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class A bias help
class A bias help
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Old 2nd March 2019, 05:29 PM   #1
ClaudeG is online now ClaudeG
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Default class A bias help

Hi

17y ago I biased in class A my op amps and was very happy with the sonic result. I used a trim pot and found the best sonic result for these to be at 2.4mA. So back then I biased the AD825 with one resistor and one 2SK170. OK, back then AD825s were very expensive and 2SK170s not !

I want to do exactly that again, still with AD825, but obviously the situation has changed. I have many AD825 handy, but no one would want to waist SK170s on that.

Could someone please point me in the right direction to get a suitable transistor replacement, non expensive and easy to get from Mouser, if possible leg / pinout compatible, and of course the associated resistor value so I get 2.4mA (under 15V) ? Having the right resistor value would help me ordering all the parts in one go, part of a bulk purchase at Mouser’s…

In short exactly the same but transposed in today’s world ę avoiding the use of SK170 :-)

Many thanks for your kind help

Claude
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Old 2nd March 2019, 05:54 PM   #2
BesPav is offline BesPav  Russian Federation
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Default class A bias help

Quote:
Originally Posted by ClaudeG View Post
Could someone please point me in the right direction to get a suitable transistor replacement, non expensive and easy to get from Mouser

MMBFJ175 to 177
2SK246
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Old 2nd March 2019, 07:25 PM   #3
ClaudeG is online now ClaudeG
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Ok, Thanks!

Let's pick either of them as an example, any idea regarding the resistor that I should order with it to get my 2.4mA?
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Old 3rd March 2019, 05:27 AM   #4
BesPav is offline BesPav  Russian Federation
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Quote:
Originally Posted by ClaudeG View Post
Ok, Thanks!

Let's pick either of them as an example, any idea regarding the resistor that I should order with it to get my 2.4mA?

In 300-1kOhm range based on specific transistor sample Idss and transconductance. They vary an order or so. Pick one, measure Idss, post there, iíll answer more precise.
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Old 3rd March 2019, 08:58 AM   #5
ClaudeG is online now ClaudeG
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Thanks for the kind reply BesPav

However, that's exactly what I wanted to avoid: having to purchase, to measure, and to purchase again. Time and money consuming.

Is there really NO way to did it in one go, as I could do it in the past? That is purchasing a given transistor and a given resistor and getting sure one gets 2.4mA (say +-10%) under 15V?

In the past it was straight forward and no need to measure and order again: I just got the 2SK170 and a 100R resistor, job done. I found it once out and it truned out to work for all other 2SK170 with sufficient precision.

Can't it be done in a similar way with today's transistor?

Many thanks again

Claude
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Old 3rd March 2019, 09:27 AM   #6
BesPav is offline BesPav  Russian Federation
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Default class A bias help

Quote:
Originally Posted by ClaudeG View Post
I just got the 2SK170 and a 100R resistor, job done.

Heh... 2SK170 was a high transconductance one, so with small deviation youíve got yours 2,4 +- 0,2 mA.

Quote:
Originally Posted by ClaudeG View Post
Can't it be done in a similar way with today's transistor?

Have no doubt, sure, it can.
Pick any you like more and iíll calculate most nominals:
IMG_4426.jpg
IMG_4723.gif

Iíll prefer 4 or 9 from first smaller collection.

Last edited by BesPav; 3rd March 2019 at 09:31 AM.
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Old 3rd March 2019, 09:51 AM   #7
ctrlx is offline ctrlx  United Kingdom
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Biasing Op-Amps into Class A
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Old 3rd March 2019, 02:40 PM   #8
BesPav is offline BesPav  Russian Federation
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Quote:
Originally Posted by ClaudeG View Post
Can't it be done in a similar way with today's transistor?

Also it can be done without additive bias and simultaneously excluding opampís OPS from shoulder switching:
IMG_0792.jpg
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Old 3rd March 2019, 03:05 PM   #9
ClaudeG is online now ClaudeG
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Ok, still confused with all the options, but I believe we progress LOL

The scheme I am following is indeed on the page ctrlx just posted, unded
Method Two: The Lone JFET

In fact one JFET and one Resistor, exactly as on this section. That's it precisely, exactly how I connect it to my op amp.

That is simple, it works in my set up and I know how to do it as I did it already, LOL


Let's assume we stick to that one please.


Transistors can be
MMBFJ175, or 176, or 177... I don't mind either, if one is easier for calculation.


They are all available. Pity they don't come in the same shape as 2SK170 with its 3 long parallel legs, but hey ho, I can solder on the tiny pin legs and voilŗ.


Now, what Transistors and what R value?


Many thanks again


Claude
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Old 3rd March 2019, 03:21 PM   #10
BesPav is offline BesPav  Russian Federation
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Quote:
Originally Posted by ClaudeG View Post
Transistors can be
MMBFJ175, or 176, or 177... I don't mind either, if one is easier for calculation.

Now, what Transistors and what R value?

Pick 175. It will require less voltage drop for reliable opamp output swing.
It needs source ~1,5 V lower than gate, so 680 Ohms will provide you with more or less precise 2-3 mA.

Also pick 470 and 860 Ohms for more precise current tuning.
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