Pass B1 pre with BF862

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#23
Read Dennis Feucht.
You need to add the source resistor to control/set the Id to BOTH upper and lower devices.
This gets the output offset to close to zero Volts. Trimming one or other source resistor as described by D.F. sets the output offset to zero.
Be careful with maximum dissipation. The B1 has 18V across the two devices. That leads to a high Pmax in the smd sot23 you are planning to use.
The DCB1 is even higher at 20V.
 

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#25
Andrew , I apologize but giving me something to read is much better. I don't even know what to read about.
I believe everyone start as a beginner. The way some treats beginners is not the nicest, at least for me. You guys have to understand not everyone here is a major in electronics or do electronics for a living. The success of a DIY forum is also based on how tolerant you all are to novices. Else it will end up being an old country club with few old people bragging about their stuff.
 
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#26
Man , I am one of those real beginners , people here are very tolerant and helpful , believe me 😉

all you have to do is , D I Y

after that you will find all the answers you look for , this forum is an mine of science , knowledge , skills ,inventors , etc... , some of those guys occurs here 😉


.
 
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#27
Fabrice,I been here at least 2 years before you.I know there is no shortage of knowledge here.But not many are willing to freely share it.Many will show up to help and leave you high and dry.They are doing because of their hightened self esteem and dont want to talk to ignorant guys like me or just because theya re unsure of their self.It is not unusal to find few who approach you seemingly to help with their buisness.
 
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#28
it's not a matter of "free sharing" , those who knows doesn't speak with "anybody" , they do with people they "like" , it all matter of passion and feeling , nothing more , and when they like they give a lot , like me , you and the all world ....

it's the first rule here , you got what you gives .

and I am ok with that 🙂

.
 
#31
AndrewT said:
Read Dennis Feucht.
You need to add the source resistor to control/set the Id to BOTH upper and lower devices.
This gets the output offset to close to zero Volts. Trimming one or other source resistor as described by D.F. sets the output offset to zero.
Be careful with maximum dissipation. The B1 has 18V across the two devices. That leads to a high Pmax in the smd sot23 you are planning to use.
The DCB1 is even higher at 20V.
Click to expand...

Andrew ,if I use trimmer resistors for both source resistors ,is it possible to avoid matching the Idss of JFETs? Or is it still needed
 
#32
You can't change Idss. That is a fixed parameter of the device as manufactured. You can choose what Id you want to operate at.

You don't need matched.
The B1 runs both jFETs at Idss, one as a constant current source and the other as a Source Follower. It does not matter that the gm is slightly different, nor that the Id vs Vgs is slightly off between the two.

You only need to select Idss that are close to each other. The closer the Idss the lower the output offset. ( as a secondary effect, the drift in offset as temperatures change is also reduced when the two devices have very close Idss).

The same will apply if you substitute a BF862 for an LSK170b
The big difference is the lower dissipation capability and the risk of excessive gate current if you run them hot and high Vds and Vgs higher than 0Vgs.

Once you have selected two BF862 that are near each other you adjust the Id down to what you want using the source resistor on BOTH devices.
Then check the output offset (because the B1 is single polarity and AC coupled this is not a case of measuring from Zero Volts to Source voltage). You can assemble a plug in breadboard with +-9Vdc with your two devices and two source resistors and two gate resistors. The BF862 will need a little temporary jig to allow plug ins.
Then as described by Feucht you trim ONE source resistor to bring output offset to zero mV. Let's suppose you have 10r in each source to get an acceptable Id and Pq
Then keep the 10r in the top device and change the 10r to 11r and add a 200ohms variable resistor in parallel. That gives you a range from 10r4 down as low as you need.
 
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#33
I have found members here to be very helpful for novices as myself. For reference material I suggest,

FIRST WATT ARTICLES

When you get to something you do not understand google.

You really cannot fault anyone for not explaining everything in detail. Some have more time to devout to research for a novice's questions than others and I value any time they have to share their knowledge. Any answer is worthy of a thank you.
 
#35
AndrewT said:
You can't change Idss. That is a fixed parameter of the device as manufactured. You can choose what Id you want to operate at. You don't need matched.
Click to expand...

A little known secret: For a buffer which uses Jfets, one as a follower
and the other as a constant current source, the way to select them is
use the Jfet with less Idss as the current source. This biases the follower
at slightly less than Idss, which is generally considered optimal, and does
do without having a Source resistor on the gain device.

This has the benefit of one less resistor, and also gives you something to
do with mismatched Jfets so that you can sleep better at night....
 
#40
Nelson Pass said:
A little known secret: For a buffer which uses Jfets, one as a follower
and the other as a constant current source, the way to select them is
use the Jfet with less Idss as the current source. This biases the follower
at slightly less than Idss, which is generally considered optimal, and does
do without having a Source resistor on the gain device.

This has the benefit of one less resistor, and also gives you something to
do with mismatched Jfets so that you can sleep better at night....
Click to expand...
It's no secret. it has been explained in this Forum many times.

It was not what our Member was asking for.
He requires a replacement for the To92 sk170b and it had been suggested that a lower power, higher Idss, BF862 be used but with source resistor to reduce the Ic and thus make the BF862 run cooler.
Once he has gone that route, I referred him to D.Feucht where he shows how it should be done, i.e. one source resistor to each device.
 
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