Bob Cordell Interview: BJT vs. MOSFET - Page 43 - diyAudio
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Old 22nd December 2006, 11:04 PM   #421
ingrast is offline ingrast  Uruguay
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Quote:
Originally posted by john curl
Ingrast, I have IC's that I have collected and used for the last 40 years, in my lab. None, to the best of my knowledge, will do as good, much less better than a good discrete design for the input stage of an audio power amp.
There are good reasons for this, because Bob Cordell and I, for example, can use j-fets that are not available to IC designers. I use complementary fets in every input stage that I design. No IC has them.
IF you are happy with IC's, then use them.

John,

While I won't argue with many years of design and production experience - which I lack - I cannot but request some more specificity as far as possible without intruding on proprietary grounds.

If the superiority of discrete designs can be qualified by measurements, and the difference in results also is above perceptual thresholds for the best listeners, then I have no argument.

If the superioriy cannot be evaluated objectively by existing measurement protocols, but can be reliably established by reputed listeners, I may not be convinced but leaving space to future developments that could fill the void.

If the superiority boils down - and I respect this - to marketing strategies, than again I will not be convinced but have no qualms regarding to each one getting what he wants better and is willing to pay for.

I am not particularly happy with IC's, but have found them nice building blocks for designs that, objectively measured, and listened by moderately discriminating people, have been quite satisfactory.

Anyway, thanks for you response.

Rodolfo
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Old 22nd December 2006, 11:04 PM   #422
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JPV, don't take Dr. Leach's comments too literally The optimum Re drops 15-25mV of voltage at idle. This was proven by a VP of HP, back in 1971 or so, in the HP Journal, also using a computer simulation. I agree with Bob on an inverting servo with a second inverter. It is less problematic than a non-inverting servo. I know from experience.
As far as IC's are concerned, I like a high slew rate of 100V/us or more, and fully balanced driver stages. IC's have a problem with this.
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Old 22nd December 2006, 11:11 PM   #423
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Quote:
small advantage of instant stable idle current, rather than stable idle current.
Quote:
proven by a VP of HP
Mr Curl,
you even have a way with words.
Thanks for the servo opamp type.
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Old 23rd December 2006, 12:20 AM   #424
mikeks is offline mikeks  United Kingdom
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Quote:
Originally posted by john curl
As far as IC's are concerned, I like a high slew rate of 100V/us or more.......
Why?

With respect to what voltage swing?
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Old 23rd December 2006, 01:31 AM   #425
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Quote:
Originally posted by suzyj


Hybrids let you use discretes in a design while forgoing many of the parasitics that plague them. You lose the leadframes and many of the PCB parasitic capacitances. Of course you've still got wirebonds to contend with, and it's just as expensive to make as a fully discrete design.

This is stuff that's interesting at gigahertz. Not at audio frequencies, where parasitics matter much less.

In the middle of 70'th we used to build hybrid power amps. Lazer trimming of resistors was a fun.
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Old 23rd December 2006, 08:24 AM   #426
ash_dac is offline ash_dac  United Kingdom
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Quote:
Originally posted by john curl

As far as IC's are concerned, I like a high slew rate of 100V/us or more, and fully balanced driver stages. IC's have a problem with this.
What gain-bandwidth product can you get with that 100v/us ?
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Old 23rd December 2006, 09:28 AM   #427
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Quote:
Originally posted by ash_dac


What gain-bandwidth product can you get with that 100v/us ?
It doesn't translate directly to GBW, since the gain depends both on input and output voltage. However, we may find the theoretical maximum amplitude bandwidth product at the amplifier output for sine waves by observing that

d/dt A sin(w t) = w A cos(w t)

That is a sine wave with amplitude A and ang. freq w needs a slew rate of at least w A. Hence, a slew rate of 100 V/us = 100 MV/s gives

w A = 2 pi f A = 10^8 => f A = 16 MVHz

So we could for instance output at most 1 V peak at 16 MHz, or 100 V peak at 160 kHz etc. To get GBW, you have to factor in also the gain of the amplifier.

Note that this is the theoretical maximum based on the slew rate figure alone.
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Old 23rd December 2006, 01:22 PM   #428
AndrewT is offline AndrewT  Scotland
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Hi Johnc,
Quote:
The optimum Vre drops 15-25mV of voltage at idle.
is there a range of Re that this rule holds true?

Presumably, this is per device, since you propose that Vre is actually measured as part of the set-up procedure.
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Old 23rd December 2006, 05:36 PM   #429
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This will work with any Re. It is derived from a computer emulation, back in 1970 or so.
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Old 23rd December 2006, 07:14 PM   #430
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Quote:
Originally posted by JPV
Concerning gm doubling

Mr Cordell, in case of complementary feedback pair it is perhaps right but in emitter follower pair like in the T topology, I do not understand gm doubling.
If you read the paper from Leach (http://users.ece.gatech.edu/~mleach/papers/classab.pdf) on gm doubling it seems undeniable with spice simulation, analytical demonstration and an explanation why the misconception .
He uses two matched transistors which is certainly not true in a real life but the argument remains.

If this is true, then the T circuit is really great not only because it loads very lightly like any darlington but also because the drivers are allways in class A ( emitters not connected to the load but between themselves through a resistor) makes them a very good low impedance to suck the charges out of the output stage. This is very good to reduce the remaining crossover spikes.

M. Curl, I understand that this topology is 40 years old but is it not the same for all of them
I am in the process of learning and I would love to understand what are the subtle distortion mecanismes that still exist in the output stage and how to solve them ( except layout of course)

When I say IC I mean front end like the LM4702. Experienced designers can certainly design discrete high voltage gain stages with good desensitivity and stability which is given for free by the LM4702 in the 100W+ ( I am not selling for National !!).
Mr Curl, what do you dislike in a specific IC like this one driving an output stage in this range of power

If all this is true, ( for the diy) making a medium power amplifier boils down to choosing between BGT and MOSfet for efficient reliability and protection mecanism. With use of servo and bridging, he can make a very high power with a BGT/MOSfet choice for ease of paralleling.

Mr Cordell, why do you prefer inverting servo and where can I read on the use of servo to remove offset.

Many Thanks to all of you

Jean-Pierre Vanderreydt

I chose the example of the CFP output stage to demonstrate gm doubling because it provides a more extreme example due to the very low output impedance of each individual CFP. However, the T circuit also demonstrates gm doubling if it is over-biased, or biased into Class AAB. It's just that in source follower output stages, the individual output impedance of each half of the source follower is a little higher and a function of current (gm of the output transistor), so proper choice of the bias current can result in a smoother crossover transition.

Neverthe less, even in a T circuit, if you bias the output stage at, say, 200 mA and use 0.33 ohm emitter resistors, you'll see gm doubling. Just note that the re of each output transistor at idle (1/gm) is only about 0.13 ohm, reasonably small compared to the emitter resistor. At idle, net output impedance is about (0.13+0.33)/2 =0.46 ohms. At high currents, where re is very small, output impedance is on the order of 0.33 ohms. Here, in this admittedly over-simplified case, gm has not doubled, but it has increased by a factor of 1.4.

I prefer the inverting servo to the non-inverting servo for at least two reasons. First, to make a non-inverting integrator, you have to use two good integrator caps. That alone is not worth the cost of a dual op amp over a single op amp. In some cases the inverting integrator makes it easier to use a smaller integrating capacitor as well. Secondly, the use of the inverting integrator followed by an inverter gives more design options and flexibility, and little or no worries about common mode effects. I usually put in a second pole to further reduce the effects of the servo in the audio band, and I also use the output of the inverting integrator as part of my dc output detect circuit.

Bob
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