John Curl's Blowtorch preamplifier part II - Page 2198 - diyAudio
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Old 2nd April 2012, 03:45 PM   #21971
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Quote:
Originally Posted by john curl View Post
What have we covered so far of the potential advantages with the complementary differential jfet input stage?
1. It is self biasing, not needing extra current sources.
2. It can have a peak current drive much higher than 2Id, that is one of the fundamental limits to slew rate limiting, and this 'advantage' can make many circuits faster.
3. The transfer function of the jfet is more linear than the bipolar transistor (without emitter resistor degeneration), and components of distortion are lower order in general.
4. With low noise jfets, this circuit can be VERY quiet, and it will NOT get noisier with higher Z drive impedances, such as a potentiometer.
5. There is virtually no input current to worry about.
6. The fet input is more immune to RF detection because there is no working diode at the input.
7. And perhaps more, but I am working on my first cup of coffee, and can't think of any.
'-)
I would add:
High Id (compared to BJT) plus peak current available widens a choice of devices for the second stage.

Best,
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Old 2nd April 2012, 03:52 PM   #21972
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Quote:
Originally Posted by elektroj View Post
I would add:
High Id (compared to BJT) plus peak current available widens a choice of devices for the second stage.

Best,
I don't believe FETs have an advantage over BJTs regarding Id or Ic.
BJTs can be driven to arbitrarily large currents to charge parasitic or real capacitances to avoid slew rate limiting.

jan
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Old 2nd April 2012, 04:01 PM   #21973
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Jan,
I ment it in the context of all other advantages outlined in John Curl's post.

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Old 2nd April 2012, 04:03 PM   #21974
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Oh sure they do, it is possible to run up to 15ma with jfets. Try that with bipolars, and you get too much self noise, and a lowish input Z.
I should point out that I have designed with this circuit since 1968, when bipolar transistors were the only realistic choice. I made dozens of different designs, both amps and preamps when working at Ampex and Alembic, using matched bipolar pairs between the years of 1968-1971, and I found that 2ma was pretty much the maximum Id for each input bipolar device.
While, with some designs, even today, I may run at 2ma with jfets, my PEAK drive current is much, much more than the 4ma that bipolars would be normally limited to.
With other designs, such as the CTC Blowtorch, I might run up to 15ma per jfet device, without any noise penalty due to extra noise generation in the input devices.
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Old 2nd April 2012, 04:06 PM   #21975
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John,

Quote:
Originally Posted by john curl View Post
Well, what do we do about it? For complete novices and people located in isolated locations across the world, it might not be practical to use a complementary differential jfet input stage, just like it would be impractical to design a car engine that needs 120 octane gas (aviation fuel) to run, for use in the USA. It might be just too difficult to source the devices.
I did some thinking about this and came up with this...

Click the image to open in full size.

I personally rejected it for use.

BTW, I cannot really claim that one, it's earlyish lower end Accuphase... Still, I can see uses...

Ciao T
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Old 2nd April 2012, 04:14 PM   #21976
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It would work, but at what 'price' in complexity and performance?
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Old 2nd April 2012, 05:10 PM   #21977
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Quote:
Originally Posted by john curl View Post
Oh sure they do, it is possible to run up to 15ma with jfets. Try that with bipolars, and you get too much self noise, and a lowish input Z.
I've seen bi-polar MC preamps with 25mA/side.
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Old 2nd April 2012, 05:41 PM   #21978
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Scott, it is obvious that you have not lived with a Quan-Tech noise analyzer on a daily basis for most of your life. You would find that for a SPECIFIC 'low noise' device that it is impractical to go above 10ma, and even then, ONLY for VERY LOW impedance drive, below 100 ohms. You are giving INFO that will only confuse, rather than give understanding. I am NOT designing MC pre-preamps here, and I would hope that you would realize that. Later, I might give an example of what happens IF you design with a bipolar input and run very high input currents, the 'trade-offs' so to speak, but not now.
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Old 2nd April 2012, 05:54 PM   #21979
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T, further consideration of the circuit that you just put up shows a true engineer's understanding of the problems, and a practical solution that could have been effected even in the middle 1960's without too much of a problem, since the 2N3819 was available even then.
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Old 2nd April 2012, 06:16 PM   #21980
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John,

Quote:
Originally Posted by john curl View Post
It would work, but at what 'price' in complexity and performance?
I agree, complexity is much up. But it allows very simple, cheap and generic parts to be used, which was my aim.

I suspect for the price of a matched Quad of 2SK170/2SJ74 we can buy a lot of 2SC2240/2SA970 and 2N3819 or similar...

Performace, I think it could be broadly comparable, if time is taken for optimisation...

Quote:
Originally Posted by john curl View Post
T, further consideration of the circuit that you just put up shows a true engineer's understanding of the problems, and a practical solution that could have been effected even in the middle 1960's without too much of a problem, since the 2N3819 was available even then.
Thank you for the flowers and yes, I agree.

It is truly a pitty that in the year 2012 we have to resort to such circuitry. I would have expected some form of monolithic J-Fet quads in big 12-Pin TO Metal Can enclosures instead of we are getting, just goes to show WTFDIK...

It sorely lacks the elegance of your complementary J-Fet solution and as it so happens, I do have hundreds of matched K170/J74 pairs (marked to 0.01mA Idss per pair) around, so I do not have to use it...

Ciao T
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