EveryCircuit - First time using FETs

rich11292000

8 years ago

They have a body diode, and only blocks current in one direction. So its important to have it correctly polarized for the intended application. http://everycircuit.com/circuit/4744755962707968

Issacsutt

8 years ago

Ok, so does current also have to travel from the source or drain through the gate or visa-versa sorta like BJT's, or no?

Issacsutt

8 years ago

Like this for Example?: http://everycircuit.com/circuit/4691915785371648

2ctiby

8 years ago

Too many questions all at one go.... I suggest that you forget all about bjt comparisons. Stick to enhancement N Mosfets (like these on EC) and then set up a simple circuit to light a LED here on EC. Then ask one or two specific queries at a time. Conventional current description, + to - does travel from drain to source there. Current does not flow through the gate basically. Instead the gate acts capacitively whereby it creates a channel between drain and source which allows the drain to source current to flow.

Issacsutt

8 years ago

Ok thanks

hurz

8 years ago

To your latest example http://everycircuit.com/circuit/5319351183081472

Addramyrz

8 years ago

http://everycircuit.com/circuit/5623026577309696

Issacsutt

8 years ago

The diodes were only to indicate direction of current flow

Issacsutt

8 years ago

I can do experimenting on my own, I was just wanting some insight on maybe some of the characteristics, like what's the width and other parameters (kinda like bjts have an Amplification factor such as Beta) and etc , how does it affect its operation, and do you need to bias FET's with a resistor like BJT's, and etc? Just advice on good practice of how to use an FET?

Issacsutt

8 years ago

I am only trying to learn everything I can, and not trying to get on anyone's nerves, honestly. I know, Hurz, you have a lot of respect for that, and only use EC nowadays to educate young possible future engineers such as Jason; and not to impress others. Maybe I'm just asking too many questions and not the right ones

2ctiby

8 years ago

@Issacsutt ... You asked about the width ... If I tell you that it forms part of the Beta so that the uCox can be ascertained once we know the Kn and the Overdrive value ..would that help? If not, then perhaps you might want to form a simple circuit as I suggested earlier, then we can move forward a step at a time so that this mentioned statement will mean something to you.

Issacsutt

8 years ago

Yes, but what is kn and overdrive and uCox?

Issacsutt

8 years ago

I have already done a good bit of trial and error circuits as you've suggested, but it seems like the MOSFET can't supply enough current to the LED without lowering the LEDs current draw or raising the gate voltage to like 20v; so that makes me think the settings have to be adjusted from the default ones, but I have no idea what any of them are, or what typical values might be in the real world, so I have no sense of how much of a difference there is between 200nm and 20nm, just that it lowers the output impedance enough to light up the LED or blow it out

2ctiby

8 years ago

Ok ... One step at a time ... Let's do Overdrive first ... http://everycircuit.com/circuit/5196827040415744

Issacsutt

8 years ago

Awesome explanation, ready to proceed to next chapter! Little review first; Vth is the threshold voltage, Vgs is like an adjustable voltage drop across gate-source, and Vov is the difference between the two (Vov being especially important)

2ctiby

8 years ago

Yes ... Now I don't know your level of Maths, but you may need to grit your teeth with a little determination for the next step... Ask at every stumbling block, and we can get you through it. It is worth the effort. Mosfet Beta here ... http://everycircuit.com/circuit/5564415574540288

Issacsutt

8 years ago

Ok, so as soon as you cross Vth, the MOSFET starts to turn on, but not instantly, it takes a lot of voltage to turn it hard on it seems, so in other words; you have to have enough overdrive for the MOSFET to have near a 0v drop across it and supply max current for the load. My best guess is: how much overdrive you need in order to turn the transistor hard on, depends upon the beta of the transistor, and that basically is determined by the relationship of KP to the ratio of its length and width...?

2ctiby

8 years ago

There is a little gap in your understanding of the two Nmos states. Let's step aside and fill that gap. We can the come back to where we were here later. That gap needs you to think about the Vds for a while in relation to the overdrive ...take a look here, then review the Beta article again with that new perspective... http://everycircuit.com/circuit/4722063570108416

Issacsutt

8 years ago

Do you think you could post an example of a simple Amplifier, and logic circuit so I can examine it?

2ctiby

8 years ago

You have done remarkably well so far, and hopefully picked up on that little side-step which was aimed at bringing your attention to the close relationship between Vov and Vds. We must not get side tracked in to Saturation at this juncture. Here is a follow up from the previous Beta article... stay with it now ... things will start falling in to place ...come back here when you are happy with the example here ... http://everycircuit.com/circuit/5930625961033728

Issacsutt

8 years ago

Ok sounds good, thank you for all your time and patience! I'll review all it once more and let you know where I'm at

Issacsutt

8 years ago

B=9/Kn=3.5

Issacsutt

8 years ago

Why does beta need halfed, what is the significant difference between MOSFET Beta and BJT Beta? Where the Beta of a BJT is an amplification factor that multiplies the current when in the active region, but for MOSFET's?

2ctiby

8 years ago

Answer is here ... http://everycircuit.com/circuit/5269292802375680

2ctiby

8 years ago

Now Let's move on to a well known Nmos Ohmic formula ... After this, we will start to use the mosfet ... http://everycircuit.com/circuit/6479932816621568

Issacsutt

8 years ago

In this post, :http://everycircuit.com/circuit/5564415574540288 you said: "You might then also see it as Kn = [μCox(W/L)]/2 Or ....... Kn = β/2" ....so that's not right?

2ctiby

8 years ago

It is right ... Kn = μCox(W/L)]/2 is the same as saying Kn = β/2 ... please re-read the article where you think there is a problem and specify again if unclear ... it may be that we are both on line at the same time at this moment, before EC updates alterations.

Issacsutt

8 years ago

Yeah, ok

Issacsutt

8 years ago

This Kn, and KP thing is really weird

2ctiby

8 years ago

Yes, it is even more confusing if we were to be dealing with Pmos, where Kp is not the same as KP ... but that is another story .... anyway, you are doing extremely well...it all becomes much easier when we get to Rds soon.

Issacsutt

8 years ago

This helps a lot: "Nmos is in its Ohmic state when Vds < (Vgs- Vth)". From this, I can easily infer that the load voltage {or really the voltage at the drain node (without arrow I think), or the voltage across drain-source when source is tied to ground} has an effect on what state it's in... and in addition to that, the difference between its applied gate voltage and threshold voltage (called overdrive voltage) will dictate when the MOSFET enters that state, as long as its voltage stays less than the voltage across its drain and source terminals.

Issacsutt

8 years ago

Using this formula: Kn = Ids/[2(Vgs-Vth)Vds], Kn should equal 4.5

Issacsutt

8 years ago

Sorry its taken so long to reply, I'm 18 and I have 4 soon to be 6 siblings... ie. a big family of 7 and soon 8, so I'm always very busy, but it varies quite a lot, and 2 of my siblings are 2 and 4, so they are screaming at the highest pitches 24/7 😁

2ctiby

8 years ago

No rush for replies, Country differences for bedtime etc, time to consolidate, and other life activities mean that we can take a casual approach here. You have progressed at a remarkable rate and you now understand that the Ohmic state is needed for switching, and that Vds < (Vgs- Vth) is the essential snippet of Mosfet knowledge that we all need to use when using any Mosfet for switching. The next part may surprise you ... http://everycircuit.com/circuit/6382784414482432

Issacsutt

8 years ago

Thanks 2ctiby!

2ctiby

8 years ago

You're welcome. That last article may have been a bit heavy, but now having come across the Kn and other parameters, it may be best to just shelve them for future reference. Instead, it is probably best now to concentrate on the task of obtaining Rds and get the Nmos easily set to replicate a SPST switch. Here is a final article to help you with that ... You can always ask questions as and when you need to. http://everycircuit.com/circuit/6687994856865792

Issacsutt

8 years ago

When I'm able to, I'll post a MOSFET circuit in the community, and you can let me know what you think if you'd like

zorgrian

8 years ago

@2cityBee, I thought you were going to refrain from pontiff posture and professor lectures

2ctiby

8 years ago

@MentioneditoncethinkIgotawaywithit ... does this mean that you are not interested in learning about normal gate switching being a lower commencing overdrive than the initial high Vds which thus takes time to be reduced in order to make the changeover from Saturation in to the necessary Ohmic needed for switching, so a ploy instead of waiting to switch the Vgs until the Vds has already dropped by other means, is employed for type E requirements with a view to incorporating the output capacitance into the resonance with the inductor and so may persuade one to consider altering the EC setup with that in mind?

zorgrian

8 years ago

And in other news professor @2CityBeeHeadmaster admitted he knew nothing about electronics! So where does the headmaster get his amazing information from?

zorgrian

8 years ago

Sources required

zorgrian

8 years ago

No?

zorgrian

8 years ago

Three points off and go to the back of the class E

zorgrian

8 years ago

@2CityBeeHeadmaster, please tell us truthfully how your illustrious worshipful entity came to bee a city bee? Freemasonry? Divine intervention? Serendipitous serpentine sausages?

2ctiby

8 years ago

I know nooothing... qué

maxmax_66

8 years ago

@zorgrian. I'm amused with your petty, schoolyard approach to a 'discussion', particularly in the context of your paranoid rantings. I was inclined to ask if this is your standard approach to discussions with your peers and colleagues, but then again this is normal peer behaviour for your typical toddler, is it not?

zorgrian

8 years ago

@MaxWarp666, I, on the other hand, feel disposed toward commending you upon your colourful language, if one can apply such terms to a mixed minded half cooked badly made puppet? It does seem odd both Semantically, logically and in most senses to apply the idea of peers and colleagues to toddlers. However, as you may be challenged in some aspects of life, conceptual incongruent mishaps are probably inevitable.

maxmax_66

8 years ago

Odd perhaps if I were convinced you were a petulant toddler. Not so if you only behave as one.

Issacsutt

8 years ago

So is all the stuff 2ctiby wrote on MOSFET's accurate or inaccurate?

jason9

8 years ago

@issacsutt, I’d say it’s probably accurate because there’s a lot of math and funny things like “Kn” and stuff that probably isn’t so easily faked. Only issue is, I have more of an intuitive understanding of MOSFETs from simulation experience rather than a mathematical understanding from looking things up so I can’t tell you whether or not any of the equations he gave are true.

zorgrian

8 years ago

I'd say its mostly true as in true from one book. There are gaps in anybody's knowledge and authors of books are no exception. This way of explaining MOSFET function is one way of looking at the subject. It may or may not suit you and its certainly not comprehensive! When it comes to class E amplifier design, for example, it lacks any direct relevance or practice. Yet, this person chooses to continue on and on as a fountain of knowledge having admitted in another thread that he/ she/ it knows very little of practical electronic development.

zorgrian

8 years ago

The whole posture of dictating like a Ponce's puppet; the way the this person chooses to conduct comment pretending to be a professional educational director is absurd, if indeed the background of this person is only an interested amateur!

2ctiby

8 years ago

@Issacsutt... You might want to look at that final help link of mine again... then ask yourself if it is useful for you in EC and on a board. There is no point in asking someone who has no idea how or why a Mosfet needs its overdrive to be considered. Here is that link ... http://everycircuit.com/circuit/6687994856865792

zorgrian

8 years ago

@2cityBees, Nobody is saying that your offerings are claptrap, balderdash, nonsense, weevil infested rot or nefarious dung! No! Just that ones way of putting things is perhaps overtly pontificate, school masterly and quite essentially over ambitious. Necessary, it is a valid way of looking at MOSFET operation. However, it appears as the insight of academic thought rather than pure experience.

2ctiby

8 years ago

@zorgrian... You come on here with nothing but venomous attacks, acting as though you know everything, when clearly you have next to no knowledge with mosfets. Now start with a full apology and we can then perhaps continue in a reasonable manner.

zorgrian

8 years ago

@2nastyBee, I dont 'come on here' with no knowledge. What knowledge I have is shared without pontificate school masterly tones or blatant vacuous rubbish. Now you start apologizing...

Issacsutt

8 years ago

Why is it, the width and length have to be adjusted sooooo much, just to fully saturate the Mosfett, are mosfett's only suitable for very low power operation? In your circuit 2ctiby, the left one has a current of 180mA flowing through it with a voltage drop of 10.8mV across the second resistor, while the Mosfett on the right has to be adjusted from it's original values of 1um and 200nm, to 31um and 1nm just to mimic it as a practical switch.

zorgrian

8 years ago

Good luck Issac

Issacsutt

8 years ago

Ok zorgrian

2ctiby

8 years ago

@issacsutt ... Good observation there, and an important point for mosfet users on EC... a normal real Nmos has a very low Rds built in, let's say about 0.06 Ohm for a general purpose Nmos as chosen in that example for the bottom left resistor simulation. To simulate that as described, the EC Nmos needs considerable adjustment as you have found. Both a real Nmos and the EC Nmos can pass high current Id, as per datasheet. Our own requirement of the Load (ie the top resistor in our example) is what we need to set in order to achieve its ensuing current Id. Try a different Load value at the top left resistor there ... say 2 Ohm, then 20k .... See the Vds for any chosen load resistor value, and adjust the right side settings to be the same, just as you did before. ... Our whole discussion has been solely to do with switching in the Ohmic state, but there are other times when we wish instead to use the Nmos in the Saturation state where the Vds is then higher than the Overdrive. The developers of EC have chosen a "middle-ground" compromise for the default settings, no doubt based on the assumption that Mosfet users will adjust the settings to their own requirements (as you are now doing). Sadly, a great many users just plonk the default Nmos in to their circuits without any thought of the Ohmic switching/Saturation necessary adjustments. Let me know if you have any further good questions like that.

2ctiby

8 years ago

Just another technical point here ... best not use the word 'saturate' for the full current flow when the Vds is below the Overdrive ... keep that word for the Nmos operation only when the Vds is higher than the Overdrive... else you/others may get confused with that specific term.

Issacsutt

8 years ago

Ok, thanks... just making sure that I wasn't doing something wrong

Issacsutt

8 years ago

So the overdrive voltage is the voltage between the threshold voltage and Vdd at the gate, and Vds is basically the voltage across the Mosfett apart from the gate... and saturation only occurs when Vds is lower than the overdrive voltage, other wise it is in the Ohmic state which is kinda like the active region of a bjt

2ctiby

8 years ago

Let's clarify some terms here.... wipe your mind clean first so that you can absorb the information fresh with no pre-fixed ideas: ...1: The volts at the gate is called Vgs... 2: The threshold turn-on volts in the settings is called VTO or Vth ... 3: (Vgs - Vth) is called the Overdrive Volts or Vov ... 4: The battery supply to the top load resistor is called Vdd ... 5: The volts at the Nmos drain terminal is called Vds ... 6: The current through the drain is called Ids ... When Vds is less than the Overdrive Volts, the Nmos operates in its Ohmic state ... ie when Vds < (Vgs-Vth). .... ..... When Vds is higher than the Overdrive Volts, the Nmos operates in its Saturation state. Only the Ohmic state is described in our entire discussion. Try to view all of those values in the schematic and let me know if anything seems confusing ...I am here to help ....forget bjt comparisons.

zorgrian

8 years ago

@2by, wow! What a difference! Apart from the punitive 'wipe your mind clean first so that you can absorb the information', the rest of this text is brilliant!

zorgrian

8 years ago

I also agree with 'forget bjt comparison' this is a very big and common mistake

zorgrian

8 years ago

Essentially, the BJT is a current amplifier, whereas a MOSFET is a voltage amplifier much like tube amplifiers (valve UK). Dont try to mix these things

Issacsutt

8 years ago

Ok, sorry. If there's a Vto/Vth of 1v, and there's 4v at the gate (Vgs of 4v) then the overdrive is 3v regardless of Vdd or Vds or Ids, right?

2ctiby

8 years ago

Correct.

Issacsutt

8 years ago

And... based on those values, the Nmos will be in its saturation sate when the voltage at the drain terminal (known as Vds; opposite the terminal tied to gnd) is greater than 3v. And, in Ohmic when Vds is less than 3v...? Also, does it matter how much greater or less Vds is than the overdrive voltage for it to enter those states, or does it take more than say, just a couple mV before the Nmos is able to tell the difference?

Issacsutt

8 years ago

One more quick question: how do you get the voltage you desire at the drain terminal, cause it varies quite a bit with the load doesn't it?

2ctiby

8 years ago

First answer: For your example value of 3v, you are correct on both counts. Theoretically that specific cut-off point is clearly defined and should be used. In reality there is a little tapering in a parabolic way but that should not concern our calculations. It can however cause some 'distortion' or noise type of problems if we are dealing with the changeover from one state to another.... it is generally best to plan usage well within one state where things are far more of a linear nature if at all possible..... don't get side tracked here for now though.... For our Ohmic action, we tend to aim for a rapid increase so that there is minimum dwell in an uncertain place. This becomes more important when high frequencies are introduced.

2ctiby

8 years ago

Second answer.... If we have a lamp or a motor etc, then that top load resistor which we place there represents whatever its true value is. The lower resistor should be a value which matches the given Rds on the datasheet of our Nmos (likewise for the Vth setting). If we are not referring to a datasheet, then we can find those values by board measurements of Ids and Vgs and Vds by using the Kn formula from earlier... or simply use hypothetical values on EC as we have done so far.

2ctiby

8 years ago

You might want to try putting a 350 Ohm resistor as the Load instead of the shown 50 Ohm (keep the same 9v Vdd), and place a standard EC LED in line. See the dim light with the default Nmos, then alter any Nmos settings as usual to obtain the correct Vds....see it glow properly now with a good Id of 20mA with a simulated Rds of 60m Ohm at Vds of 1.2mV ...... starting to get the hang of it?... and notice how you are becoming familiar with all these terms now.... many users would just lower the LED resistance here in a poor effort to get it glowing better...at the expense of creating unwanted power waste in the Nmos with too much Vds across it, and an unrealistic Rds.

Issacsutt

8 years ago

Thanks for taking the time to write all that!

Issacsutt

8 years ago

And everything else! I think I got it, but it'll take a little while to get used to it, since I've been using nothing but bjts for quite a while

2ctiby

8 years ago

You're welcome. You could watch a scope trace of the overdrive move across the Vds trace up or down in to Ohmic/Saturation by altering the far bottom left d.c. volt source from its present 5v d.c. here on one of my Mosfet amplifier Saturation pages, or play there with other alterations: http://everycircuit.com/circuit/6347349325774848

Issacsutt

8 years ago

Good idea!

2ctiby

8 years ago

Thanks... eg: Alter the present Vgs at the far bottom left volt source from its present d.c. 5v in that link to become d.c. 4.6v (thus giving a blue trace overdrive Vov of 3.6v + 200mv a.c. = 3.8v max)... see then how the green trace Vds is nicely above that 3.8v max overdrive (ie Vds min being a consistent 4.16v as seen) ... and so shows less distortion in that green Vds trace now that the mosfet is comfortably in Saturation rather than alternating between the two states .... with just the green saturation LED lit, rather than that green LED flashing alternately with the Ohmic red LED.

Issacsutt

8 years ago

Yeah, I'll try that. Sorry it took me so long to reply.