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CMOS Inverter as an Analog Circuit

A CMOS inverter is an integrated circuit, or IC, designed to manipulate digital data in computers and related equipment. Its transistors have complementary metal-oxide semiconductor technology for very low power consumption. Though it uses and produces digital on and off states, these are measurable voltages in the analog realm. Nothing prevents its use in interesting analog circuits, generating and processing signals for audio and other applications.

Inverter Action
- Computers process data as strings of binary digits -- 1s and 0s. Computer circuits work on voltages: A high voltage represents a 1, and a low voltage represents a 0. "High" and "Low" are relative terms, however; the circuit determines the exact voltages needed to signify a 1 or 0. An inverter takes a 1, or high-logic state, present at the inverter's input, and produces a 0, or low-logic state, at its output. In other words, it inverts or "flips" an individual binary digit. With a 0 at its input, it produces a 1 output. With a 10-volt power supply, the CMOS inverter recognizes 0 as any voltage from 0 to 2 volts, and 1 as any voltage higher than 8 volts. The power-supply voltage affects the input voltage sensitivity; lower supply voltages lead to lower thresholds for low- and high-logic voltages.
Power Supply Voltage Range
- Unlike a transistor-transistor logic IC, which works at 5 volts DC, CMOS ICs can run on a wide range of power supply voltages, typically between 3 and 15 volts. This flexibility makes CMOS easier to work with analog circuits, which have a similar working voltage range. Because the CMOS inverter's sensitivity varies with the supply voltage, you can, for example, use a variable resistor to fine-tune the inverter's characteristics in an analog circuit.
Pulse Generator
- The CD4069 hex inverter integrated circuit has six inverters in its 14-pin package. By connecting three of these inverters along with a few resistors and capacitors, you can build a pulse generator circuit able to work in either digital or analog applications. As an analog circuit, it produces square and rectangular pulses in a frequency range of a few hz to several hundred kHz. It serves as a low-cost audio tone source or noisemaker. An even simpler pulse generator, using a 74VHC14 hex Schmitt inverter, employs a single inverter, resistor and capacitor.
Comparator
- A CMOS hex inverter normally processes a digital, high-low voltage as an input, though it can just as easily use a smoothly changing analog voltage. The inverter converts a sine, triangle or other analog waveform into a pulse wave of equal frequency. It can also use a DC input, flipping into a high state when the voltage passes beneath a threshold value, and going low when the input voltage exceeds the threshold. In the analog world, this circuit is called a comparator, comparing the input to a voltage value and outputting a simple high or low state based on the result of the comparison.

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