CMOS

CMOS families are extremely important and widely used, especially in large scale integrated circuits (e.g. microprocessors). CMOS is low power (significantly lower than TTL), and while older CMOS families were slow, newer ones have a speed comparable to TTL. Noise margin is high, up to 40% of V_DD. Due to high MOSFET input resistance, fan-out is high but loads affect switching speed (due to channel capacitance).

Figure 5 shows a CMOS inverter, and Figure 6 shows the CMOS inverter characteristics.

**Figure 5:** CMOS inverter gate.
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**Figure 6:** CMOS inverter gate characterics.
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Both transistors are enhancement types. The top transistor is p channel (PMOS), while the bottom transistor is n channel (NMOS).

When the input A is L, Q₁ is turned on, while Q₂ is off, and so Y is H. When A is H, Q₁ is off, and Q₂ is on, so Y is L.

In both operating states (a-b and e-f) one of the transistors is always off, and this keeps current flows and hence power consumption low. In the region c-d, both transistors are saturated.

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