Bob Cordell Interview: BJT vs. MOSFET

[GK]

Originally posted by Wavebourn


We don't need to be musically talented to understand that length of piano's legs matter less than other parameters, right? And suppose, the whole industry is working hard to create perfect legs for piano because people think that length of piano legs impact significantly on sound quality because it have to stay strictly horizontally...

...and if ignorant in perfect piano design people subjective evaluate and say that piano with bad legs sounds better, shall we show them measurements of legs, or try to figure out what parameters are more significant to measure?

Umm.....0k. To me, it's easier to determine which of two power amplifiers, one with a 20kHz THD of 0.01% and the other with a 20kHz THD of 0.001%, is more "perfect" in terms of THD performance than the other by testing them on a distortion analyser than listening to them.

[Wavebourn]

Originally posted by G.Kleinschmidt



Umm.....0k. To me, it's easier to determine which of two power amplifiers, one with a 20kHz THD of 0.01% and the other with a 20kHz THD of 0.001%, is more "perfect" in terms of THD performance than the other by testing them on a distortion analyser than listening to them.

What happen to measured differences on low output power, if they were measured on high power? How capacitive character of load change them?

Why BJT, MOSFET, tube outputs with the same THD level on relatively high power sound so different on low power?

The question was, if such non-significant according to measurements differences are audible....

[lineup]

Originally posted by G.Kleinschmidt

Umm.....0k. To me, it's easier to determine which of two power amplifiers, one with a 20kHz THD of 0.01% and the other with a 20kHz THD of 0.001%, is more "perfect" in terms of THD performance than the other by testing them on a distortion analyser than listening to them.

Hello all readers and music lovers!

I have read at several different sources that
0.1% THD is at the limit to human hearing.

This means both those amplifiers are perfect
by THD criteria only
at a specific level, at a specific test frequency and with a specific test load
and no one will truly be able to hear anything different under those specific circumstances

When we talk levels of 0.50% THD or more, in relation to one other 0.01% THD amplifier
we can most probably say one is more HiFi, high fidelity,
( in true meaning of word, which is like: more accurate in signal reproduction )
than the other

--------------

Note!
High Fidelity does not necessary mean sounding better, a more pleasing sound.
It means you get output in accordance what you input.
For better or for worse.

--------------

Say an artist produce a recorded song, which is very dissonantly mixed and produced
and both lyrics and musical sound is intended to express a worrying feeling - discomfort.

Now some NOT HiFi amplifier may add some even harmonies,
2nd 4th harmonics into this song.
This would destroy the intended artwork in its original form.

An HiFi, High Fidelity, amplifier is fidel, =true, to the original at such level
it is impossible to tell difference from original recording by ears.

-------------

A very good sounding amplifier may well be Not HiFi - high fidelity,
but InFi - infidelity, which also is called LowFi - low fidelity.
But this infidelity is such, that in most listened kinds of music, it is more pleasing.
Easier to your ears and mind.

-------------

Finally, we can not judge either HiFi or LowFi amplifiers as being all bad.
As long as anyone likes one or the other
and wants to use them, it is good to those who enjoys them.

Taste is difference, thinking is different and isn't this very good and interesting thing!
Indifference would be extremely boring
One might just as well go put an end to such an excuse for a life.


thinks lineup you may think totally different

[GK]

Originally posted by G.Kleinschmidt
Bob:
Glenn, If you've designed amplifiers up to 5000W in Class AB, rather than H or G, it seems inconsistent that you would be worrying that much about idle bias power consumption.


Glen:
Absolutely not! Had I designed the same amplifiers with MOSFET’s in the output stages the dissipation would have been a great deal higher. It’s my very choice of class AB instead of the more efficient G or H configurations that has dictated my need to use BJT’s - the more efficient output devices.


Bob:
Unless your amplifiers are not rated for continuous operation at 1/3 power or even 1/8 power, the dissipation of the amplifier under those conditions is far more than that of idle bias.


Glen:
I’m sorry, but that’s just plain wrong. A quick example with some basic calculations to put things into perspective - a 1kW RMS in 4 ohms AB amplifier would be required to deliver a peak voltage of SQRT(1000*4*2) = 89V with a peak load current of 89/4 = 22A. The average haft sinusoid current for one half of the output stage working at full output power is equal to Ipeak/pi = 22A/3.14 = 7A.
Lets assume this amplifier was made with a MOSFET output stage with 10 paralleled output pairs biased at 200mA each. That would give an idle current of 10*0.2A = 2A.

That’s an idle current of 2A Vs 7A average current when delivering full output power.

At 1/3 power output, the peak output voltage is equal to SQRT((1000/3)*2*4) = 51.6V, giving an average half sinusoid current for one half of the output stage of (51.6/4)/pi = 4.1A

Now we’ve got an idle current of 2A Vs 4.1A average current when delivering 1/3 of full output power.

At 1/8 power output, the peak output voltage is equal to SQRT((1000/8)*2*4) = 32V, giving an average half sinusoid current for one half of the output stage of (32/4)/pi = 2.5A.

Now we’ve got an idle current of 2A Vs 2.5A average current when delivering 1/8 full output power.

As you can deduce from these figures, the 1/8 power dissipation is by no means far less than the idle dissipation by any stretch of the imagination. Incidentally, my really high power designs were designed for continuous operation at 1/8 output power, as that was adequate for their intended application. I feel entirely justified in my choice of bipolar transistors in order to keep dissipation levels at a minimum. Even at 1/3 continuous output power, I believe I have shown that dissipation at idle conditions can be rather significant.


Bob:
You don't tell us your THD number, but assert it is "well below perceptible limits". Maybe you assert that 0.1% of THD-20 is below perceptible limits. How should I know? There are those who do. While 0.1% THD-20 may be OK for a tube amp, it is atrocious for a solid state high end amp.


Glen:
OK, I wouldn’t be particularly enamoured with 0.1% either.


Bob:
First of all, one can make a very fine amplifier out of BJTs, and there are plenty of great ones out there. John Curl and I may disagree on a few things, but he makes a very fine BJT amplifier. He has his own good reasons for choosing that technology and knows how to tame its individual challenges. But you can bet he does not achieve his good sonics by biasing his output stage at only 20 mA.


Glen:
Good. I don’t bias my output stages as low as 20mA either.


Bob:
I merely quoted my MOSFET non-EC 0.02% THD-20 to demonstrate that at only 150 mA, MOSFETs can do quite well, even without EC. By making the point that one cannot hear the harmonics of 20 kHz, and that 20 kHz THD does not matter so much, you are showing that you do not understand that THD-20 is a symptom of underlying HF nonlinearity, not what one hears as the resulting sonic degradation. This is a very important distinction that you seem not to grasp. THD-20 is just a very convenient and well-known measurement technique, but what you actually are hearing when the cymbal spits rather than shimmers is intermodulation products, but the tendency to producing those IM products is quite well correlated to THD-20.


Glen:
You sure do infer a lot. I didn’t say that THD at 20kHz isn’t important, or to have a significant relation to an amplifiers IMD performance. I questioned the validity of attempting to achieve ultra low THD figures at 20kHz. In my opinion low THD is of lesser importance at 20kHz than at 10kHz or 1kHz. Many audio designers agree, many don’t. That’s what it’s like in the audio engineering domain.


Me:
OK, but I don’t design for the esoteric audiophile market. I’m interested in producing efficient cost effective designs. Heat dissipation must be kept to a minimum and in that regard BJT’s with their much lower bias current requirements and ability to run at high temperatures without throttling back rule supreme.

Bob:
This is baloney in super high power amplifiers if you are still using Class-AB rather than Class H or Class G. Do the power dissipation math at 1/3 power or 1/8 power. You never did tell us if your designs are able to operate continuously at 1/3 power or 1/8 power without overheating.


Glen:
No it isn’t baloney. Did the math. Addressed this contention above. My super high power design had to fulfil a requirement of being powered by a remote 12Vlead acid battery bank with a commercially available SMPS at the battery bank end delivering the rail voltages. It wasn’t an economical proposition to provide for the multiple rail voltages required for either class G or H at the time, so I was stuck with class AB. Using BJT’s, I designed my amp as economically as I could.


Bob:
It sounds to me like you've never bothered to build a MOSFET amplifier. The relationship of Vgs to Id at higher currents in MOSFETs is not a problem at all.


Glen:
I’ve used Exicon, Hitachi and Tosbiba MOSFETs all with a great deal of success in low (<100W designs). I did not say that the relationship of Vgs to Id was a very significant problem or insurmountable problem - I was just pointing out the fact that many MOSFETs have a lower intrinsic linearity in source follower applications than do many high power BJT’s.


Cheers,
Glen

I’d like to elaborate a bit further in the levels of power dissipation under quiescent conditions.
That 1kW into 4 ohm hypothetical design with a 89V peak voltage swing may have +/-110V rail voltages. With 10 paralleled MOSFET pairs biased to 200mA each we have a total of 440W of power dissipation when the amplifier is just idling – that’s a hell of a lot of quiescent power dissipation for an amplifier only rated at 1000W RMS into the load.
Using BJT devices of similar current and voltage ratings instead, that bias current could easily be cut down to one-fifth this level without sacrificing crossover distortion performance, giving an idle dissipation of 88W. That is a dramatic increase in efficiency.

Another thing that should be said, is that just because an amplifier may be rated to deliver 1/8 of maximum rated power on a continuous basis (endless hours) without over heating, it does not mean that such an amplifier does not have reserves to operate at much higher ‘continuous’ levels for briefer periods of time – say 10 or thirty minutes.

Now the hypothetical 1kW amplifier, with a BJT output stage and 88W of quiescent power dissipation would have much greater reserves to deliver high average power levels to the load before over heating than the equivalent MOSFET design with 440W of quiescent power dissipation - assuming both amplifiers have the same degree of heatsinking.
The MOSFET design will idle much hotter and will therefore thermally limit much sooner when called upon to deliver high output power levels. The recovery time of the MOSFET amplifier after thermally limiting will also be much greater than the BJT design, as at 5 times the quiescent power dissipation, it will take much longer to cool down.

The only way to alleviate this disadvantage of the MOSFET design is to heat sink it much more heavily than the equivalent BJT design – and that is a significant economical disadvantage.

[Wavebourn]

The entire symmetrical-follower design is one big disadvantage in order to save electricity.

[Bob Cordell]

Originally posted by john curl
Bob, when it comes to distortion, itself: It is almost impossible to set a single number of what 20KHz thd should be with any amp. at full output. Actually, who cares? Who listens to 20KHz sine waves (or their equivalent) at full power at some very low distortion, just to have the amp clip with a 1dB increase in the output?
Personally, I like to concentrate on high frequency distortion at relatively high listening levels which are 1-25W for example. I usually use 5KHz, because my oscillator is happy at that frequency, it is in a more realistic high frequency range, and it is easy for my FFT analyser that follows my THD analyser to see all of the harmonics up to the 20th. Of course, I usually can do well enough with just 10 harmonics, but sometimes higher is useful.
It is in my GREATEST interest to note any 7th or 9th harmonic distortion, especially at listening levels. This is what I think separates the solid state sound from the tube sound, all else being equal. Your opinion may differ.
I must make good measurements at 20KHz, because Tom Holman insisted on putting that into his THX specs, and I make THX rated amps. I think that it is a poor choice of a standard of quality, but I commend you for doing so well with it.
I do not criticise Charles Hansen for not caring about it, because he is after a more 'authentic' sound than you or I are trying for in power amp design. Does he go too far, perhaps? Probably, but what the heck. "You pay your money and you take your choice.";-)

I think we are in pretty good agreement here, John. I have liked 20 kHz THD in the past because it is a tougher test, and it correlates well with HF non linearity that will also result in in-band IM. But you are absolutely right about the difficulty of seeing the higher-order harmonics of THD-20 with ordinarily-available spectral analysis tools. You are also right in suggesting that not all THD-20 is the same; the same number composed of benign 2nd and 3rd is much less objectionable than the same number containing equal amounts of harmonics out to the 7th or 10th.

For this reason, these days I tend to focus more on CCIF 19 kHz + 20 kHz with full spectral analysis, since it still has reasonably good slew rate and HF stress, and readily shows the high-order IM products.

Bob

[lineup]

Originally posted by lineup


A question.
I have run some discrete amplifier simulation tests.

And a Fourier analys at 1 kHz, can show 2nd, 3rd and 4th harmonic in falling steps nicely
and 5th, 6th can be very low.

But the 9th suddenly can be comparatively high, almost like 4th.
Also sometimes 7th sticks up a bit higher than 5th-6th.

But one preamplifier I simulated had very low of almost every harmonic 3-10, except for this 9th.
Didn't matter what levels, loads I used for running analysis.
And there was no way I could tweak away that 9th peak.

Some other circuits does not have this 9th problem, at least in simulations.


Question now.
What can cause this 9th harmonics distortion?
Power supply, some type of feedback or some special topology?

As I recall it, it was a no global feedback that had most of this constant high 9th.
But, then again, most circuits I put together are no feedback and all stages in Class A.


lineup

Well, I guess neither Bob Cordell or John Curl, two very well known audio designers,
can answer my question.
Or they just wont bother. Not even a comment ....

Maybe because my, lineup status, in the inner circle of audio society
is close to ZERO = 0.000000001.



Humbly while under a deep bow to these cruel facts
And with 1000 Regards
lineup

[Bob Cordell]

Originally posted by G.Kleinschmidt
Bob:
Glenn, If you've designed amplifiers up to 5000W in Class AB, rather than H or G, it seems inconsistent that you would be worrying that much about idle bias power consumption.


Absolutely not! Had I designed the same amplifiers with MOSFET’s in the output stages the dissipation would have been a great deal higher. It’s my very choice of class AB instead of the more efficient G or H configurations that has dictated my need to use BJT’s - the more efficient output devices.


Bob:
Unless your amplifiers are not rated for continuous operation at 1/3 power or even 1/8 power, the dissipation of the amplifier under those conditions is far more than that of idle bias.


I’m sorry, but that’s just plain wrong. A quick example with some basic calculations to put things into perspective - a 1kW RMS in 4 ohms AB amplifier would be required to deliver a peak voltage of SQRT(1000*4*2) = 89V with a peak load current of 89/4 = 22A. The average haft sinusoid current for one half of the output stage working at full output power is equal to Ipeak/pi = 22A/3.14 = 7A.
Lets assume this amplifier was made with a MOSFET output stage with 10 paralleled output pairs biased at 200mA each. That would give an idle current of 10*0.2A = 2A.

That’s an idle current of 2A Vs 7A average current when delivering full output power.

At 1/3 power output, the peak output voltage is equal to SQRT((1000/3)*2*4) = 51.6V, giving an average half sinusoid current for one half of the output stage of (51.6/4)/pi = 4.1A

Now we’ve got an idle current of 2A Vs 4.1A average current when delivering 1/3 of full output power.

At 1/8 power output, the peak output voltage is equal to SQRT((1000/8)*2*4) = 32V, giving an average half sinusoid current for one half of the output stage of (32/4)/pi = 2.5A.

Now we’ve got an idle current of 2A Vs 2.5A average current when delivering 1/8 full output power.

As you can deduce from these figures, the 1/8 power dissipation is by no means far less than the idle dissipation by any stretch of the imagination. Incidentally, my really high power designs were designed for continuous operation at 1/8 output power, as that was adequate for their intended application. I feel entirely justified in my choice of bipolar transistors in order to keep dissipation levels at a minimum. Even at 1/3 continuous output power, I believe I have shown that dissipation at idle conditions can be rather significant.


Bob:
You don't tell us your THD number, but assert it is "well below perceptible limits". Maybe you assert that 0.1% of THD-20 is below perceptible limits. How should I know? There are those who do. While 0.1% THD-20 may be OK for a tube amp, it is atrocious for a solid state high end amp.


OK, I wouldn’t be particularly enamoured with 0.1% either.


Bob:
First of all, one can make a very fine amplifier out of BJTs, and there are plenty of great ones out there. John Curl and I may disagree on a few things, but he makes a very fine BJT amplifier. He has his own good reasons for choosing that technology and knows how to tame its individual challenges. But you can bet he does not achieve his good sonics by biasing his output stage at only 20 mA.


Good. I don’t bias my output stages as low as 20mA either.


Bob:
I merely quoted my MOSFET non-EC 0.02% THD-20 to demonstrate that at only 150 mA, MOSFETs can do quite well, even without EC. By making the point that one cannot hear the harmonics of 20 kHz, and that 20 kHz THD does not matter so much, you are showing that you do not understand that THD-20 is a symptom of underlying HF nonlinearity, not what one hears as the resulting sonic degradation. This is a very important distinction that you seem not to grasp. THD-20 is just a very convenient and well-known measurement technique, but what you actually are hearing when the cymbal spits rather than shimmers is intermodulation products, but the tendency to producing those IM products is quite well correlated to THD-20.


You sure do infer a lot. I didn’t say that THD at 20kHz isn’t important, or to have a significant relation to an amplifiers IMD performance. I questioned the validity of attempting to achieve ultra low THD figures at 20kHz. In my opinion low THD is of lesser importance at 20kHz than at 10kHz or 1kHz. Many audio designers agree, many don’t. That’s what it’s like in the audio engineering domain.


Me:
OK, but I don’t design for the esoteric audiophile market. I’m interested in producing efficient cost effective designs. Heat dissipation must be kept to a minimum and in that regard BJT’s with their much lower bias current requirements and ability to run at high temperatures without throttling back rule supreme.

Bob:
This is baloney in super high power amplifiers if you are still using Class-AB rather than Class H or Class G. Do the power dissipation math at 1/3 power or 1/8 power. You never did tell us if your designs are able to operate continuously at 1/3 power or 1/8 power without overheating.


No it isn’t baloney. Did the math. Addressed this contention above. My super high power design had to fulfil a requirement of being powered by a remote 12Vlead acid battery bank with a commercially available SMPS at the battery bank end delivering the rail voltages. It wasn’t an economical proposition to provide for the multiple rail voltages required for either class G or H at the time, so I was stuck with class AB. Using BJT’s, I designed my amp as economically as I could.


Bob:
It sounds to me like you've never bothered to build a MOSFET amplifier. The relationship of Vgs to Id at higher currents in MOSFETs is not a problem at all.


I’ve used Exicon, Hitachi and Tosbiba MOSFETs all with a great deal of success in low (<100W designs). I did not say that the relationship of Vgs to Id was a very significant problem or insurmountable problem - I was just pointing out the fact that many MOSFETs have a lower intrinsic linearity in source follower applications than do many high power BJT’s.


Cheers,
Glen

Glenn,

You've made a HUGE mistake here. Your assumption that 10 pairs of paralleled MOSFETs would have to have a bias of 200 mA each for reasonable crossover distortion into 4 ohms is completely wrong.

What matters in the creation of static crossover distortion is the variation in total transconductance of the output stage in relation to the impedance of the load. In very rough terms, the re if a bipolar transistor is on the order of 26 ohms/Ic in mA, and that of a MOSFET is on the order of 250 ohms/Id in mA. That is why we normally say that a MOSFET needs about ten times the bias current, all else remaining equal.

Transconductance of both types of devices is roughtly proportional to current in this current range. A single MOSFET biased at 200 mA will have rs on the order of 1.2 ohm, the complementary pair on the order of 0.6 ohm. Let's say this ratio of 0.6 ohm against 4 ohms was OK for a 50W amplifier using a single pair of MOSFETs. The interesting thing is that this TOTAL amount of idle bias is STILL just as OK for the 1000 W amplifier using 10 pairs of MOSFETs paralleled each idling at 20 mA. Your logic in assuming that each of the ten pairs of MOSFETs would have to be biased at 200 mA was totally wrong.

I'm not saying I'd bias them that low in practice in a 1000W amplifier, but I hope you get the point. I would probably bias them at some level such that the idle bias dissipation was still below the 1/8 power dissipation. That is why I asked you about whether your amplifier was rated to work continuously at 1/8 power. If it has to be adequately heat-sinked to withstand 1/8 power continuously, then it can ceratinly withstand a few hundred mA of idle bias.

Cheers,
Bob

[GK]

Originally posted by Bob Cordell



Glenn,

You've made a HUGE mistake here. Your assumption that 10 pairs of paralleled MOSFETs would have to have a bias of 200 mA each for reasonable crossover distortion into 4 ohms is completely wrong.

What matters in the creation of static crossover distortion is the variation in total transconductance of the output stage in relation to the impedance of the load. In very rough terms, the re if a bipolar transistor is on the order of 26 ohms/Ic in mA, and that of a MOSFET is on the order of 250 ohms/Id in mA. That is why we normally say that a MOSFET needs about ten times the bias current, all else remaining equal.

Transconductance of both types of devices is roughtly proportional to current in this current range. A single MOSFET biased at 200 mA will have rs on the order of 1.2 ohm, the complementary pair on the order of 0.6 ohm. Let's say this ratio of 0.6 ohm against 4 ohms was OK for a 50W amplifier using a single pair of MOSFETs. The interesting thing is that this TOTAL amount of idle bias is STILL just as OK for the 1000 W amplifier using 10 pairs of MOSFETs paralleled each idling at 20 mA. Your logic in assuming that each of the ten pairs of MOSFETs would have to be biased at 200 mA was totally wrong.

I'm not saying I'd bias them that low in practice in a 1000W amplifier, but I hope you get the point. I would probably bias them at some level such that the idle bias dissipation was still below the 1/8 power dissipation. That is why I asked you about whether your amplifier was rated to work continuously at 1/8 power. If it has to be adequately heat-sinked to withstand 1/8 power continuously, then it can ceratinly withstand a few hundred mA of idle bias.

Cheers,
Bob

Sorry, I still disagree! I've worked on commercial high power MOSFET amps and have several articles of published designs which all run the individual bias currents of multiple parallel connected MOSFET pairs at anywhere from 100-200mA each. Perhaps my experience is limited, but I've never seen a high power MOSFET amp biased anywhere near as low as what you are suggesting.
I've always seen the need for 100-200mA of bias current for each audio MOSFET not strictly for the reduction of x-over distortion but to bias each device nearest possible to the zero temperature coefficent point of vgs Vs Id so as to keep the bias current stable with the wild temperature variations that can occur in an abused high power amplifier.

[john curl]

I tend to agree Glen. It would be almost impossible for me to insert Mosfets where I now have power transistors and get the same power in a practical way.
Also, I would think that I would want to bias the power Mosfets at 80ma ea or more, just to keep them from increasing the higher order distortion component due to the rate of change of the Gm at very low currents. What is the advantage of power mosfets in high powered amps anyway?
Lineup, I don't know exactly why you should get very high 9 harmonic distortion in a class A design. It would seem to be impossible. Maybe it is a computer program glitch.