Offset of JFet OP Amp

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spencer

Member
Joined 2004
fetaudio.com
Hi,

The Jfets is required to have following Idss.

Q1 & Q2 = Select Idss from 6 to 8mA but matched to within 0.2mA if possible.

Q6 and Q4 must be with Idss >10mA, matched them if possible.

Q5 and Q6 should be 7-10mA, matched to within 0.2mA.

DC Currents:

Current through Q1 and Q2 add up and through into Q3 (R2).

Current through Q6 (R7) is same as current through Q4 (R4).

Current through Q5 (R5) should also be same as current throug Q7 (R6).

All the clues are here. Good luck.
 
H

HKC

Member
Joined 2003
Hi Spencer

Thank you for prompt reply. In order to make the amp meets the original standard I am looking for the JFets with correct Idss in the market which I wanna use for the next version of this pre-amp. I will keep posted please stay with me to share your valuable suggestion.
 
H

HKC

Member
Joined 2003
Hi PP

I added two 0.033uF Polyester caps (this is what I have on hand) to each channel today. After adding these caps the sound is more clear and detail than before. The improvement is very noticeable. I will try the 1000P in the next few days when I have time to buy it from the shops.

Thanks
Chan
 
bear

bear

Member
Joined 2002
bearlabs.com
Look at ur JFETs.

They will say the number and below some letters, or they will have a color dot on them. That tells you the range of Idss for the batch they come out of.

If it is letters, likely: B, G, V, BL

Blue, Green, Violet, Black... there may be other color ranges.
Look up the mfrs sheet to see the Idss range they are in.

You can also discover the Idss of the JFET by using it as a current source.
Look up the circuit - can't be simpler... place JFET in series with a resistor of a value that will draw just more than the IDSS you want when in series with a given voltage, check the vdrop (current) of just the resistor. Then insert the JFET current source measure the vdrop, that equals the IDSS.

_-_-bear
 
H

hags

Member
Joined 2004
jfet testing

Another way is to simply tie the source and gate pins together to ground and connect the drain through an ampmeter to say +15vdc. The current you read will be the device's Idss.
You do have to be careful not to exceed max. dissipation for any device you test though.
 
H

HKC

Member
Joined 2003
Hi hags

The link has the same problem. I got the following message:

"You are not authorized to view this page
The Web server you are attempting to reach has a list of IP addresses that are not allowed to access the Web site, and the IP address of your browsing computer is on this list."

Best Regards
 
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