| Bakmeel |
As my first posting I would like to ask this question:
I am planning to build my own Class A power amplifier, and with my current knowledge of electronics, I should be able to do such a thing. I -of course- want this amp to be better than best, and take the good things of all components... And so there begins the battle between MOSFET and BJT.
Both have advantages, as wel as disadvantages. But in some cases these (dis)advantages change. For example, a MOSFET would do good in a power stage, because it handles power better than a BJT (correct me if i'm wrong), while a BJT would do nice in a line stage.
Who can tell me when to use what device where and why? What exactly are the advantages of the one device over the other? When are those advantages best utilised, and when manifest the disadvantages themselves strongest?
I'm eager to know :)
Bouke |
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| subwo1 |
| Sorry for the brevity in answering such a intricate question. But I have a hunch that for class A, all mosfets could be the best way to go since there is no need to make a fast transition from one half of the power output stage to the other. I agree that mosfets handle power better too. |
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| Chucko |
Here's my take on things. I don't claim this is right or wrong, just my opinion. I would be delighted to hear other opinions.
There isn't a really good device for the output stage of a power amp. They all stink. :D
BJTs tend to be slow, they're subject to thermal runaway, and they have safe-operating-area (SOA) restrictions that often spells doom with highly reactive loads. Beta droop in most power BJTs is a major source of output stage non-linearity. Push-pull biasing is moderately tricky and the proper bias point is not widely known. But they can produce output voltages very close to the supply rails if properly applied, the Vbe-Ic relationship is very predictable and uniform across devices, and they tend to be cheap.
MOSFETs are fast - sometimes too fast. There's a lot of variation in threshold voltage in production devices, and it can be difficult to devise a push-pull biasing scheme that properly tracks threshold voltage changes with temperature. The gate oxide can be fragile (this is mostly a concern for handling prior to installation). Gate drive is primarily capacitive, and this can be a problem if you don't design for it. Rds(on) limits how close to the rails you can take the output voltage under heavy loads - and it increases as the devices warm up. But they don't have SOA problems, and don't play the thermal runaway game. Oh, and they tend to be more expensive than BJTs.
Both device types have the problem that complementary pairs tend not to be well matched.
For small signal gain stages, it's a case of horses for courses. BJTs can work very nicely in differential pairs for low impedance sources. JFETs are better for hi-Z sources, and for one-transistor gain stages if you don't mind (or are actively seeking) even-harmonic distortion. JFETs make excellent current source loads. I'm not up to speed on MOSFET noise behavior at these levels, but I imagine they would be fine for extremely high impedances - e.g. piezo transducers. MOS certainly seems to work fine in radio frequency gain stages.
At line levels it's your call. I lean towards BJT input op amps because I'm lazy, and because JFET and MOSFET op amps tend to have input stage shortcomings (poor common mode rejection, high input capacitance, etc.). If you were looking for major headroom, I imagine high-voltage MOS in line stages would be a good choice, but mostly because high-voltage BJTs are getting harder to find (SOA is a problem here too), and JFETs have never been common as high-voltage devices. The fact that MOSFETs are available in both enhancement and depletion mode types adds flexibility.
So what have I left out, and where am I wrong? There must be something bogus in this post. |
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| P.Lacombe |
If you want to build a pure class A amplifier, BJTs are more linear than MOSFETS if you use very large BJTs, having very high power handling capacity, and Ft greater than 10 Mhz, for instance 2SC3281/2SA1302 (Toshiba), which are specially designed for this purpose.
But BJTs which are designed for general industrial use (TIP 36...) are quite unusable for very high quality service, standard MOSFETS gives better results than such cheap devices, at the same cost.
In class AB amplifiers, it is very difficult to eliminate totally the crossover distortion , even with good BJTs, and MOSFETS gives better results, because of their "soft start" transfer characteristics.
P.Lacombe |
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| traderbam |
I agree with Chucko's comparison. I would just add some more flavouring. What device you choose should be driven by what you want the device to do in the circuit topology you choose. Understanding what you want it to do is essential - at an electrical level.
Mosfets are transconductance devices: you must charge their relatively high input capacitances up and down in order to vary their drain current. They are majority carrier, resistive devices. Their transconductance is non-linearly related to drain current (approximately a square law). Outside hifi they tend to be used in high-speed switching applications, like switch mode psus, beacuse they have negligible turn-on and turn-off delays and do not suffer from secondary breakdown (tend to be reliable). Transconductance tends to decrease with temperature.
Bipolars are current gain, minority carrier devices. Collector current is related to base current, often fairly linearly over a certain range of collector currents. The devices have input capacitance although much less than comparable FETs. Base-emitter voltage is non-linearly (exponentially) related to base current. Current gain tends to increase with temperature.
In my experience, bipolars are used most often in high-end power amps (with all due respect to Mr Pass :) ). I would speculate that this is because it is easier to achieve linear current gain, their transconductance is higher in the usual operating region, they enable greater phase margin with less devices because of their lower capacitances. Bipolar designs need more care wrt exceeding safe current operating limits and often have carefully designed current limiting circuitry. A great sounding amp can be made with either device provided you understand how they work and design circuitry around them to compensate for their foibles. |
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| Chucko |
| quote: | Originally posted by traderbam
Bipolars are current gain, minority carrier devices. Collector current is related to base current, often fairly linearly over a certain range of collector currents. The devices have input capacitance although much less than comparable FETs. Base-emitter voltage is non-linearly (exponentially) related to base current. Current gain tends to increase with temperature. |
Lots of people seem to believe that BJTs are current-driven devices. Doug Self has convinced me that they are instead transconductance devices.
Self asserts that the relationship between Vbe and Ic is the main driver, and that Hfe (current gain, a.k.a. beta) is merely a side effect. He points to the fact that the exponential relationship Ic = e^(Vbe/k) is a very good approximation across several decades of collector current for the vast majority of silicon BJTs, and that there is little consistency between the beta curves of most BJTs - even of the same type!
This doesn't mean that you have to live with an exponential transfer curve; there are lots of tricks, using current mirrors and the like, to enforce a more linear relationship.
I agree with the rest of your comments, traderbam, but I didn't want to leave this assertion unchallenged. |
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| DarkOne |
Linearity of BJTs begins in less current than in FET(needs to be biased for high currents to be linear). BJTs have smaller tolerances than FETs. FETs don't have current noise (near to 0), so they are best for phono stages and similar low voltage applications. Fets have higher capacitances than BJTs, so they need more current for higher slew rates.
And it is not true that FETs are faster than BJTs. This could be true about 10 years ago.;)
Finally both are good for audio (FETs - Nelson Pass - great amps)(BJTs - Dieter Burmester, Mark Levinson and many others - also great amps). You have to know everything about device before you use it. |
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| R. McAnally |
True, true... designing with mosfets requires a lot more trial and error in my experiences. I have successfully designed and built many full BJT amplifiers -- and getting mosfets to work well in the OPS instead of BJT's took about as long as the entire design.
BJT's are better for the first time designer/builder, much more forgiving when it comes to stability and biasing, ect...
| quote: | Originally posted by DarkOne
Finally both are good for audio (FETs - Nelson Pass - great amps)(BJTs - Dieter Burmester, Mark Levinson and many others - also great amps). You have to know everything about device before you use it. |
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| traderbam |
"Self asserts that the relationship between Vbe and Ic is the main driver, and that Hfe (current gain, a.k.a. beta) is merely a side effect. "
Isn't that a little like saying a resistor is a transconductance device since its current is related to its voltage? I guess it depends how you choose to look at it. I guess Mr. Self is saying that the exponential relationship between Vbe and Ic is more consistent across devices than hfe. Hideous isn't it? In many applications it is desirable to think of a bipolar having a small-signal gm of roughly 40Ic - such as when calculating the transconductance of a long-tailed pair. There are now a range of power BJT where the hfe is very constant with Ic and it may therefore be more useful to design around the hfe rather than the exponential gm. Horses for courses. |
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| hugobross |
Here's just my opinion:
Have you ever considered IGBT devices?
And I don't know if someone of this forum has experience with room acoustics, but it's proved that the acoustic characteristics of the room are dominant at one or two meters distance of the speakers you're playing with (in an average room, not in "dead" rooms). This is because of the facts like reverberations, echo's, etc... So at this distance these characteristics are more important than the way your amp and speakers will play the music.
Of course it's nice to start with a proper signal, and maybe you'll see the difference between mosfets and BJTs on a scoop, but I wonder if anyone can hear the differences between a good mosfet amp and a good BJT amp!!! This is all called relativity, :) .
This is just my way to analyse such problems, I don't want to be the anarchist here!!
Best regards,
HB. |
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| Bakmeel |
| quote: | Originally posted by hugobross
Here's just my opinion:
Have you ever considered IGBT devices?
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Actually, I have... and already have sniffed this forum for info about that. I have learned that IGBT's aren't very suitable for the job. In Fact, they suffer from the disadvantages from BJT's and MOSFETs. High input capacitance, and nonlinearity. IGBT's were made for switching applications, and they perform perfectly there. I could use IGBT's in controlling biascurrents, i've heard the perform well in that too....
Reading all opinions posted so far, it seems that MOSFET's have most advantages over BJT's, and can be used best in DIY designs.
Does anyone know a little more about the application of JFETs instead of regular MOSFET's? And how about HEXFET's?
Bouke |
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| Morrist |
It has been stated previously in this thread but I don't think that there has been enough emphasis placed on this point: MOSFETs are not linear devices.
As mentioned above by traderbam, the relationship between the applied voltage and the resultant drain current is approximately a square: Id = K*Vgs^2 (Where K is a constant). Therefore, theoretically, no matter how much feedback you apply you will be unable to reproduce the signal linearly.
You might now be saying 'Oh, this guy's a MOSFET hater' but that's not true. MOSFETs have their advantages: Little to no input current being one advantage, which would perhaps make them very useful as the input device of a line stage amplifier.
As for BJTs, they aren't linear devices either but they are more linear than MOSFETs. Note that not all BJTs are equal, some are more linear than others. If one takes care in both choosing appropriate BJTs and biasing in class-A, they stand a much better chance of amplifying the input signal linearly.
With that being said I believe that one can biuld a good amplifier using MOSFETs but could just as easily build an exceptional amplifier using BJTs.
Thanks,
Morrist |
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| subwo1 |
| One useful feature about JFETs is that the pinchoff voltage is below the source voltage for an N-channel device, and the other way around for P-channel. This characteristic could help in driving them as voltage followers up to the power supply rails. Otherwise, I believe they behave much like mosfets. |
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| seangoesbonk |
"Isn't that a little like saying a resistor is a transconductance device since its current is related to its voltage?"
A resistor is simply "conductive", because the change in voltage across the resistor results in a change of current through the resisitor. Devices are described as "transconductive" only when the change that occurs happens in a different part of the circuit. A bjt IS a transconductive device because the change in Vbe results in a change in Ic, and Ic does not travel into or out of the base.
Also someone stated an equation in correctly.
It should read Ic = Is*e^(Vbe/Vt) where Vt is the thermal voltage (26mV at room temp for all trans) and Is is the saturation current(typically 10^-12 A to 10^-14 A). Is is device dependent.
Well, that was my first post. I hope I didn't **** anyone off. |
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| Chucko |
| quote: | Originally posted by traderbam
I guess Mr. Self is saying that the exponential relationship between Vbe and Ic is more consistent across devices than hfe. Hideous isn't it? |
That's certainly the way I read it.
And no, I don't find it hideous. It's a challenge. If anyone ever finds a solid-state device (or electronic amplifying device, period) that is inherently linear, audiophiles everywhere will rejoice, and engineers everywhere will be out of work.
Until then, we have to make the best of what we've got.
There are ways to use the BJT's exponential Vbe->Ic relationship that are inherently linear. They're just not easy to apply unless you're a chip designer. |
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| P.Lacombe |
"Have you ever considered IGBT devices? "
Hugo,
IGBT are specially designed for switching purposes, at high currents and high frequency, not for linear amplification at audio frequencies. In my opinion it is impossible to obtain a clear sound with such devices, which are more suitable for cooking electrical appliance...
Regards, P.Lacombe. |
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| sonnya |
To add some salt... What about GT20D101 and D201 from Toshiba? They are made for audio. I have tried them out and the do sound nice to.
And the are IGBT types.
;)
Sonny |
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| sonnya |
Hint of the day!!
Take one VQ1000J from Vishay.
Use only 2 of the 4 MOSFET's
Set an idle current of 90mA and a VDS of 10V.
Then check the "Transfer Characteristics" in the datasheet.
Also check the values of Ciss, Coss, Crss.
Add some degeneration sourceresistance in the size of 10 - 20 Ohm.
I must say this is better than BJT and even nearly 100% matched.
The perfect one would be a ZVN3310 idling at .2 - .4 Amp and 25 Vds. but it cant handle the heat. :(
Sonny |
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| sonnya |
Forgotten!!! Add a heatsink to the VQ1000J
Sonny |
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| traderbam |
| Dear power MOSFET experts, just out of interest (I don't have any datasheets handy), how linear is Id to total gate charge? |
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| sonnya |
Id is dependent on Vgs. You want to have as low gatecharge as possible to reduce the gatecharge before the draincurrent starts to rise. A high impedance driver will raise the Turn off and on time.
That is the reason why the driverstage should be able to sink or source as much current as possible.
I think ... I think some times the problem with mosfet and IGBT outputstage (The sound quality) is addressed to the gatecharge.
If your driverstage cant remove the gatecharge fast enough you will a short shortcut in the outputstage when one mosfet turns off and another turns on wich again can be the reason why MOSFET class A sounds a lot better than MOSFET class AB.
The size off gatecharge is dependent of Vgs and Vds. It would desirable go get as low Vgs and Vds chage as possible.
I dont know if i am right? Ask Nelson ... He does nothing but designing with mosfet... Or ask Elso.
Sonny |
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| Nelson Pass |
Ok, the manuals only offer Gate Charge versus
Vgs, but from these we infer that the Ids is
a reasonably linear function of Gate Charge
at currents significantly above 0. Below that it
is not so good, and that is why we like lots of
bias on our MOSFETs. |
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| sonnya |
Have i understand it right?
You draw a "parallel" line between "Vgs versus gatecharge" and "Vgs versus Id"?
Yes i can You are interrested in running the mosfet at a high Vgs as possible. The Drain current will be high. If you make a change in Id in the order of 2%. Then the Vgs will only change a small amount wich again will result in a small change in gatecharge?
The perfect would then be a mosfet with really high "gfs" to get the Vgs change as small as possible?
Sonny |
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| sonnya |
The first link is a design from elektor some years ago. They use Gt20D101 and D201 but the amp was in the begining made to work with MOSFET. This Amp do have higher power rating thoug.
The UNETTO amp uses the same IGBT.
One question i have on my mind. Why do they depent their frequency compensation (miller cap) in the UNE TTO on Q7 or Q8 b-c capacitance!?!?!? You will not get two amps with the same Phasemargin (read sound)!!!!!!!!!
Sonny |
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| P.Lacombe |
Hugo,
Thank you for the two links. The first schematic diagram seems to be correctly designed, and perhaps I will try to build this amplifier for evaluation purpose.
But all IGBT based amplifiers I have heard as yet, sounds not very well...
Regards, P.Lacombe. |
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| traderbam |
Thanks for looking up the Id/Qgs for me, Nelson. :D This is interesting - in the "active region" the drain current is approximately linear to the integral of gate current.
Some more thoughts on the use of FETs:
I notice quite often designers treating FETs like substitutes for BJTs, especially in emitter-follower output stages. This really doesn't make much sense to me. Using a FET as a source-follower is undermined because of the transconductance characteristic and the very high Cgs and Cdg. So they don't work as well as BJTs as emitter-follower buffers. However, they could be used as current amps as part of a BJT emitter-follower output. Using power FETs in common-source is more likely to be fruitful. Again, remember the Cdg which is usually very large (>200pF) and varies non-linearly and inversely with Vdg. This can cause some interesting "Miller" effects if not dealt with. And the transconductance is non-linear too. Uncompensated, these characteristics will degrade the sound quality, especially so if loop feedback is used (yes, feedback can make the sound much worse and/or cause instability). Another pointer is to not assume a mfrs complementary device is truely complementary - with some power BJTs they are pretty darned close but power FETs tend to be miles apart on capacitances and transconductance curves, so look for the best complement not just the one with the same serial no preceeded by a "9". Of course, if you choose a single-ended stage you don't have to worry about this - instead you have to worry about heat. |
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| FEThead |
[QUOTE]Originally posted by Morrist
[B]It has been stated previously in this thread but I don't think that there has been enough emphasis placed on this point: MOSFETs are not linear devices.... Note that not all BJTs are equal, some are more linear than others.
(Morrist's words above, mine below) I edited his post for brevity and it did not come out too well. Sorry about that.
Just as some BJTs are more linear than others, some FETs are more linear than others. MOST BJTs and FETs are made for some kind of switching application where linearity is not wanted. For linear FETs I suggest you go to the following links: http://www.hitachisemiconductor.com...1241_2sk213.pdf
http://www.hitachisemiconductor.com...244_2sk1056.pdf
Go to the third page and look at the "Typical Output Characteristics".
FWIW, I run 2SK1058s in my version of Nelson's Zen amp and I am using 2SK216s in my version of the Pearl phono preamp. That is still in progress . . . but the prototype works fine. The '216s are quiet and linear even using just one '216 non-cascode for the first device! I would use the '216s in a line amp if I had one of those.
FETs are prone to oscillate at a very high frequency, but a couple hundred ohm resistor (or ferrite bead) in series with the gate ends that problem.
I run the '1058s SE, class A, and my speakers are quiet efficent, (SP!) so I don't need a "lot" of power.
Nelson, if you find this I am curious as to your opinion of the above devices.
To repeat, this is just my opinion, I do not mean to "put down" anybody else including Morrist and especially Nelson Pass. |
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| Nelson Pass |
I haven't used the 1058's, but looking at the sheet
I see that it's transconductance is quite a bit lower
than the IRF's I favor, and is more similar to the earlier
generation of MOSFETs.
Does your Zen amp have less distortion for using them,
or does it simply sound better to you? |
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| FEThead |
| quote: | Originally posted by Nelson Pass
Does your Zen amp have less distortion for using them, or does it simply sound better to you? |
Nelson,
Thank you for your reply!
I really don't have any way to measure distortion except to put the amp on an oscilloscope and do an X-Y plot. I built a Zen using an IRF130 a couple of years ago. (because that device was on hand) A few months I built my present amp using the 1058's which is actually two Zen amps bridged to eliminate DC across the speaker with using a blocking capacitor. (and fed push-pull) To me the newer amp sounds better, but there is more than one variable here (lack of output cap - which was a cheapo I admit, less voltage swing per device, etc.) and of course I am not without bias. I still have the single ended Zen and will put the new and old on the 'scope in the next day or two to see if I can see any difference.
Thanks again for your time and all you have done for us DIYers! |
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| Nelson Pass |
I would be willing to bet that with a single amp
you'll see more measured distortion. I have seen
and built Zens with the lower transconductance devices
and they all measured greater distortion, and to my ear,
did not sound as good.
One of the trade-offs is the lower capacitance, so that
with lower transcondance devices you can use a higher
input impedance, and this sometimes helps with sources
that have trouble driving the low input impedance.
If you are driving the speaker with 2 balanced Zens
(or better yet 2 pairs of parallel balanced Zens) you
can get considerably less distortion due to cancellation
of the dominant second harmonic. |
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| Circlotron |
| quote: | Originally posted by subwo1
One useful feature about JFETs is that the pinchoff voltage is below the source voltage for an N-channel device, and the other way around for P-channel. This characteristic could help in driving them as voltage followers up to the power supply rails. Otherwise, I believe they behave much like mosfets. |
Are JFETs only small signal devices or is there such a thing as a power JFET too? |
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| jcarr |
Circlotron:
>is there such a thing as a power JFET?<
At least there used to be, from the likes of Yamaha, Sony and NEC. These devices were referred to as vertical power FETs, and date back to the mid-70s, right around the time that Hitachi was introducing its MOSFETs (which originally had a completely different designation than 2SK134/2SJ48 et al). As far as I know, there were a number of application issues relating primarily to the gate drive which made these vertical power FETs tricky to use and easy to break.
In most cases, the devices were featured in commercial power amps from their respective manufacturers, and I would not be surprised if the audio divisions in each company had a major influence in getting these devices produced.
A quick list of commercial power amps incorporating these devices would include JVC's JM-S7, Sony's TA-5650 and TAN-5550, Sansui's BA-1000, and of course the B-1 from Yamaha. I am sure that there are many more that I have forgotten.
A few years ago, Tokin started manufacturing a line of static-induction transistors (SIT), which behave very similarly to the older Sony, Yamaha and NEC devices. However, I think that Tokin was subsequently acquired by NEC, and I haven't kept track of what happened to the SIT lineup.
2SK60/2SJ18 (Sony), 2SK70/2SJ20 (NEC), and 2SK77 (Yamaha) are some of the device codes that I remember. Apologies for any memory lapses.
regards, jonathan carr |
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| MarcelvdG |
| Apparently everyone here is convinced that power bipolar junction transistor device capacitances are smaller than power MOSFET device capacitances. This may be true when you compare the bipolar part's junction capacitances to the MOSFET's gate and overlap capacitances, but as soon as you forward bias a bipolar device, you get a large diffusion capacitance between base and emitter (this capacitance models the minority charge storage in the base). For a typical epitaxial-base power transistor with 10MHz fT biased at 1A, its value is around 600nF, much greater than the oxide capacitance of a normal power MOSFET. In fact, this is why the fT of the transistor is only 10MHz despite of its huge transconductance. |
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| traderbam |
Interesting argument, Marcel. You may be right about the base capacitance being very large: how did you arrive at these figures?
One thing that may be of more importance is the energy required to cause a change in output current. For example, for a FET and BJT with similar Pd and Imax, how much charge is required to change their output currents from, say, 1A to 2A in 1us? You would have to make some assumption about the change in collector-base/drain-gate voltage to take account of charging Ccb/Cdg.
Figuring out the gate charge is relatively easy because the datasheets usually show a graph of Id vs Q as FETs are most often used in switching applications. Trickier to find the equivalent for a BJT, perhaps. |
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| AKSA |
Marcel,
Accepted, but you refer only to base/emitter capacitance, which is not too significant, particularly in a common collector since the Vbe does not much change during operation.
Of far greater significance is the depletion capacitance across the base/collector, the so-called Miller capacitance, which is profoundly influential in the common emitter configuration.
Your comment is interesting, but less relevant in this context, since the gate capacitance of a mosfet is far higher wrt the drain, and this effectively mandates use of muscular drive, not so much a problem with a bipolar output stage and in any case ameliorated by use of a double emitter follower.....
In closing I'd suggest the propensity to self-oscillation of the mosfet introduces other problems, necessitating a gate stopper which to some extent negates the use of strong drive at the gate.
I apologize for the subjective comments; I don't have capacitance figures to hand, but am flying blind. I do know the gate capacitance of a P type IRF9140 is 600pF, and this capacitance is against the drain, a real PITA for a source follower.
Cheers,
Hugh |
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| subwo1 |
| Hello, I noticed I mistakenly said go with all MOSFETs. I meant use them in the power output stage. |
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| MarcelvdG |
Regarding Traderbam's question: in the following I will assume you bias your transistor in the normal forward active region, not in saturation or in reverse or anything funny. When you know a transistor's transconductance and fT for a given bias point, the sum of the base-emitter and base-collector capacitances is given by the equation:
cpi+cmu~=gm/(2*pi*fT),
where cpi is the base-emitter capacitance, cmu is the base-collector capacitance, gm is the transconductance, fT is the extrapolated frequency at which the current gain drops to unity and pi is 3.14159265358979....
Ideally, neglecting some second-order effects which occur at high current densities, the transconductance of a bipolar transistor equals:
gm=IC*q/(kT),
where IC is the collector bias current, q is the electron charge (1.6022E-19 C), k Boltzmann's constant (1.38065E-23 J/K) and T is the absolute temperature.
Assuming 10MHz fT, 1A bias current and kT/q=26mV (which is true just above room temperature):
gm~=38.4615 S,
cpi+cmu~=612.134nF.
The collector-base capacitance cmu is just pure junction capacitance, the base-emitter capacitance cpi is a combination of junction and diffusion capacitance. So after subtracting the junction capacitances, you are left with the base-emitter diffusion capacitance.
Regarding Hugh's/AKSA's remarks, it is certainly true that capacitance between base and collector is much worse than an equal capacitance between base and emitter. This does, however, not mean that diffusion capacitance is necessarily negligible.
For example, without any cascoding, base-collector capacitance is gm*Rload+1 times as bad as base-emitter capacitance, due to the Miller effect in common-emitter stages or the bootstrapping effect in emitter followers. When gm=38 siemens and the load resistance Rload=8 ohm, gm*Rload+1=305. But a 600nF cpi then still has a comparable influence as a cmu of 600nF/305, or almost 2nF. The situation gets worse at lower load resistances.
To make matters worse, I think that in most practical cases in audio amplifiers the static base current required by bipolar output transistors is even larger than the capacitive currents. Of course you can solve that by using double emitter followers, at the expense of a more complicated high frequency behaviour of the feedback loop, but you could do the same in an amplifier with a MOSFET common-drain output stage as long as gate stoppers don't spoil the fun.
By the way, has anyone ever tried RC damping networks between gate and source instead of gate series resistors to stop parasitic oscillations in power MOSFET's? I mean connecting a resistor in series with a capacitor and then putting the whole thing between gate and source. I haven't tried it, but I have often used circuits like that in unstable RF IC's.
Yours sincerely,
Marcel |
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| subwo1 |
| Hello all again. I have not noticed mention of an advantage of using mosfets instead of BJTs as class A output devices. They are much more tolerant of operating at high temperatures than bipolar transistors. |
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| x-pro |
Hi all,
for 10 years I was making amplifiers with N-channel only MOSFETs in the output and really like these devices. Main reason is that the modern "switching" logic level MOSFETS, like BUK555-100 and later similar devices, are incredibly tightly matched inside one batch (less than 1% difference in capacitances, threshold voltage and transconductance). So you have nice symmetry for push-pull without spending much money (BUK555 was about $1 each) . The main advantage of the MOSFET ouput stage, in my view, is that it has low output impedance even without NFB, and does not change the load of the voltage amplifier as much as BJT output does with the load change.
Al |
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| Circlotron |
| quote: | Originally posted by x-pro
Hi all, for 10 years I was making amplifiers with N-channel only MOSFETs in the output and really like these devices. |
Hi x-pro. I like them very much too. would you like to post some info on your knowledge of them, particularly the gate drive / phase spltting circuitry needed to drive the lower mosfet. I think this kind of cct is becoming a dying art.
Right - back to the thread. |
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| x-pro |
Check the link in my post in the "Quasi-complementary Hexfet monstrosity" thread
Al |
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| Circlotron |
| quote: | Originally posted by x-pro
The main advantage of the MOSFET ouput stage, in my view, is that it has low output impedance even without NFB, and does not change the load of the voltage amplifier as much as BJT output does with the load change. Al |
Could this be a property peculiar to logic level fets? My experience with normal n-channel type switching hexfets in linear mode is that they have a fairly "soft" and squishy output impedance with voltage changing under load from zero to full maybe 5 to 10% depending on output level. This improves greatly though when you crank up the bias to something like Class A levels because the transconductance increases with current so the output impedance drops a fair bit. |
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| x-pro |
| quote: | Originally posted by Circlotron
Could this be a property peculiar to logic level fets? My experience with normal n-channel type switching hexfets in linear mode is that they have a fairly "soft" and squishy output impedance with voltage changing under load from zero to full maybe 5 to 10% depending on output level. This improves greatly though when you crank up the bias to something like Class A levels because the transconductance increases with current so the output impedance drops a fair bit. |
In my circuit I did measure less than 0.5 Om output impedance with about 100 mA idle. Logic level devices certanly have higher transconductance - it helps. However they also much easier destroyed by an overvoltage on the gate. Thankfully, in my design it never goes over 2xVth.
Al |
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| gwolf |
Al,
I never found a Spice model for the BUK (555, ...) devices. Do you have them?
Cheers,
Gerhard |
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| x-pro |
BUK555 are obsolete anyway. Try BUK9540-100A as a replacement
x-pro |
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| gwolf |
| quote: | Originally posted by x-pro
BUK555 are obsolete anyway. Try BUK9540-100A as a replacement
x-pro |
Yes, I know. But I still have a bunch of BUK555-100B in my drawer. But even from the BUK9540 I could find no usable spice model. Any idea?
Gerhard |
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