Jfet/bipolar PCB and Schematic (finaly)

[sonnya]

I have read you thread. And i will get back later today. with some answers.

First : Yes! Supply voltage is +/-15V that is right and i have forgotten to mention that!.

About the gain in the circuit. It depends a lot on the Q5 and Q8 output admittance. Does Q7 and Q4 only do a current "transfer" to Q8.
The gain is also set by the two resistor R1 and R2. The smaller they are the bigger the current difference will be at a given voltage diffence at the gate of Q1 and Q2.

So the dc-gain is nearly a product of : gm in Q1 and Q2, R1 and R2 (is the source resistance on the fet's) and the output admittance of Q5 and Q8.

Q5 is coupled as a commonbase configuration and the output admittance will then be : hob = hoe/(1+hfe).
example :

hfe = 100;
hoe = 10uS; (100KOhm)
hob = 99nS => 10.1MOhm.

I can't remember how to calculate the output admittance of a currentmirror right know .. So it have to wait.

It is right that Q4 and Q5 is a current generator in it self, but the configuration with the LED makes up a constant voltage across R3 and R4. So there will "always" be ~ 1.2Volt across R3 and R4 wich in return will generate a current through R3 and R4 (200 ohm) of ~6mA in each resistor.
So if Q1 sinks 4mA through R3 then Q4 can only sink 6 - 4 = ~ 2mA.

So if Q7 was a resistor the voltage across it would be 180 degress out of phase with the inputvoltage on Q1. Like it would i a normal cascode stage.

By the way Thanks for the links. I will get back to current source thing! There is something to change at this point.

By the way : Parts like ztx651 from zetex should have noise figures as small as 1nV/Squareroot(Hz)!!! Check there appl.note DN11.

Sonny

[hifidaddy]

When I had a look at on old cheap Luxman integrated amplifier from 1984, called L410, I realized that it had cascoded jfets int he input stage, and also a folded cascode in the voltage amplification stage, of the discrete power amp section.

And that folded cascode was designed quite good: they use current sources for biasing instead of resistors R3, R4 in the circuit presented in this thread.

I tried a simple folded cascode some years ago, when I had a lot of talk with Jonathan Carr (designer of Connoisseur preamp and Lyra cartridges). I tried it in phono and line stage of a preamp. While treble was very good indeed, stability in the bass and soundstage was too vague for my taste. I assume there is a lot of thermic distortion when biasing folded cascodes with resistors only. Any voltage drift will transform into a current drift, and then the poor input transistors have to rule that out, you see ...

Jonathan Carr uses current sources, too.

regards,
Hartmut from Munich

[sonnya]

Yep and they have been used in opamp for ~ 10 - 20 years maybe more!
But i like to test different circuits!, and thats why i made this one!

" I assume there is a lot of thermic distortion when biasing folded cascodes with resistors only" ... You are right about it ... Resistors change value 50ppm/degress celcius (typical). So big current change -> big power change -> big value change!

I like the last link Elso send about "memory effect" they point out that high openloop gain may be the solution to some of the problem. The higher gain => lower current change => lower change in power discipation => less thermal distortion!!!

I am going to like this!!

I find this forum good, cause you get new input!

Nice to hear others opinion!

Sonny

[Elso Kwak]

Hi Hartmut,
Please post the schematic of the Luxman and I will comment on that. I don't agree with you about the "thermic" distortion. If you use monilitic duals for the input FET's and the current mirror you have absolutely no drift and can omit all coupling caps at the input and output, leaving another layer of grunge!

[Elso Kwak]

Hi Sonnya,
Did you see the Great River Electronics Opamp by Dan Kennedy? Just a hint.
http://www.forsselltech.com/schematics.htm

[sonnya]

Hi Elso

It is funny!!!, i did fall over his page i think 3 - 4 days ago.. haha

mmhhh i like the 3 - transistor currentmirror he is using ... it have less current difference (if there is any ) than a 2 transistor i am using.

The thing about currentsource.. what should i say .. After som measurement .. Yes you are right about using bjt as currentsources .. They are faster and more stable .. They get there desired current faster.

And the difference from device to device is smaller than with JFET's

I took 200 pcs. more of J113 and tested the Idss .. Min : ~ 8mA - ~21mA it is far to big tolerance from one batch!!!!!

I think i will trymount a 2 transistor currentsource and also a zenerbiased transistorcurrentsource..

2.7V zener have nearly the same temp. coefficient (~-2mV/degress) as a transistor.

By the way what have you been up to ... you did mention some thing about your own design .. If want to tell

- Sonny

[Elso Kwak]

Hi Sonnya,
In the MAT03 datasheet fig. 14a & 15 you will find current sources, or sinks if you want, of a bipolar transistor and a LED. These have less noise than the 2.7V zener and have about zero tepmco. Both the NPN type and PNP type are illustrated. You can cascode these with a extra transistor like PeuFeu to eliminate thermal distortion or the Early effect or whatever effect.
Laymen are always very surprised and amused seeing those red lights glowing on the PCB when they are peeping through a ventilation hole.
http://products.analog.com/products/...?product=MAT03

In the MAT02 datasheet a two transistor current source is pictured. I am not so fond of these but these are used by Metaxas Audio in Australia.
http://www.metaxas.com/pages/masnewfiles/index.html

[sonnya]

Good point about using LED's (GaAs types!? I am going to find out why GaAs!?).
The two transistor currentsource is not as stable as the LED-Transistor solution, but the noise is as low as the one generated from the transistors and the surrounding resistors.

Passlabs have done a smart thing to lower the noise from the LED. Check : http://www.passlabs.com/pdf/aleph/aonoserv0.pdf

and : http://www.analog.com/library/techAr...tion_V-Ref.pdf

The last one is written by Walt Jung and on page 2.18 the make there low noise circuits the same way!

By the way : half the production of MAT03 is Obsolete. it is the same with the SSMXXXX line.

Sidetrack : You have to check the new AD8610AR .. The datasheet speaks for itself (How many opamp's can drive a 2uF load!!!??? Yes 2uF and not 2nF!!!)

Sonny

[Elso Kwak]

Hi Sonnya,
Thanks for the links; two were new to me!
Please find also below a link to a article by Walt Jung. He uses a similar buffer you do but with LED's in the current sources!
GaAsP is just chemical shorthand for the chemical elements of gallium, arsenic and phosphorus used in the semiconductor. It looks very scientific but it is just a normal red LED [I am a chemist BTW]
http://www.planetee.com/planetee/ser...ArticleID=2800
I was familiar with the Passlabs article, but not sure what you mean by <I>"lowering the noise of the LED's"</I>. Should I read <B>FET's</B>? Makes more sense to me. Paralleling active devices like bipolar transistors or FET's is a old trick from the conjuring-box of the solidstate engineer-magician. But you can also look at a large geometryFET like the 2SK389 or 2SK 147 as several small FET's paralleled.
I also was familiar with the low pass filtering of the reference noise, though from a other source (OP275 datasheet).
I do not understand: <I>"half the production of MAT03 is obsolete'</I>? Are they no longer manufactured? The EH and FH types are still available! I do know the MAT02 & MAT 03 are better than the SSM2210 and SSM 2220 although the datasheets claim they are the same. They are not, the MAT's have much higher Beta! As found out MarkLevinson (the Company). They changed to the MAT type's with much ADO.
I am sorry but since I work with discrete circuits I am hardly interested anymore in IC's. Why would you want to drive 2µF?. I have a long cable to the poweramp but that's only a few 100 pF.

[sonnya]

Sorry about that obsolete thing .. It is only the military types you can't get any longer or for a limited time...

And about the passlink: Sorry i have just looked at it again and i am wrong.

But you can make the noisereduction on a LED like it is done with a Voltagereference. By making a lowpasfilter before entering basis on a transistor (1K + 100uF like fig.2.17 in the AD note).

mmmhhh you are a real discrete freak it is interresting to see that it is possible to make "opamp" stage wich is stable with large phaseshift at the output.