Bob Cordell Interview: BJT vs. MOSFET - Page 4 - diyAudio
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Old 15th November 2006, 12:44 PM   #31
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Quote:
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.

-Chris
I have to disagree with you on this Chris....
Again its dependant on the designer itself..........
Think about SKA[remember Greg whom you guy's banned] its much superior than Bipolar designs on this forum
Think about Anthony's Design work AV1000/800, very much appriciated in VFET's amps....
And now again BOB Cordell design with VFET's and you guy's have hard time to digest it...............
No offense Chris
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Old 15th November 2006, 12:52 PM   #32
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Cds

Kanwar - this is good. A healthy debate makes the forum more interesting.
I agree that FETs are better on reliability and some ease of low frequency drive and such. That's why they are so popular in SMPS apps.
I can't say how many better designs there are that forum members have developed than the ones you've quoted among forum members...can you?
I wouldn't be so bold as to assume the designs you mentioned are the best nor that they cannot be bettered without also using VFETs.
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Old 15th November 2006, 12:56 PM   #33
mikeks is offline mikeks  United Kingdom
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Originally posted by Workhorse


...provided that the designer has required skillset to implement the vertical Mosfets in an excellent manner in his/her designs




K a n w a r

This is :bs:

Nothing wrong with my ''skillset'', and i still think MOSFETs are inferior to modern BJTs in audio frequency applications.
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Old 15th November 2006, 01:01 PM   #34
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Quote:
Originally posted by traderbam
Cds

Kanwar - this is good. A healthy debate makes the forum more interesting.
I agree that FETs are better on reliability and some ease of low frequency drive and such. That's why they are so popular in SMPS apps.
I can't say how many better designs there are that forum members have developed than the ones you've quoted among forum members...can you?
I wouldn't be so bold as to assume those designs are the best nor that they cannot be bettered without also using VFETs.
Yeah Forum needs to be Healthy with Good Debates....

For VFET there were few like SKA, AV800, ALEPH[Nelson Pass-Pass labs]
For BJT there are numerous.....
BJT amp is much easier to design than VFET amp for a DIYER's to build ..dont you think so...




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Old 15th November 2006, 01:03 PM   #35
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Originally posted by john curl
That is a good idea, Wavebourn. I was tempted to use something like that for a PA power amp. It has the problem of a mismatch in impedance, at both input and output, which could cause some real problems. Still, it can be an interesting approach.
Personally, I use FET's when they will allow the power output that I need to achieve. Most vertical FET's won't handle difficult loads or a short circuit, for more than an instant.

John,

Your comments above about difficult loads and short circuits got me to thinking. You've obviously had a lot more experience than most of us building reliable, bullet-proof commercial amplifiers. I have two questions about your experience that I'd like you to share with us the answers on if you are comfortable with it.

First, when you are building and testing a new amplifier, what is your favorite difficult load and how do you torture test your amplifiers?

Second, what kind of protection do you use in your amplifiers? I understand the details may not be something you'd like to share, but I'd still be interested in knowing the general types of protection you use, e.g., just fuses, crowbar based on peak or short-term current, V-I, etc.

Bob
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Old 15th November 2006, 01:52 PM   #36
anatech is offline anatech  Canada
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Hi Kanwar,
I've no doubt that mosfets are the best for your application. I also think that for what you would consider flea powered amplifiers (100W +/- 50 W) that bipolars might be the way to go unless some error correcting circuitry is used. So it's more a case of using the correct tool for the job at hand.

You must admit that you design amplifiers that are not suited to the average living room. They are concert sized.

-Chris

Edit: Good article Bob, thanks for pointing it out Jacco.
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Old 15th November 2006, 02:48 PM   #37
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BJT amp is much easier to design than VFET amp for a DIYER's to build ..dont you think so...
I have designed with both types . I guess it depends what you mean by "easier" and how capable the DIY'er is. Do you mean easier to get good sound quality or do you mean ease of getting it to work reliability?

The first ever circuit I built was a MOSFET circuit out of some magazine...it was very easy to build and make functional because of few parts and the bomb-proof nature. For novices I'd say a FET design is easier. I'd say this ease is also a reason why many diy kits use FETs. But Hugh Dean uses bipolars, I believe.
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Old 15th November 2006, 03:15 PM   #38
ilimzn is offline ilimzn  Croatia
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Originally posted by Wavebourn

Click the image to open in full size.
Now this idea, I like
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Old 15th November 2006, 03:18 PM   #39
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That's one way to settle the argument. Very witty. Perhaps you'd better throw a tube in there just in case.
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Old 15th November 2006, 03:22 PM   #40
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Originally posted by andy_c


Please be aware that the SPICE models named NMOS and PMOS, levels 1, 2 and 3 assume CGS, CGD and CDS are constant with VGS, VGD and VDS respectively. These models were not meant for vertical power MOSFETs. International Rectifier provide some subcircuit-based models for the IRFP244 and IRFP9240 that use diodes to approximate the junction capacitance variations with varying voltage. I haven't compared their data with the datasheets carefully yet though. LTSpice has a model specifically for vertical power MOSFETs called VDMOS that models the variations of CGD and CDS with VGD and VDS respectively, and assumes CGS does not vary with VGS. I'm in the process of doing some model extraction for some vertical MOSFETs that uses the Excel solver to do a best-fit match for CGD and CDS vs VGD and VDS respectively. So far, I've done the Fairchild fqp2p25 and fqp3n25 TO-220 devices. I hope to do the International Rectifier IRFP244 and Fairchild fqa12P20 TO-3P devices soon.

Andy, this is a good point, and the results of your investigation should be valuable.

Although these capacitances change with the voltages, I don't think this changes the overall conclusion about speed of vertical MOSFETs vs bipolars.

A good example is Cgd as a function of Vds. It can get fairly large, up to about 1000 pf at very low Vds below 10V, but it gets quite small at higher Vds, e.g., about 50 pF at 100V. So it only gets bad when the output is near the rails, and it gets better fast as you turn the transistor off. Collector-base capacitance in a bipolar behaves in a similar fashion. Anyway, we certainly need to take these variations into account, but I'm not sure they change the big picture that much. Keep us posted on your simulation results. I'm particularly interested in what you find in regard to Cgs vs Vgs, as IR does not seem to explicitly show this. They focus mostly on capacitance variation as a function of Vds.

The main points I believe we need to keep in minds are:

1) what is the small-signal bandwidth of the MOSFET source follower as a function of output voltage, and how does that compare to a bipolar?

2) what is the small-signal bandwidth of the MOSFET source follower as a function of output current, and how does that compare to a bipolar?

3) How much current does it take to turn a MOSFET off at a given rate of change (amps/us) under the different voltage and current conditions, and how does that compare to a bipolar?

Cheers,
Bob
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