Jfet/bipolar PCB and Schematic (finaly) - diyAudio
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Old 8th January 2002, 11:21 AM   #1
sonnya is offline sonnya  Denmark
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Smile Jfet/bipolar PCB and Schematic (finaly)

First the schematic...

Match Q1 and Q2 (J111 - J113) to a current difference of 100uA when measuring IDSS.

Change R1 and R2 to 470R.
Change R3, R4, R14 and R15 to ~ 200R
Change R10 and R13 to 500R (multiturn)
Change R18 to 2K0

Set ID on Q3 to 6 - 8 mA.

Set ID on Q6 to ~ 6 mA.

Set ID on Q2 o ~ 2 mA.

Then it should idle around 30 - 35 mA.

The feedback is outside the PCB. Remember short tracks!.

Slewrate is ~+/- 75V/usec.

at unitygain -3dB is ~4MHz.

It is stable with 1nF load (No overshoot! at squarewave signal).

Try it!, i have mounted it inside my CD player and it have in my system better treble perfomance than OP275GP when driving
1K || 220pF load.

Sonny
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File Type: zip scm.zip (77.2 KB, 7866 views)
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Old 8th January 2002, 11:39 AM   #2
sonnya is offline sonnya  Denmark
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ups!

I meant :

Set ID on Q10 to ~ 2 mA.

If you can't reach the ID settings change the source resistance on Q3, Q6 or Q10.

The Pcb will come later this week!

Sonny
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Old 8th January 2002, 01:27 PM   #3
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Default Where is the schematic you are referring to???(NT)

nt
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Old 8th January 2002, 01:45 PM   #4
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In SCM.ZIP.

There you will find a file called scm.tif.

Sonny
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Old 8th January 2002, 01:54 PM   #5
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I forgot to say :

You need to glue the following together to minimize drift:

Q1 and Q2
Q4 and Q5
Q7 and Q8
Q11 and Q12
Q13 and Q14
Q15 and Q16

And when you match Q1 and Q2 to around 100uA you will get an offset on VGS around 10 - 20mV.

Buy 100 J113(It will cost you about 500 DKR.) and you will get lets say 20 pairs! It is cheaper then buying U401,U404 etc....

And the noise level is typical around 3nV/Squareroot(HZ) @ 1KHz.
Not as good as 2SK170 but cheaper!

Sonny
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Old 9th January 2002, 08:55 AM   #6
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Default The Last part!

Here comes the PCB and PCB placement on top and bottom (SMD resistors in 1206 housing).

Happy DIY to your all!

Is anyone interrested if iwas going to make it smaller? Lets say 2 - 3 times (in both direction) the size of a DIP8 house used by the most opamp with 8 pins, so it could be mounted instead of an OPAMP.
The it would be made with SMD bipolar and maybe SMD JFET's.
It will be a doublesided PCB.

Sonny
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Old 10th January 2002, 09:39 AM   #7
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Lightbulb Nice Circuit!

Hi Sonnya,
Congratulations with your circuit. It is the first discrete <I>folded cascode</I> design I see on the net!
I have a few suggestion and questions:
I think it is better for the <B>sound </B> to use a monolitic dual JFET like 2SK389 or industrystandard 2N5912. Any reason why you use FET's as current sources? I my own design I use LED/transistor current sources. Personally I would raise R3 & R4 more to say 2k and lower R1 & R2 to 10 Ohm but that's only a minor detail. The matching transistor issue also applies to the current mirror Q7 and 8. Would use a dual here too.
Are you familiar with the Application Note High speed Amplifier Techniques? It contains the the same type buffer you use and some very practical tips to prevent oscillations
http://www.linear.com/pub/document.h...pp&document=50


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Old 10th January 2002, 12:20 PM   #8
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You are right about the Fet's. 2sk389 is a good choice .. lower ciss and crss. Lower noise (1/3 of J113). better cmrr and higher drain voltage (J113 : VD = 35V).

But the 2n5912 is easy to get here in denmark.
Same noiselevel as J113.
Matched to 15mV.
Drawback!? drainvoltage of 25Volt.

I think i will try 2n5912 in the next weeks.

Why i use fet's as currentsource?
1. to test how stable they are. And the J113 has a good temperature stability at current level around 2 - 8 mA.
2. the have a high gos (output admitans) when the have a Rs in commonsource configuration. Higher than bipolar types.

rd = 1/gos.
Ro = rd*(1+gm*rs).

The low voltage noise of fet's will also show of in the current noise when connected as a current source.

But please correct me if i am wrong.

The thing about r3 and r4. If you raise them to 2k then the current for Q1||Q4 and Q2||Q5 is as low as ~ 550uA. If we do this we have to lower the current generator Q3 to ~ 550uA.
Then the current flow through Q1,Q2,Q4,Q5 is 275uA each.

I have tried started out with r1 and r2 set to 2k each. It gets slower, and the open loop gain gets lower also. So 10R0 should make it faster!. But i haven't tried it out.

mmmhhh Q7 and Q8 i have also thought of using a mathed pair like dual's but then i am limited to only a few ones. But your are right about matching them.

Other types then 2n5551 and 2n5401 will maybe have a higher output admitans. Something to try out!

The reason why i only have matched the J113 Q1 and Q2 is to see how it behaves before i go to sot23 housing(Try match smd transistors ... yerk!!!)

Thank's for the app.note. I have made some study about what analog devices says about highspeed design's. But Linear does have some know how to!!!

Thanks a lot for the feedback!
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Old 10th January 2002, 01:23 PM   #9
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okay! the last post from me. I write when the idears come to me... obvious not fast?!

board size for a full smd layout as single opamp dropin replacement.

25mm * 35mm * 5mm.. small enough?

Sonny
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Old 11th January 2002, 11:51 AM   #10
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Lightbulb JFET input/folded cascode design

Hi Sonnya,
<B><I>InputFETs:</I></B> You can also use the 2N3958 with 50V breakdown voltage. It is in a small TO-71 housing. But I wouldn't be too concerned on the 25V breakdown voltage of the 2N5912 as the current source takes a large part of the voltage drop.
<I><B>JFET's as current sources:</I></B> May I quote from Horowitz and Hill, the Art of Electronics , Chapter3 $3.06 about JFET current sources? "It is important to realize that a good bipolar transistor current source will give far better predictability and stability than a JFET current source" See also the link to PeuFeu France for cascoding current sources.
http://peufeu.free.fr/audio/

<B><I>Analysing the currents through R3 & R4</I></B> As I see it the curent through the resistor R3 is the sum of the currents through Q1 & Q4. The current through Q1 is determined by current source Q3 and is just half of it. The current through Q4 is determined by the voltage drop of the LED and the rest of the "divider" LED/R11, Q6. In fact Q4 and Q5 are a current source in itself! I would omit Q6 making the voltage at the base of Q4 & Q5 adjustable by means of a adjustable resistor (pot) in series with the <B>LED</B> or use a string of LED's. To give a example. If the current source Q3 provides 2mA Q1 passes 1mA. And if the current through Q4 is 0.5mA. the current through R3 is 1+0.5=1.5mA giving a voltage drop of 1.5mx100=150mV below the supply voltage. Also note that the voltage at the bases of Q4 & Q5 should be 0.65V lower than the voltage of there emittors, point 2 at R3 & R4.
You see the example I gave is not correct as the voltage calculated by substraction of 0.65V of the emittor voltage will give 15-0.15-0.65=14.2V. While the LED will give rise to 15-1.6=13.4V !. I assumed =/-15V supply's

Interestingly most of the gain of the circuit comes from the current mirror Q7 & Q8............

It must have been a revelation for you to that a simple circuit like this is sounding much better than a highly praised opamp OP275!
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