Bob Cordell Interview: BJT vs. MOSFET - Page 5 - diyAudio
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Old 15th November 2006, 03:36 PM   #41
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Quote:
Originally posted by traderbam
That's one way to settle the argument. Very witty. Perhaps you'd better throw a tube in there just in case.
You are right. Tube drives this device. 12L6 in triode connection loaded by transistor CCS.

Speaking of testing, it's pitty that I did not take a picture of evaporized tip of scredriver and binding posts. Also, fuses throw glass peaces inside of the amp.
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Old 15th November 2006, 03:45 PM   #42
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Quote:
Originally posted by ilimzn


Now this idea, I like
Thanks.

Here is the old version, without tubes. 3-step approximation of transfer function.

Click the image to open in full size.
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Old 15th November 2006, 03:45 PM   #43
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Quote:
Originally posted by Bob Cordell
Although these capacitances change with the voltages, I don't think this changes the overall conclusion about speed of vertical MOSFETs vs bipolars.
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.
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Old 15th November 2006, 05:07 PM   #44
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Quote:
Originally posted by traderbam
I've seen many people use FETs because their assessment of the parameters of importance to sound quality are superficial, often due to bad advice from "experts", and they end up with sub-optimal sound.
Heresy, and please don't bring what it sounds like into the discussion.
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Old 15th November 2006, 05:35 PM   #45
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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!
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Old 15th November 2006, 06:09 PM   #46
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Quote:
Originally posted by traderbam
Bob,
My comment was "comparing fts is not valid".
You mis-interpreted this as meaning ft is not a valid speed measure of a FET. Which is not what I said.
The speed of a FET is related to Cgs and gm, which is related to its ft. For a given ft, the speed of a bipolar is related to gm and beta. So you miss a trick if you compare only on ft.
I don't think your emitter/source follower simulation is apt. Perhaps if you compared the output Z of the two followers. It may be necessary to have a 50 ohm gate resistor for a FET but no series resistance is needed for a BJT.
Of course SMPS use power FETs because they make reliable and efficient switches. A linear amplifier has different requirements.
Brian
Brian,

Sorry if I mis-interpreted what you said.

I guess I shouldn't be surprized that my mention of differences between MOSFETs and bipolars sparked a big debate, but I'm frankly surprized that my assertion that MOSFETs are faster than bipolars was so controversial. Help me to better understand your point, and let me clarify my position and try to better understand yours.

If I read you right, you agree that ft is an appropriate measure of speed for a MOSFET. I think you also agree that the ft of a MOSFET is quite a bit higher than that of a ring emitter bipolar (on the order of 300 MHz vs 30 MHz under a reasonable set of operating conditions, give or take a factor of two).

What it sounds like you are disagreeing with is the assertion that ft is an appropriate measure of the "speed" of an output device for an audio power amplifier. Am I correct so far?

I need to better understand what your preferred metric for relevant speed is.

I believe that, regardless of which device type we are talking about, we need to look at small-signal bandwidth of the output stage in its circuit environment, and we need to look at the large-signal turn-off rate of the device in its circuit environment. These two may be different, but they are both important. Although they are different, they are usually related, since device capacitances influence both of them.

In the small-signal simulation I suggested, the 50 ohm resistor was not put there for stability reasons, but rather to assess the transfer bandwith of the stage under a reasonable set of conditions. Driving both devices with a voltage source, for example, would give a meaningless result. There is no VAS-driver combination that I know of that has a zero output impedance. Having the 50 ohm resistor there allows the a.c. current gain of the device to properly come into play in the analysis. I'm OK with it if you disagree with this approach to asessment of the small-signal speed, but tell me what you propose as an alternative and why, so I'll be able to understand and evaluate your point.

For large-signal speed, what I care about most is being able to turn the device off fast enough, especially in the crossover region. For bipolars, this means sweeping out the minority carrier charge which is incrementally represented by the hybrid-pi base-emitter capacitance, which is in turn determined by the ft of the device. For a bipolar, the available turn-off current is usually set by the standing current of the driver, and will typically be between 10 mA and 50 mA (assuming no cross-coupling capacitor tricks, for now, to keep things simple).

So to me, the amount of reverse base current needed to take the device from one amp down to 0.1 amp is a good metric. This is basically how long it takes the reverse base current to discharge the average base-emitter capacitance, over this current range, by 60 mV. For a 30 Mhz ft bipolar starting at 1A, the Cbe is 0.2 uF and ends up at 0.02 uF when we reach 0.1A. A working average of 0.1 uF is probably reasonable for back of the envelope stuff. With a 10 mA reverse base current, we end up with about 600 ns.

For a MOSFET with the same available turn-off current, it is the time it takes to move the gate by about 700 mV against the Cgs and Cgd capacitances. If those capacitances add up to about 1300 pf average over that 700 mV Vgs range, then we get about 90 ns. That is why I think MOSFETs are faster. If I've screwed up here, please let me know how. It certainly wouldn't be the first time I screwed up a back-of-the-envelope calculation.

One thing that you said I would like you to explain better. You said, "for a given ft the speed of a bipolar is related to gm and beta. So you miss a trick if you compare only on ft." What is the trick we are missing? It sounds like you are somehow concerned about f-beta affecting speed for a given ft. This leaves only beta affecting speed, since f-beta ~ ft/beta. But beta has very little influence on how long it takes to pull the minority carrier charge out of the base. Are you perhaps suggesting that you want lower beta for higher speed, like we did in the old days when we used a little bit of gold doping in bipolar digital ICs to kill the minority carrier lifetime? Help me out here.

Finally, I completely agree that, in the absence of error correction, you have to run MOSFET output stages significantly hotter than what you can get away with for bipolars. That is a result of the big drawback of MOSFETs called transconductance droop. That is probably the single biggest reason I pursued the error correction scheme. In fairness, however, recognize that there are many very high-quality bipolar amplifiers that run their output stages hot also, in Class A-AB, so they are in Class A up to perhaps a couple watts.

Cheers,
Bob
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Old 15th November 2006, 06:58 PM   #47
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Hai Bob,

Thanks for sharing so much with us. Just glanced through your website and the Mosfet amp there.

Could you tell us how it sounds in comparison with other popular amps. Could you also give us more modern parts that could be employed in your design.

Thanks again,
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Old 15th November 2006, 07:27 PM   #48
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Folks, if you want to understand why power vertical mosfets tend to break with higher voltages, just compare the 10ms safe area of the best high speed bipolars to any other pair of practical vertical mosfets.
What I have found: 10ms-100V

2SK3264---5A
IRF130----1.7A
IRF140----2.8A
2SJ201-----3A
Please show me a better vertical power mos pair!
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Old 15th November 2006, 07:29 PM   #49
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Quote:
Originally posted by john curl
In fact, most manufacturers do not recommend vertical power mosfets for linear operation these days. I wonder why?



Article on power MOSFET reliability referring to linear applications
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Old 15th November 2006, 07:41 PM   #50
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Quote:
Originally posted by john curl
Folks, if you want to understand why power vertical mosfets tend to break with higher voltages, just compare the 10ms safe area of the best high speed bipolars to any other pair of practical vertical mosfets.
What I have found: 10ms-100V

2SK3264---5A
IRF130----1.7A
IRF140----2.8A
2SJ201-----3A
Please show me a better vertical power mos pair!
I don't understand...

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

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