Can mosfet work as amplifiers? - Page 1
Forum section: electronics / beginners

author: little_carlos
author_rank: Regular Contributor
timestamp: August 20, 2014, 02:09:24 pm
content: Hey guys, me agaaainive been wondering if mosfets (n type and p type) can work as amplifiers? i mean analog amps, not those d class stuffand if they can, it would be better than bjt type? worse? or same crap?thanks for your time

author: XFDDesign
author_rank: Frequent Contributor
timestamp: August 20, 2014, 02:24:55 pm
content: There is no reason why not. The main PITA is that FET datasheets don't hand over a few parameters you want for Small Signal amplifier design, which complicates things. None the less, you can collect the data needed to derive your own model.In HF Amplifier design, the RD16HHF1 is a fine example of an N-Ch Enhancement mode MOSFET used for its linearity in Side-band radio design (where linearity actually matters).Are they better than BJTs? How long is a piece of string? -- It depends on what you're willing to trade off or what your emphasis of design is.

author: c4757p
author_rank: Super Contributor
timestamp: August 20, 2014, 02:33:46 pm
content: Quote from: XFDDesign on August 20, 2014, 02:24:55 pmNot only that, but the process variation tends to behuge. There's no reason you can't use a MOSFET as a linear output device, for instance (except something something SOA), but if you want to use them in lots of (lowercase) small signal applications, you'll be matching the fuckers like they're vacuum tubes...

author: Monkeh
author_rank: Super Contributor
timestamp: August 20, 2014, 02:34:44 pm
content: Quote from: c4757p on August 20, 2014, 02:33:46 pmThey're just missing the pilot light.

author: Richard Crowley
author_rank: Super Contributor
timestamp: August 20, 2014, 02:40:16 pm
content: Perhaps it would be instructive to review this discussion....https://www.eevblog.com/forum/projects/a-good-audio-amplifier-based-around-2n3055(s)/msg473250

author: WarSim
author_rank: Frequent Contributor
timestamp: August 20, 2014, 04:48:10 pm
content: Yes you can. It has been done with great results.There are even matched pairs for class B.Here is one thing you need to know first. Just like BJTs each one is engineered for a purpose. Because MOSFet's most common use is switching relatively much fewer MOSFET are designed with a viable linear range. The last time I had to find a class B pair they cost $36, much more than the more common avalanche mode MOSFets.Sorry the amp was not my design and I don't remember where I sourced the transistors. All I can offer is that they are out there, and at the time they cost more than a BJT equivalent.Sent from my iPad using Tapatalk

author: diyaudio
author_rank: Frequent Contributor
timestamp: August 20, 2014, 05:14:50 pm
content: Quote from: little_carlos on August 20, 2014, 02:09:24 pmyes the best ones ive seen can bed found here.http://www.aussieamplifiers.com/5 years ago, some of the diagrams was available, if you interested PM me and I will send you some of them.

author: DanielS
author_rank: Frequent Contributor
timestamp: August 20, 2014, 05:30:25 pm
content: As others have said, most MOSFETs out there now are for PWM purposes.Most switching MOSFETs do not specify SOA for pulse widths wider than 1ms but for analog use, you need power devices able to go all the way to DC. I was looking at MOSFETs to build a programmable electronic load and was surprised to see how few of the MOSFETs I looked at had DC-rated SOA.Once your have your NMOS and PMOS, they work almost exactly the same as a bipolar power output stage; it just need a higher bias voltage difference between gates than bipolars do. You will need thermal compensation on the bias voltage since the MOS' threshold voltage drops with temperature and can cause thermal runaway.

author: tszaboo
author_rank: Super Contributor
timestamp: August 20, 2014, 07:09:01 pm
content: Circuits exist in the same basic applications as BJTs. Common source, common drain, common gate. You can make the small signal equlations it will be similar to the BJTs. Gate current Zero, the other current depends on the Vgs. You can add the gate capacitance for better frequency dependent results.Although most MOSFET does not specify the SOA for DC. It is kinda an art to find the ones which can work with it. Thermal runaway, hotshots and other nasty stuff can happen. If you want this for audio (why else) they can work in class A. Also, since the output characteristic is square of the voltage, not exponential, you will have more second order harmonics than third (Tubes have also more second order), which is better for the ear (psycho-acoustics).

author: diyaudio
author_rank: Frequent Contributor
timestamp: August 20, 2014, 07:14:29 pm
content: Quote from: DanielS on August 20, 2014, 05:30:25 pmWhy do you need the SOA for MOSFETs for a DC load??, I'm curious (standard ohms law power equations will be suitable for now, transient response maybe simulated and left lastly as the DC electronic load is DC-biased,like you I am also looking at building one, I have studied the bk precession tear down video, they use standard IRFP250, I'm plaining to use 6 irfp250n with a PIC24 and the same Analog devices DAC. I have some of the firmware running im about to embark on the analogue section soon.see thesehttp://www.irf.com/product-info/datasheets/data/irfp260n.pdfhttp://www.irf.com/product-info/datasheets/data/irfp250n.pdf

author: David Hess
author_rank: Super Contributor
timestamp: August 20, 2014, 07:32:09 pm
content: Some linear rated MOSFETs are still available. Semelab comes to mind but I think IXYS has some also:http://products.semelab-tt.com/magnatec/alfet.shtmlThe IXYS web site is a disaster so I cannot provide a link to the proper page.While they are not characterized for it, vertical MOSFETs intended for switching applications have been successfully used in HF and audio amplifiers.

author: DanielS
author_rank: Frequent Contributor
timestamp: August 20, 2014, 09:05:54 pm
content: Quote from: diyaudio on August 20, 2014, 07:14:29 pmBecause I do not want to blow up a "300W" MOSFET that is rated 300W only for one-shot 1ms pulses by trying to put 300W continuous through it.Pulse-rated MOSFETs are based on off-time letting heat spread through the junction to keep Vgs relatively even across the die. In DC applications, the MOSFET does not get that courtesy and you run a high risk of encountering hotspot from local Vgs runaway since Vgs drops as temperature rises... hotspot gets hotter, local Vgs becomes lower, even more current runs through the channel's hotspot, local Vgs goes even lower, rinse and repeat until failure. DC-rated MOSFETS are designed to be much less vulnerable to this.If you look at the IRFP260N's datasheet, you lose about half your peak SOA current every time you increase pulse with by 10X so if we extrapolate from the 10ms curve to 1s, that MOSFET would only be good for about 40W continuous. What do you get in that BK Precision video? The 300W DC load uses eight MOSFETs, which is about 40W peak per device even though the datasheet says these devices are rated up to 300W each.If BK had used 150W DC-SOA-rated devices, they would have needed only three to reach 300W dissipation with a 50% margin or four 100W-DC-SOA-rated devices for a 33% margin.

author: vk6zgo
author_rank: Super Contributor
timestamp: August 21, 2014, 12:42:11 am
content: For many years,the term "MOSFET" meant a small signal device used as a linear amplifier,or a switching device,in many of the same applications as BJTs.It is only in fairly recent years that most references to MOSFETs have been to high power switching devices.A similar situation exists wirh LEDs---newbies are unaware that they were ever anything other than relatively high power lighting devices.In both cases it gives us Greybeards a headache!.

author: David Hess
author_rank: Super Contributor
timestamp: August 21, 2014, 01:07:19 am
content: Quote from: vk6zgo on August 21, 2014, 12:42:11 amThe change was that almost all power MOSFETs now are vertical instead of lateral devices. Vertical MOSFET structures have lower Rds for a given area but worse SOA curves and are more likely to suffer from problems very similar to secondary breakdown in bipolar transistors.

author: owiecc
author_rank: Frequent Contributor
timestamp: August 21, 2014, 05:46:11 am
content: Quote from: DanielS on August 20, 2014, 09:05:54 pmWhich Vgs are you talking about? Threshold? Can you give some references? What about the effect of Rds(on) change in temperature, won't it dominate?Quote from: David Hess on August 21, 2014, 01:07:19 amIs this what DanielS is talking about?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 21, 2014, 07:41:42 am
content: Yes, he meant Vgs(th). The tempco is negative (I think it goes as something like 1/T_abs, but it's usually just expressed as a roughly-constant tempco around room temperature).Rds(on) is the characteristic of the left-hand (voltage saturation) region on the drain (output) curves of the device. The flat (constant current, linear) region has nothing to do with Rds(on), the voltage drop is >> Id*Rds(on).Hot-spotting starts like this:- The die is dissipating heat. The die is a finite area, so some points will be more deeply surrounded by nothing but heat generation (towards the center), and others will be partially away from heat generation (points along the edges). Thus, the temperature is simply lower on the edges than in the center, because there is less power density there. This very broad hot spot (warm spot, really) is inevitable, and normal.- So the center heats up. So its Vgs(th) drops, causing more current density. The temperature warms up a little more. At low power density, the hot spot is exaggerated a little bit, but it still remains fairly broad. Some manufacturing variation will drive local areas slightly hotter (i.e., points in the silicon with slightly more doping or diffusion or whatever).- Beyond a certain critical power density, temperature will begin to run away: Vgs(th) continues dropping, local temperature continues rising, and seeded by manufacturing variation, the spot becomes ever smaller, and the center becomes ever hotter. Eventually, that small spot hogs all the current of the transistor (surrounding areas actually cool down during the short moments this occurs in, because their Vgs(th) is now several volts above that of the spot!), and becomes so hot that two things happen: one, the silicon becomes completely resistive, and all transistor action ceases; two, diffusion sets in, scrambling the junction (permanent damage).If power is limited at this point, you may get a silent failure, where the transistor becomes a two or three way short circuit, but does not explode. If not, then the fault current delivered by the circuit causes the plastic above the die hotspot to decompose and bubble up, the silicon or bondwires to melt, vaporize, and ionize (arc), and after a few miliseconds, the pressure is large enough to blow a hole through the plastic case, or crack it in half, or blast a flaming jet, etc. Either way, the magic smoke finds its way out. During this time, you can count on the voltages on all three pins to be near each other in some way or another, so... if you weren't expecting full drain voltage to appear on the gate terminal ever...guess what part of your surrounding circuit is exploded now?The difficulty is not intrinsic freedom from runaway: that is a fundamental aspect of both BJTs and MOSFETs. The difficulty lies in that critical power density. In the old days of lateral MOS, the cell density was so low that it couldn't be reached: drain current just wheezes if you try to short it out, and drain voltage was limited by process limitations (it was hard to make reliable transistors over, say, 100V, or 200V*), and P = V*I over that wide (lateral!) area means... the transistors literally just suck too much to blow up!Modern devices push current density as much as possible, at the expense of anything else: and especially in high voltage devices, there is more than enough opportunity to exceed that limit and destroy the transistor, while staying well below the nameplate rated dissipation. It used to be said that only BJTs exhibit this 'second breakdown' phenomenon, but that was because only BJTs had the power density to make that a problem; nowadays, MOSFETs are more than powerful enough to be dangers to themselves, and exhibit second breakdown.*Say, are there any old timers here that might remember when sales literature / databooks / magazine ads started proclaiming high voltage BJTs or MOSFETs? Guessing it was during the 60s and 70s. I don't know when higher and higher voltage devices were introduced, would be interesting to get some history on that.In short: the phenomenon is exactly the same reason why you can't go paralleling transistors willy-nilly, in linear operation (and often, the same is still true of switched operation as well). Internally, a transistor is just made up of millions of tiny transistors all in parallel; they're better matched, and better thermally coupled (silicon is a great conductor) than loose, random devices, but that doesn't preclude anything, it just means it takes more power density to get there. Transistors designed to tolerate this (i.e., usually by designing in the same features that help you parallel discrete transistors -- emitter / source degeneration) will perform relatively poorly under saturated conditions, but will handle the linear range very well.Both BJTs and MOSFETs are current available in useful sizes for linear use: BJTs up to, say, 10-20A and 200-400V, and MOSFETs up to similar currents, and 1200V or maybe more (with some derating at the highest voltages I think).Tim

author: DanielS
author_rank: Frequent Contributor
timestamp: August 21, 2014, 12:15:06 pm
content: Quote from: owiecc on August 21, 2014, 05:46:11 amRds(on) is only relevant for switching applications where the MOSFET is either on or off; it is meaningless in linear applications where the MOSFET spends most of its time somewhere in-between. In linear applications, the MOSFET is effectively behaving somewhat like a voltage-controlled resistor and Rds(on) merely indicates the lowest achievable resistance.As for the negative coefficient on Vgs, pull out any random MOSFET datasheet, look at the Vgs vs Ids vs temperature plots and you can see that Ids for a given Vgs changes considerably from 25C to 175C - particularly at the lower end of the curve. If you look at the IRFP260N linked earlier in this thread, at 4.5V Vgs, current goes from 3.5A at 25C to 15A at 175C.

author: owiecc
author_rank: Frequent Contributor
timestamp: August 21, 2014, 05:01:53 pm
content: Thanks for the clarification. I didn't know second breakdown exists in MOSFETs.

author: Refrigerator
author_rank: Super Contributor
timestamp: August 21, 2014, 05:34:53 pm
content: Wow, for a long time i thought that mosfets were used for linear regulation and transistors could only go ON or OFF.

author: DanielS
author_rank: Frequent Contributor
timestamp: August 21, 2014, 07:24:31 pm
content: Quote from: owiecc on August 21, 2014, 05:01:53 pmIt mostly came to be due to modern devices with ultra-fast switching times and high current densities.For fun, go visit DigiKey or other favorite component distributor and search for the highest-powered TO-220 MOSFET you can find. Can you honestly tell me nothing fishy is going on when manufacturers quote 500W in that package? Before manufacturers started quoting power based on one-shot 1µs pulses, 1% duty-cycle at 25C Tcase or similar, the TO-220 package was usually rated only up to 75W. It still is in the 75-100W range for true sustainable heat output once you average those 300-500W rise/fall peaks, Rds(on) and leakage losses along with de-rating and off-time between pulses.As Dave likes to say: traps for young players!

author: owiecc
author_rank: Frequent Contributor
timestamp: August 21, 2014, 07:34:02 pm
content: I saw many of them rated to absurd values but all of them have a *package limited to 50W. Never really cared about this value. I only used to PWM them.

author: SeanB
author_rank: Super Contributor
timestamp: August 21, 2014, 07:47:44 pm
content: I just got a scrap yard amplifier, HMA9500 MOSFET amplifier. Who knows, it might work once I dry it out a little, or I might just tear it down for the scrap value, and see if I can get more than I paid for this 29kg monster. It uses 2SK135 and 2SJ50 power mosfets in the output stage, and runs off 70V rails with 15000uF smoothing capacitors. 120W RMS per channel and it is rated to deliver this all month long, with distortion specced to 100kHz. Wonder how big an ultrasonic speaker I could drive with this thing, as it is rated to full power almost from DC to 300kHz. no wonder it was known as a speaker killer.This weekend a look inside to see if the FETs survive.

author: Refrigerator
author_rank: Super Contributor
timestamp: August 21, 2014, 08:21:55 pm
content: Quote from: SeanB on August 21, 2014, 07:47:44 pmThese kind of amps should be robust enough to handle a little splash, just make sure there's no crud or bugs on the pcb. I remember once i heard a relatively loud pop in my old pure A class transistor amp just to find some burnt remains of a spider.

author: David Hess
author_rank: Super Contributor
timestamp: August 21, 2014, 09:15:06 pm
content: Quote from: owiecc on August 21, 2014, 05:46:11 amIXYS and Semelab have application notes discussing the problem. My old Siliconix power MOSFET application book discusses the theory but it was only a potential issue at the time.The problem comes down to the Vgs temperature coefficient reversing at high Vds (which may not be as high as you think) so separate MOSFETs and even cells in the same MOSFET no longer share current leading to thermal runaway.It is not that vertical MOSFETs are necessarily poor in this regard but that they are worse than lateral MOSFETs and poorly characterized.QuoteYes, it is the same problem.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 22, 2014, 08:35:05 am
content: Quote from: DanielS on August 21, 2014, 07:24:31 pmOh, they're not pulsing them or anything, watts is watts. But what are the chances you have your devices submerged in a nucleated boiling liquid at 25.0°C?Tim

author: SeanB
author_rank: Super Contributor
timestamp: August 22, 2014, 05:38:52 pm
content: You can do the simple thing and use a pure silver heat spreader to transfer the heat to a room temperature superconductor that then is attached to a cubic metre copper fin heatsink. Otherwise that is just marketing BS and the real rating is going to be somewhere around 50W in real world apps. They get that from the peak current multipled by the saturation voltage at that peak current. Keep it up for even 10mS and the device will turn into a small bright ball of plasma.

author: David Hess
author_rank: Super Contributor
timestamp: August 22, 2014, 06:05:49 pm
content: It is a little more complicated than that.Peak current may be limited by metalization as well as power dissipation.At high Vds which can be surprisingly low in MOSFETs optimized for switching applications, the maximum power dissipation is lowered because of thermal runaway issues not unlike secondary breakdown in bipolar transistors. This is where MOSFETs optimized for linear operation have a real advantage.Some "large die" TO-220 packages are good to at least 125 watts but again, not at high Vds.Also temperature derating applies in all cases so 1/2 to 2/3rd of the maximum power rating at 25C is more realistic.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 22, 2014, 07:19:48 pm
content: Quote from: SeanB on August 22, 2014, 05:38:52 pmThat would actually do worse. More than a few watts come off the front (plastic) of the package, and the leads (especially the drain lead, but also the source if it's heavily bonded, which should be the case with those big-amp parts).QuoteAgreed. Under normal mounting conditions (finite heatsink, elevated ambient, insulating pads, etc.), 50W is usually pushing it.Almost all the thermal resistance causing that limit is external to the part -- only in very small devices (e.g., IRF540 and down?) will the device limitations become apparent, and the power dissipation has to be derated further (30W, 20W...).Or for lower junction temperatures, like 7805 regulators, which have limited die size to begin with, but a fairly conservative thermal limit built in as well. They're often put on paper-thin (not really) tabbed TO-220s, because they're only good for 10W to begin with.QuoteNo, peak current is generally well into the desaturated region (assuming by "saturation" you mean the conventional sense, "Rds(on) limited region", and not "FET  saturation", which alas, is a thing, as confusing as that sounds), at which point the power dissipation is massive: 10 microseconds to failure, not milliseconds.This may be slightly different for those really massive ones, the kind with lower Rds(on) in the die than in the bondwires. Though the outcome is still the same.Tim

author: sorin
author_rank: Frequent Contributor
timestamp: August 23, 2014, 10:56:48 pm
content: I have read that hight voltage (D-S) mosfet perform better on linear applications