Interview Questions.

Top 100+ Cmos Interview Questions And Answers

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Top 100+ Cmos Interview Questions And Answers

Question 1. What Is Latch Up?

Answer :

Latch-up pertains to a failure mechanism in which a parasitic thyristor (including a parasitic silicon managed rectifier, or SCR) is inadvertently created inside a circuit, causing a high amount of modern to constantly drift through it once it's miles by chance prompted or grew to become on. Depending at the circuits worried, the amount of present day drift produced through this mechanism may be massive enough to result in permanent destruction of the device due to electrical overstress (EOS).

Question 2. Why Is Nand Gate Preferred Over Nor Gate For Fabrication?

Answer :

NAND is a better gate for layout than NOR due to the fact on the transistor degree the mobility of electrons is generally 3 instances that of holes as compared to NOR and accordingly the NAND is a quicker gate.

Additionally, the gate-leakage in NAND systems is an awful lot decrease. If you don't forget t_phl and t_plh delays you will find that it's miles extra symmetric in case of NAND ( the delay profile), but for NOR, one put off is a good deal higher than the other(obviously t_plh is better because the higher resistance p mos's are in series connection which again increases the resistance).

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Question 3. What Is Noise Margin? Explain The Procedure To Determine Noise Margin?

Answer :

The minimum quantity of noise that may be allowed at the enter stage for which the output will now not be effected.

Question 4. Explain Sizing Of The Inverter?

Answer :

In order to drive the desired load capacitance we ought to growth the scale (width) of the inverters to get an optimized performance.

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Question five. What Happens To Delay If You Increase Load Capacitance?

Answer :

put off increases.

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Question 6. What Happens To Delay If We Include A Resistance At The Output Of A Cmos Circuit?

Answer :

Increases. (RC postpone)

Question 7. What Are The Limitations In Increasing The Power Supply To Reduce Delay?

Answer :

The put off may be decreased by increasing the power deliver however if we achieve this the heating impact comes due to excessive power, to compensate this we have to increase the die size which isn't practical.

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Question 8. For Cmos Logic, Give The Various Techniques You Know To Minimize Power Consumption?

Answer :

Power dissipation=CV2f ,from this limit the burden capacitance, dc voltage and the running frequency.

Question nine. What Is Charge Sharing? Explain The Charge Sharing Problem While Sampling Data From A Bus?

Answer :

In the serially connected NMOS good judgment the input capacitance of every gate shares the rate with the burden capacitance with the aid of which the logical levels appreciably mismatched than that of the desired once. To take away this load capacitance need to be very excessive in comparison to the input capacitance of the gates (approximately 10 times).

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Question 10. Why Do We Gradually Increase The Size Of Inverters In Buffer Design? Why Not Give The Output Of A Circuit To One Large Inverter?

Answer :

Because it cannot force the output load at once, so we gradually increase the size to get an optimized performance.

Question eleven. What Is Latch Up? Explain Latch Up With Cross Section Of A Cmos Inverter. How Do You Avoid Latch Up?

Answer :

Latch-up is a condition in which the parasitic components give upward push to the Establishment of low resistance undertaking course between VDD and VSS with Disastrous consequences.

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Question 12. Give The Expression For Cmos Switching Power Dissipation?

Answer :

CV2

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Question thirteen. What Is Body Effect?

Answer :

In widespread more than one MOS gadgets are made on a commonplace substrate. As a result, the substrate voltage of all gadgets is normally same. However at the same time as connecting the gadgets serially this could result in an growth in supply-to-substrate voltage as we proceed vertically alongside the series chain (Vsb1=0, Vsb2 zero).Which results Vth2>Vth1.

Question 14. Why Is The Substrate In Nmos Connected To Ground And In Pmos To Vdd?

Answer :

we strive to opposite bias now not the channel and the substrate but we try to preserve the drain,supply junctions reverse biased with admire to the substrate so that we dont unfastened our current into the substrate.

Question 15. What Is The Fundamental Difference Between A Mosfet And Bjt?

Answer :

In MOSFET, modern waft is either because of electrons(n-channel MOS) or because of holes(p-channel MOS) - In BJT, we see cutting-edge because of both the providers.. Electrons and holes. BJT is a contemporary controlled device and MOSFET is a voltage managed device.

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Question 16. Which Transistor Has Higher Gain. Bjt Or Mos And Why?

Answer :

BJT has higher advantage because it has higher transconductance.This is because the modern-day in BJT is exponentially dependent on input in which as in MOSFET it's far square law.

Question 17. In Cmos Technology, In Digital Design, Why Do We Design The Size Of Pmos To Be Higher Than The Nmos.What Determines The Size Of Pmos Wrt Nmos. Though This Is A Simple Question Try To List All The Reasons Possible?

Answer :

In PMOS the carriers are holes whose mobility is less[ aprrox half ] than the electrons, the vendors in NMOS. That manner PMOS is slower than an NMOS. In CMOS generation, nmos helps in pulling down the output to ground ann PMOS helps in pulling up the output to Vdd. If the sizes of PMOS and NMOS are the identical, then PMOS takes long term to price up the output node. If we have a bigger PMOS than there could be extra providers to charge the node fast and triumph over the sluggish nature of PMOS . Basically we do all this to get identical upward thrust and fall instances for the output node.

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Question 18. Why Pmos And Nmos Are Sized Equally In A Transmission Gates?

Answer :

In Transmission Gate, PMOS and NMOS useful resource each other alternatively competing with each different. That's the cause why we need no longer length them like in CMOS. In CMOS layout we've NMOS and PMOS competing that is the reason we strive to size them proportional to their mobility.

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Question 19. All Of Us Know How An Inverter Works. What Happens When The Pmos And Nmos Are Interchanged With One Another In An Inverter?

Answer :

I actually have seen similar Qs in a number of the discussions. If the supply & drain also linked well...It acts as a buffer. But think enter is common sense 1 O/P will be degraded 1 Similarly degraded 0;

Question 20. Give 5 Important Design Techniques You Would Follow When Doing A Layout For Digital Circuits?

Answer :

In virtual design, decide the height of widespread cells you need to layout.It relies upon upon how massive your transistors can be.Have reasonable width for VDD and GND steel paths.Maintaining uniform Height for all of the mobile is very critical considering this can assist you operate vicinity direction tool easily and additionally incase you want to do manual connection of all of the blocks it saves on lot of vicinity.
Use one metal in a single course simplest, This does now not apply for metal 1. Say you are the use of metal 2 to do horizontal connections, then use metal three for vertical connections, metal4 for horizontal, metallic 5 vertical and so forth...
Place as many substrate contact as possible inside the empty spaces of the format.
Do not use poly over lengthy distances as it has big resistances except you have no different choice.
Use fingered transistors as and whilst you feel essential.
Try maintaining symmetry on your layout. Try to get the layout in BIT Sliced way.
Question 21. What Is Metastability? When/why It Will Occur?One-of-a-kind Ways To Avoid This?

Answer :

Metastable nation: A un-recognized nation in among the two logical acknowledged states.This will happen if the O/P cap isn't allowed to charge/discharge completely to the required logical ranges.

One of the instances is: If there may be a setup time violation, metastability will occur,To avoid this, a chain of FFs is used (generally 2 or three) that allows you to get rid of the intermediate states.

Question 22. Let A And B Be Two Inputs Of The Nand Gate. Say Signal A Arrives At The Nand Gate Later Than Signal B. To Optimize Delay Of The Two Series Nmos Inputs A And B Which One Would You Place Near To The Output?

Answer :

The overdue coming signals are to be placed closer to the output node ie A ought to go to the nmos this is toward the output.




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