Why logic gates work only on DC?
Why do logic gates only work on DC? Why can't we use AC in those components? I want to know the reasons behind it. Please explain in detail.
Answer: Why Logic Gates Only Work on DC and Not AC
TLDR; logic gates are electronic switches and work only on DC because they need distinct voltage along with “on” and “off” behavior, which is possible only with DC and not AC.
Let me explain this a bit in more detail.
Logic gates, the fundamental building blocks of digital systems, work on the principle of binary logic -- that is, they interpret inputs and produce outputs based on two distinct voltage levels that correspond to "0" (often representing low or ground voltage) and "1" (often representing a higher voltage, such as 5V, 3.3V, or 1.8V, depending on the technology).
Let's delve into why Direct Current (DC) is more suitable for these logic gates rather than Alternating Current (AC).
Distinct voltage levels: In a DC system, voltage levels are constant over time. These steady-state levels can be used to represent binary "0" and "1" values clearly. On the other hand, AC voltage continuously varies with time in a sinusoidal (or similar) manner, going from positive to negative. The changing nature of AC voltage can make it challenging to consistently and clearly represent binary values.
Switching behavior: Logic gates are essentially electronic switches. The transistors within these gates switch between "on" and "off" states to represent different logic levels. With DC, this switching behavior can be precisely controlled. However, due to the alternating nature of AC, the transistors' behavior would be unpredictable and inappropriate for the deterministic functioning of logic gates.
Power dissipation: In digital logic circuits, power dissipation happens mostly during the switching of the logic gates. With DC, the switching can be controlled to be only when necessary (i.e., when the logic state needs to change). However, with AC, the continuous changing of voltage levels would cause constant switching, leading to excessive power dissipation.
Noise and Signal Integrity: Digital circuits are designed with a certain noise margin to ensure reliable operation. The constant voltage levels of DC make it easier to maintain signal integrity and manage noise. AC signals, with their continuous voltage changes, could introduce more noise and make maintaining signal integrity challenging.
Technological Constraints: The design and functioning of logic gates are based on semiconductors like silicon. The behavior of these materials, and hence the devices made from them, is highly dependent on the applied voltage and current characteristics. The devices have been designed and optimized for DC operation due to its steady and predictable nature.
Can AC be used for Logic Gates?
While standard logic gates are designed to operate using DC because of the reasons described earlier, it is technically possible to design systems that can interpret AC signals.
But these wouldn't really be what we typically think of as "logic gates."
Consider, for example, a system where you define two different frequency bands of an AC signal as logic '0' and '1'.
It's conceivable that you could create an analog circuit that operates on this principle, where different frequency bands correspond to different logical states. However, this quickly becomes more complex and less efficient than using DC, especially given the demands of digital systems in terms of speed, miniaturization, and power efficiency.
For these reasons and more, AC is not typically used to represent binary states in digital logic systems. In addition, the nature of AC signals – which include a constantly changing amplitude and direction (positive and negative) – makes them unsuitable for logic gates, which need a clear, unambiguous distinction between the binary '0' and '1' states.
Therefore, while one can theoretically design systems that use AC signals for logic operations, these would generally be more complex and less efficient than equivalent DC systems. Hence, we don't typically see AC being used for logic gates in the real world.
Still, it's a fascinating area to explore from a theoretical perspective and could potentially have applications in certain niche areas. For example, some types of quantum computing may involve concepts somewhat similar to this, albeit at a very different scale and with different types of technology.
Replies
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Abhishek Rawalgates are micro components and AC can damage or even burn/melt the CMOS transistors of logic gates. -
Jeffrey Arulrajwhat is DC it has zero frequency that is it
making a IC work in such conditions is much simpler than making it work in AC pulses
in general IC work only in DIGITAL PULSES and not in DC to make my point clear
the variations in DIGITAL pulses make them work not DC supply
we just power the IC with DC SUPPLY if we do it with AC the ic turns on and off n number of times within a second -
Nishant Patelits not like we can't use gates in AC.... because they are made up of CMOS,FET or BJT etc. they can function in AC as well.
but our all application is related to digital now-a-days,
so gates(uni. nand,xor;and,or,not) are made for specially for digital circuits..where decision has to be made b/w only true or false(according t diff. logic)..both have dc value(or range)..that's it.
hope it is justified. -
Guru KarthikIn digital ICs there are just two levels of operation. A '1' and a '0' which is set by the presence and absence of a DC signal(supply and ground).
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rohit_jun22you could apply AC as well but DC is required to bias or to "turn on" the internal devices such as CMOS, FET's, transistors etc.
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kiransabarishThe gates are mostly ICs that can work at a maximum of 5V, not more than that. They usually see their input as HIGH or LOW signals. Mostly the voltage between 4.5-5V is taken as High, and the voltage between 0-1V will be taken as Low. So, if you use an AC (Even if its range is between -5V to 5V), then it has several states in between these two peaks. The IC cannot understand its input so, it can't work.
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Jeffrey Arulrajit is better said the frequency of signal and the time for state change is lower and so the response of ICS never are accurate in AC supply
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Ramani AswathIf you look at it purely from the logic point of view, a negative going potential is the opposite of the statement and so goes against the logic.
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Jeffrey ArulrajDo you refer the Negative half cycle of any AC signal here sir
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Ramani Aswath
That is right.jeffrey samuelDo you refer the Negative half cycle of any AC signal here sir
However, as you and Patel said above, it is the truth table that is relevant. So '0 V' (actually low voltage) is one state and high (usually above 4 V) is the other state. The negative half cycle does not convey any logical meaning.
Analog audio circuits only use AC signals because the full wave form is needed for this. -
aruncharylogic gate are developed using different technologies like ECL,TTL,CMOS etc..
accordingly the voltage levels for on and off switching will varies.all are works only on DC not on AC.very less amount of currents and voltages are required because of TRANSISTOR configuration.so basically transistors(not power transistors) are opereated in DC votages. -
Jeffrey ArulrajNo transistor is trigerred by a AC signal we use a DC or square pulse to trigger a transistor
Transistors amplify the AC signals
Kindly don't confuse your self between them -
aruncharybut i already mentioned not signal transistor ,though your right my pal...
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aruncharyone more thing bjt is used to give phase reversal output so when we are giving positive (logic 1) means negative peak ( logic 0) will come its in TTL like that each family is varis with component configuration...
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Jeffrey ArulrajNo that is not the case
In a CE amplifiers the input Analog signal is given as a op with an 180 degree phase shift
And this phase reversal does not have any role in the reversal of the Supply voltage given at the collector terminal (Vcc)
And so there is a slight error in your statement -
aruncharyok.
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