Bob Cordell Interview: BJT vs. MOSFET - Page 12 - diyAudio
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Old 23rd November 2006, 10:50 PM   #111
GK is offline GK  Australia
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First, tell us more about your amplifier. What is its rated power into what load? Is it Class AB, G, or H, or something else? What is its rated THD-1 and THD-20 at full power? Is it specified to deliver 1/3 or 1/8 power continuously into its rated load without overheating? I am familiar with high-power amplifier design, having been involved in the design of the Crest 8001 (bipolar, Class-H).


Iíve designed and built various bipolar A, AB designs from 500-5000W with THD levels well below perceptible limits. These are not mass produced units, but mostly one-off designs either for my own use or for specific applications.


Your point about the bias current for MOSFETs vs bipolar is well-taken, one of the reasons I designed my MOSFET amplifier with error correction back in 1982. The EC almost eliminates the problem of MOSFET transconductance droop in the crossover region. You can definitely get away with 20-50 mA of bias on a BJT if you're willing to accept only fair sonics. At 150 mA bias, my MOSFET amplifier did 0.02% THD-20 into 8 ohms without EC.


Iím sorry, but I donít agree with your assertion that a BJT amplifier biased to 20-50mA per device will only deliver ďfair sonicsĒ. Thatís just plain absurd. The THD figure you quote isnít particularly difficult to achieve in non-EC amplifiers with bipolar output stages and ultra low THD figures at 20kHz are superfluous anyway. If you can hear the second or third harmonic of 20kHz then youíre doing better than me.


The Public Address, Pro Audio and Audiophile markets are very different. In the latter, it is quite common for high-end amplifiers to operate in Class AAB, with output devices somewhat over-biased, for best sonics. There is less concern about idle dissipation in the audiophile arena. At the same time, there is very much more concern for high-frequency linearity.


OK, but I donít design for the esoteric audiophile market. Iím interested in producing efficient cost effective designs. Heat dissipation must be kept to a minimum and in that regard BJTís with their much lower bias current requirements and ability to run at high temperatures without throttling back rule supreme.


Please define better what you mean by compression distortion. I don't believe my MOSFET amplifier suffers from it, be it from reduced transconductance or not. At high currents, gm of MOSFETs is plenty.


I didnít say that the gm of Mosfets wasnít plenty for linear amplification, I was making a comparison to BJTís which have a much higher transconductance and therefore suffer from a particular distortion mechanism to a lesser degree than Mosfets devices. In source follower configurations the instantaneous output voltage is equal to the voltage at the gate minus the gate to source voltage, which for some devices can become significantly non linear at higher currents.


The MJL21193 transistors you mention are very slow devices by today's standards, with an ft of only 4 MHz. That is adequate for PA applications, but certainly not for high-end audio.


I didnít cite the MJL21193 as the best audio transistor out there, but as a unit with comparable robustness, as that is what you had asked for.


They are also not less expensive than IRFP240s. I don't know where you got your numbers, but at DigiKey, the MJL21193 is $3.78 in QTY 100, while the IRFP240 is only $1.71.


Things are obviously better where you are. I was comparing it to the TO3 IRF240, which from an Australian supplier costs approximately 50% more than both the MJ21193 and MJL21193. I didnít get a price on the IRFP240.


It's unfortunate that the MJL21193 is not rated for SOA at 10 ms, which is what is the more important number for audio.


The data sheets only provide limited information. That does not mean the MJL21193 will not withstand much high dissipation levels for brief 10mS bursts. I havenít subjected one to destructive testing, but going by itís robust 1 second rating I doubt that it would be much less able, if at all, than the IRF240 device.


Cheers,
Glen
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Old 23rd November 2006, 11:15 PM   #112
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Quote:
Originally posted by Bob Cordell
Among all these power MOSFETs, one of the things one needs to understand is that they ALL start out with a positive temperature coefficient of drain current at a given gate voltage. BUT, they all eventually have that temperature coefficient eventually go through zero and then negative. The main difference is at what value of drain current they do so.
Bob
I really hate it if a control loop switches direction somewhere in the middle..

regards, Gerhard
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Old 24th November 2006, 04:29 AM   #113
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At 150 mA bias, my MOSFET amplifier did 0.02% THD-20 into 8 ohms without EC.




Bob, Just to further elaborate on the results that can be achieved with bipolar output stages, I provide a link to a rather basic, albeit refined Douglas Self design that does not use error correction and is of comparable power specification to your MOSFET design:


http://www.dself.dsl.pipex.com/ampins/dipa/dipa.htm


It produces less than 0.02% THD at 40kHz delivering 40W into 8 ohms.
I have never published my designs, but I have achieved similar results using paralleled MJL21193/MJL21194 complementary pairs at 15 times this power figure - devices which you assert are not suitable for high-end audio.

Cheers,
Glen
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Old 24th November 2006, 04:47 AM   #114
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Quote:
Originally posted by G.Kleinschmidt
[If you can hear the second or third harmonic of 20kHz then youíre doing better than me.
I can hear intermodulation between 20 KHz and 20100Hz well.
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Old 24th November 2006, 04:54 AM   #115
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Wavebourn:
I can hear intermodulation between 20 KHz and 20100Hz well.



OK, but how well can you hear the intermodulation products between the 2nd harmonic of 20kHz and 20100Hz?
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Old 24th November 2006, 05:23 AM   #116
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Quote:
Originally posted by G.Kleinschmidt
Wavebourn:
I can hear intermodulation between 20 KHz and 20100Hz well.



OK, but how well can you hear the intermodulation products between the 2nd harmonic of 20kHz and 20100Hz?
Do you mean how I can hear products with 100 and 200 Hz frequencies?
I don't know.

Also, I can distinguish natural sibiliant consonants from distorted.
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Old 24th November 2006, 06:15 AM   #117
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Wavebourn:
Also, I can distinguish natural sibiliant consonants from distorted.




Man, I don't even know what sibiliant consonants are, but that sounds impressive. I can identify between the 400Hz and 1kHz test tones of my RF amp test rig without looking at the frequency switch.....usually.

Am I similarly gifted?
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Old 24th November 2006, 08:18 AM   #118
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Quote:
Originally posted by G.Kleinschmidt
At 150 mA bias, my MOSFET amplifier did 0.02% THD-20 into 8 ohms without EC.




Bob, Just to further elaborate on the results that can be achieved with bipolar output stages, I provide a link to a rather basic, albeit refined Douglas Self design that does not use error correction and is of comparable power specification to your MOSFET design:


http://www.dself.dsl.pipex.com/ampins/dipa/dipa.htm


It produces less than 0.02% THD at 40kHz delivering 40W into 8 ohms.
I have never published my designs, but I have achieved similar results using paralleled MJL21193/MJL21194 complementary pairs at 15 times this power figure - devices which you assert are not suitable for high-end audio.

Cheers,
Glen

Another thing which is worth noting is that in this design D.Self used the now rather old hat Bipolar MJ802/MJ4502 complementary pair - transistors which only have an fT of 2MHz - half that of the MJL21193/MJL21194 complementary pairs 4MHz fT. The MJ802/MJ4502 transistors also have quite inferior hfe Vs collector current linearity.
In my designs using the MJL21193/MJL21194 pairs I run larger emitter resistors and a bias current of 40-50mA.


Cheers,
Glen
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Old 24th November 2006, 03:28 PM   #119
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Old 24th November 2006, 04:03 PM   #120
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Hi, Bob Cordell,

What is your opinion about quasi complementary output stage (that uses all NPN devices)?

It can be seen that the positive part is common collector, while the negative part is common emitor usage mode of output transistors, so it is not balanced between positive and negative half.

What is the merit and drawback of this quasi complementary output stage?
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