = B = logic 1 = V_{DD} = 2.5 V and the output voltage can be found using Equation (16.29), which is
(−V_{T N L})^{2} = 2 \left(\frac{K_{D}}{K_{L}} \right) [2(V_{DD} − V_{T N D})v_{O} − v^{2}_{O} ]
or
[−(−0.6)]^{2} = 2 \left(\frac{4}{1} \right) [2(2.5 − 0.4)v_{O} − v^{2}_{O} ]The output voltage is found to be v_{O} = 10.7 mV.
Comment: An NMOS NOR gate must be designed to achieve a specified V_{O L} output voltage when only one input is high. This will give the largest logic 0 value.
When more than one input is high, the output voltage is smaller than the specified V_{O L} value, since the effective width-to-length ratio of the composite driver transistor increases.