Bob Cordell Interview: BJT vs. MOSFET

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#1,861
Hi John,
After seeing Dr. Lipshitz at an AES meeting, I was struck by how much he seemed to enjoy loosing people with the math. Never the subject or understanding, only the math.
For example, in the '70's Dr. Lipshitz et al used a Shure V15 (of the time) to measure slew rate. I proved in my IEEE paper that this was ridiculous, and I had given him a copy of my paper, so there was really no excuse.
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One wonders why they didn't use real lab equipment with known characteristics. Even the readings would have been easier to take. Another case of a flawed experiment?

-Chris
 
#1,864
john curl said:
I just spoke to Ed Oxner. Yes, the parasitic transistor is still very important and they even test for its potential effect at Siliconix.

If you are of the opinion that the parasitic transistor is not very important, then you should show some evidence to prove it to be as you think, not just dismiss it.
You should talk to some real mosfet designers, like Ed Oxner, and get the inside info.
You never know, we all might learn something.
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Hi John,

I have been in touch with Ed Oxner, and he was kind enough to give me a very detailed answer to my questions and also to provide a Siliconix app note, AN89-6 on unclamped inductive switching.

He confirmed my understanding of what he was saying in his other paper, namely that the parasitic bipolar transistor is not a problem for audio applications.

The reason it is important, and that they continue to test the parasitic transistor, is for Unclamped Inductive Switching (UIS) withstanding capabilities of FETs. This comes into play in, for example, motor drive and relay drive applications. In these applications, the FET may be drawing a lot of current through an inductive load, and then suddenly switch off. The inductor will try to maintain this current, producing a voltage spike that will drive the MOSFET into avalanche via its built-in zener diode, which is designed to handle currents on the order of the maximum rated Id of the MOSFET (or more), with a breakdown somewhat in excess of the Vds rating of the device.

Under conditions of very high peak UIS current, there can develop enough voltage across the parasitic transistor base resistance to turn on the parastic bipolar device, which will result in destruction. This is why they test the parastic device for its Rb, beta and Vbe (at a separate test site on the wafer).

These UIS conditions do not occur in a properly designed audio amplifier, even with inductive speaker loads (think about the push-pull topology and what the built-in drain-source body zener diodes do), so this parasitic transistor won't turn on.

Moreover, at frequencies below 80 MHz, the parasitics of the parasitic transistor are already adequately accounted for in the specified drain-source capacitance.

No problem for audio amplifiers here.

Cheers,
Bob
 
#1,872
john curl said:
The same thing happened to me, decades ago with worst case dV/dt in audio circuitry.
People often used the wrong phono cartridge, microphone, or other input source, and would then establish a slightly to ridiculously low slew rate criterion. For example, in the '70's Dr. Lipshitz et al used a Shure V15 (of the time) to measure slew rate. I proved in my IEEE paper that this was ridiculous, and I had given him a copy of my paper, so there was really no excuse. Oh well! Some folks might use a bandwidth limited transformer with an MC cartridge and get less than worst case results.
Much of the criticism of Otala was based on these half baked efforts to find true worst case for all practical conditions.
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Hi John,

Was that your IEEE paper in which you demonstrated high slew rates coming from a moving coil cartridge when it was in severe mis-tracking?

If so, I honestly think that that was as misguided as the work of those using the Shure V15 as the input source.

Worrying about a little bit of TIM when your cartridge is severely mis-tracking is like worrying about your car's headlamp alignment after having had a head-on collision 🙂.

Although the slew rate of real sources is certainly relevant and of interest to any TIM discussion, quibbles about real slew rates were the least of the reasons that Otala got roundly hammered by professionals and academics alike.

The first major reason why he got hammered was his steadfast refusal to acknowledge that all that he was talking about was distortion resulting from the onset of slew rate limting, even after that was brought to his attention.

The second reason he got hammered was for wrongly asserting that it was large amounts of negative feedback that led to TIM (and, of course, too-low slew rate).

The third reason he got hammered was for wrongly asserting that low open loop bandwidth led to TIM (and, of course, too-low slew rate).

Although situations can be contrived where these assertions may be true in a limited context, designers nowadays know that these assertions are simply not generally true, and that extremely low TIM can be achieved in amplifiers with extraordinarily high amounts of NFB and very low open-loop frequency response corners.

Cheers,
Bob
 
#1,873
Hi Scott. I still have that computer simulation that you sent me back in the early '80's, emulating DA in caps using your differential subtraction method. I sure could have used SPICE in those days, although I did get an early version of Microcap that did about the same thing about that time.
Personally, I'm not against SPICE, but OVERDEPENDENCE on it over real circuits. Some of these guys couldn't mow a lawn without a SPICE emulation first. 😉
I just might be able to get LT Spice running today, as an old schoolmate, Mac-PC expert, and former Analog Devices emplyee as well, is visiting me today. IF I could just get my PC-Mac emulator going properly, I would be OK.
I just like to tease these guys, because they tend to depend on it so much.
 
#1,874
Bob, I tend to design my audio products for REASONABLE WORST CASE. This is not typical, BUT realistic worst case reproduction.
Think of the difference in design between a Renault Dauphine and a Porsche, even from the same time period.
The Porsche is designed to go into a controlled skid, and a Renault is designed to roll over.
You or I cannot control how the car is used, but I would rather skid, than roll.
This is why I found REAL RECORDS available at the time mistracking REAL phono cartridges at their specified tracking weight in a good arm. THEN, and only then, I took snapshots of the action, and analyzed it.
How about putting your car in a controlled skid. Ever done it? Did it roll over?
Audio design must also incorporate cartridge mistracking and handle it well. If that is done, it can be barely noticable. You might be surprised how the annoyance factor of ticks on a record is reduced as well. It is my job to make quality audio products, not typical ones.
 
#1,875
john curl said:
This is why I found REAL RECORDS available at the time mistracking REAL phono cartridges at their specified tracking weight in a good arm. THEN, and only then, I took snapshots of the action, and analyzed it.
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If anyone still has the old Telarc OMNIDISK you will find in the liner notes a comment to the effect, "the horrible crackling sound you hear when playing the 5k and up 1/3 octave noise bands is normal".
 
#1,876
Scott, my job in life is to reduce that crackling sound to something managable and hopefully ignorable. I used a Sheffield #3, direct disc with a piece called 'The Perfect Song'. Did it every time!
I didn't know that you knew Dr. Lipshitz. You might remember that Walt sent the WORST tantalum caps that he had ever found, to Dr. Lipshitz and they could not detect a difference. Oh well!: :apathic:
 
#1,877
john curl said:
Congratulations, Bob! You have discovered the 2SJ201 !!!





:bigeyes:
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Thanks, John. I'm proud to be an old dog who can still learn new tricks. These do look like nice parts.

Where do you get your 2SJ201's from?

BTW, have you used the ON ThermalTrak devices yet? Even as a transistor apart from the thermal tracking diode, they look better overall than the Sanken RETs. Don't go by what it says in the spec sheet table; look at the actual typical beta vs IC curves and the ft vs IC curves.

High-current beta falloff appears about the same, but Beta appears a tad flatter at lower currents.

Ft at 1 amp of the Sanken 2SA1295 is typically 30 MHz, while the NJL1302D is at about 41 MHz. The typical ft peak of the 1295 is 38 MHz at 4A, while that for the 1302 is 45 MHz at 4A. They are about the same at 6A, then beyond that the 1295 is better. At a low typical idle bias current of 100 mA, the 1295 is only at 12 MHz, while the 1302 is at 17 MHz. As always, there are tradeoffs.

Cob at 10V is about the same for both devices, at about 500 pF.

SOA at 10 ms, 100V appears comparable at about 3.5A.

Also, they have the friendlier TO-264 package (BTW, is it hard to get the insulators for those Sanken two-screw packages?).

What do you think of these ThermalTrak devices?

Cheers
Bob
 
#1,880
Re: Re: Toshiba 2SJ201 & 2SK1530

andy_c said:


Hi Bob,

Did you mean the 2SK1530? The models for these devices were discussed earlier in this thread here . I haven't done extensive verification of these, but I have done some capacitance sims. Also, in private email to Edmond, who came up with the original models, I sent him some plots of Cgd vs Vgd. These are in Excel spreadsheets attached to this post.
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Hi Andy. Yes, I meant the 1530. Thanks! I had forgotten that part of the thread.

Cheers,
Bob
 
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