Bob Cordell Interview: BJT vs. MOSFET

[john curl]

MikeB, you are correct! I accidently used the 100ms line with that part, and it is comparable to the bipolars that we use, but do look at the IRF's Very bad, still.
We do use the 2SK1530-J201 pair for the output driver devices to drive the bipolars in the JC-1 power amp.

[MikeB]

Hi John, yes, the IRFs are not too impressive here... But the Toshiba mosfets are ! (Not to forget, the Toshibas are intended for power amps, the irfs are typically for switching)

Mike

[Bob Cordell]

Originally posted by john curl
Bob, I normally use fets everywhere, except at the very output. I have also designed power amps that are 100% fet, with Vfets, vertical mosfets, and lateral mosfets as output stages for different designs. I did this between 20 and 30 years ago.
This is what I found:
Vfets are now unavailable, but interesting devices.
Lateral mosfets are rugged, fairly linear, but have too low of transconductance to be optimum.
Vertical mosfets have much promise but they won't hold together when an amp is accidently shorted, at voltages over about +/- 35V.
In fact, most manufacturers do not recommend vertical power mosfets for linear operation these days. I wonder why?
IR makes a lousy power p channel mosfet.
The breakdown mode in mosfets occurs faster than second breakdown in a well designed transistor output amplifier, and requires at least as much protection as a transistor output amplfier.
I have been using an output stage that is a combination of vertical power fets as drivers and multiple pairs of ring emitter transistors. I use 9 pairs of output transistors in the JC-1 power amp. I can produce more than 800W into 4 ohms.
My slew rate, without an output coil, is more than 100V/us
I have made a ring emitter based power amp in the past that has single sided slew rate of greater than 500V/ us. I used a 2uH inductor to achieve this slew rate and still have it stable.
I run +/- 90V on my 8 amp driver vertical mosfets. Can anyone show me how these devices can be connected directly to the output, without blowing up the first instant someone accidently shorts the amp?
Inquiring minds what to know!

John,

Thanks for this detailed answer. I agree, when MOSFETs go, for whatever reason, they go fast. Hey, I've blown up a lot of them! My experience has been that they will usually not outlast a fuse; they definitely need some sort of last-resort protection against short circuits, such as an electronic crowbar. In some respects, in a short circuit condition, they may be their own worst enemy, since they will readily each attempt to deliver 50 amps or more; there is not much of a mitigating mechanism to reduce such high current flowing in the presence of a sudden short. I don't know how much of a role it mught play, but high-current beta degradation in bipolars might be an example of such a mitigating factor (as long as the driver transistor survives - in this sense, your choice of MOSFETs as drivers makes sense).

As far as device destruction goes, sometimes it is a chicken-egg situation, and it is hard to tell what the initial proximate cause of the destruction was. For example, often in conventional output stages the driver will die with the output transistor, and one doesn't always know which one went first. Another way in which MOSFETs will go very fast (fast devices blow faster?) is if for any reason the oxide is punctured. For example, in the case of a short circuit, if the driver successfully applies more than 20V forward bias to the MOSFET gate, poof!! That wasn't the fault of the MOSFET, but rather an ill-controlled driver. Similarly, if the application of the short circuit causes operating points or circuit parameters to shift enough to cause the MOSFET to go into a parasitic oscillation at 100 MHz, poof!! All I'm saying here is that we don't always know that the cause of the destruction was the die temperature getting up to beyond, say, 175C.

The p-channel devices are usually inferior to the n-channel devices, but what specifically do you especially not like about the IRF p-channel MOSFETs?

Here's a trivia question for you that I honestly don't know the answer to. Which came first, the HEXFET or the ring emitter transistor? Both were born of the same essential idea: use integrated circuit-like technology to effectively build a transistor that consists of hundreds or thousands or tens of thousands of little transistors in parallel.

I'm still interested in learning what your favorite difficult load is for evaluating your amplifiers, and what general type of output stage protection circuit you use.

Bob

[mikeks]

Originally posted by Bob Cordell

............ MOSFETs........definitely need some sort of last-resort protection against short circuits, such as an electronic crowbar.
Bob

Beg to differ: i would suggest a full-blown SOA protection regime is mandatory.

[Nelson Pass]

Originally posted by Bob Cordell
Here's a trivia question for you that I honestly don't know the answer to. Which came first, the HEXFET or the ring emitter transistor? Both were born of the same essential idea: use integrated circuit-like technology to effectively build a transistor that consists of hundreds or thousands or tens of thousands of little transistors in parallel.

It's a horse race - Hexfets in '78 and a patent application for
ring emitters in 1977.

[Bob Cordell]

Originally posted by andy_c


I certainly agree there. I just wanted to point out a caveat of using the NMOS and PMOS models for vertical MOSFETs.

In general, there's some really bad models floating around. The MJL3281A and MJL1302A models from ON Semiconductor have a number of problems that I've documented here. For the MJL3281A model, the simulated fT is low by a factor of six, and the beta vs current is all wrong.

In another case, the Fairchild models for the KSA1220A and KSC2690A have TF not specified, making it default to zero. This gives simulated fT values of about 1 GHz at midrange currents. A quick way to see fT in LTSpice is to do an operating point sim, then View, SPICE error log. There's lots of other handy info in that log for an operating point sim as well.

Andy,

Nice web site. Thanks for pointing this out. I too really like LTSPICE.

Bob

[Bob Cordell]

Originally posted by mikeks



This is :bs:

Nothing wrong with my ''skillset'', and i still think MOSFETs are inferior to modern BJTs in audio frequency applications.

Mike,

There is nothing wrong with the skillset of most of the people here. I continue to be impressed by everyone's intelligence. And there is nothing wrong with intelligent, well-intentioned people disagreeing. Indeed, I have always asserted that what influences an EE's design the most is what he or she FEARS the most. This is certainly the case with me, and most of the engineers I work with agree with this observation.

So it is not so much skillset, or intelligence or even diligence. It has a lot to do with experience with a particular thing. We pick our poison. We find our comfort zone.

Anyway, can you elaborate on why you think that MOSFETs are inferior to modern BJTs in audio frequency applications?

We ALL learn from these lively discussions.

Thanks,
Bob

[Bob Cordell]

Originally posted by anatech
Hi traderbam,
I have to agree with you with regard to your last comments, no offense Kanwar. I find that so far, I have to run mosfets pretty hot to get decent performance out of them, and they still don't sound right. Could be my skills, but I've also redesigned a few commercial mosfet output stages to use bipolar types and have ended up with better sound quality and lower distortion every single time. I am talking about vertical type mosfets here.

Bob, I think your error correction approach works well and is necessary for mosfets. I am trying to adapt the idea to a bipolar design I've been working on. I think it holds great promise. I really ought to read your papers on this.

-Chris

Chris,

I agree, there are a PLETHORA of really bad MOSFET designs out there. As is often the case with any "new" technology there are those numb-nuts who ignornatly exploit it because it is the latest buzz-word (digital, anyone?). There are also those ignoramuses (sp?) who see that the beta of a MOSFET is infinite and think that they therefore can make a good amplifier without using a driver stage between the VAS and the output MOSFETs.

Bipolar transistors suffered from these same sorts of things for years in the early days (indeed, some still do).

Bob

[ilimzn]

Originally posted by MikeB

2SC5200---2.2A
MJL4281---4.5A
2SC2922---4.5A
2SK1530---5A
Looks quite comparable, BJT <-> Mosfets
Mike

And I think that should have been 2SC3264 in John's post (not 2SK).

There is substantial difference between MOSFET technologies. For instance, take IRFP240 (keep in mind this is in a smaller case compared to the bipolars mentioned, and specified 150W at 25C), we have at 100V / 10ms:
Original IR: 3A
Fairchild IRFP240B (Specced 180W @25C): 4.7A
Interestingly:
IR IRFP140: 3.1A
IR IRFP140N: 2A (it is specced 140W though).
It should also be noted that IR does not publich DC SOA for their parts at all (but Fairchild does). I'm not even going to begin comparing prices for 2SC3264 (which is no doubt an excellent BJT) and the lowly IRFP parts.
You could look at this in another way: make a single pair output stage. Take nearly any sensible device and you are limited to about +-40V rails calculating for a nominal 8 and actual 4 ohm load. Then, compare taking care of the price bracket, and you will find that it is very similar - in fact, although HEXFETs sometimes get a fraction of an amp ahead, in either case, you are well advised to provide some sort of proper SOA limiting. Basically, comparing IRFP240 and 2SC5200 and similar devices is a fair comparison.
That being said, I have never had a current limited MOSFET not survive a fuse (sensibly chosen, of course!), but I do take care of one thing: avoiding gate punchthrough either by oscillation of overvoltage. The failure rate of HEXFETs in designs where this is not assured, is in my experience 100%.

[john curl]

What is important is what works. Sometimes, a Japanese part will be expensive, but since I have my power amps made in Taiwan, the parts are what the Taiwanese select as what is easily available. Fairchild (IR) devices are superior, and what I normally use for audio applications.
I am happy that the Toshiba 2SJ201 parts are really better than I first thought. That leaves it easier for me to use them in future.
I would be careful however, I have been 'burned' before.