Theory of MOS Transistor Inverter and Inverter Dynamic Characteristics

MOS Transistor Inverter:

The inverter is globally accepted as the most fundamental logic gate doing a Boolean operation on a single input variable. Figure below depicts the symbol, truth table and a common structure of a CMOS inverter. As shown, the simple structure comprises of a combination of a pMOS transistor at the top and an nMOS transistor at bottom.

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CMOS is as well sometimes termed to as complementary-symmetry metal oxide semiconductor. The term "complementary-symmetry" refer to the fact that the usual digital design style with CMOS employs complementary and symmetrical pairs of n-type and p-type metal oxide semiconductor field effect transistors (or MOSFETs) for logic functions. Two significant characteristics of CMOS devices are high noise immunity and low static power consumption. Significant power is merely drawn while the transistors in CMOS device are switching between on and off states. As a result, CMOS devices do not generate as much waste heat as other forms of logic, for illustration transistor-transistor logic (TTL) or NMOS logic that uses all n-channel devices devoid of p-channel devices.

Inverter Dynamic Characteristics:

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Figure above shows the dynamic characteristics of CMOS inverter. The following are a few formal definitions of temporal parameters of the digital circuits. All percentages are of steady state values.

Rise Time (tr): Time taken to mount from 10% to 90%.

Fall Time (tf): Time taken to drop from 90% to 10%.

Edge Rate (trf): (tr + tf )/2.

High-to-Low propagation delay (tpHL): Time taken to drop from VOH to 50%.

Low-to-High propagation delay (tpLH): Time taken to mount from 50% to VOL.

Propagation Delay (tp): (tpHL + tpLH)/2.

Contamination Delay (tcd): It is the minimum time from the input crossing 50% to the output crossing of 50%.

Power dissipation analysis of the CMOS inverter

As we know that, the CMOS inverter exhibits very low power dissipation whenever in proper operation. However, the power dissipation is virtually zero (0) if operating close to VOH and VOL. The following graph exhibits the drain to source current (efficiently the total current of the inverter) of NMOS as a function of input voltage. Note that the current in far left and right regions (low and high VIN correspondingly) encompass low current, and the peak current in middle is just .232mA (1.16mW power dissipation).

Conclusion:

The CMOS inverter is a significant circuit device that gives high buffer margins, quick transition time, and low power dissipation: all three of such are desired qualities in inverters for most of the circuit design. It is quite obvious why this inverter has become as admired as it is.

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