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Old 12th September 2003, 08:58 AM   #1
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Question increase the bias current of an opamp final stage

please can you help me?

I want to increase the bias current of my final stage.
But it is not necessary to have a powerful output.


What can i do?

-Do nothing,it's a stupid idea and let the opamp like it is.

-Do a discrete opamp with class-a output like here:

http://sound.westhost.com/project07.htm

-Driving a buffer with an opamp bias in class A

like ppl do with opa637,el-2002,and mpf-103
(http://www4.head-fi.org/forums/showt...current+source)

or like linear technology do in the data sheet of the LT-1115

Thank you,herve.
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Old 12th September 2003, 01:30 PM   #2
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Default Re: increase the bias current of an opamp final stage

Quote:
Originally posted by herve

-Do a discrete opamp with class-a output like here:

http://sound.westhost.com/project07.htm

That discrete amp certainly looks interesting.

But perhaps I'd leave an option on the pcb to add a 68 ohm resistor between Q3's emitter and the V- supply. You can use the resistor or a wire later.


Carlos
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Old 12th September 2003, 01:38 PM   #3
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herve, what is the whole purpose of this? Signal, load etc?

If you use any good opamp (AD8610, OPA134, OPA627 etc.) plus a buffer like BUF634 you'll get very good results.

Click the image to open in full size.
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Old 12th September 2003, 01:55 PM   #4
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If you want to make a discrete opamp, then take a look here:

http://www.passdiy.com/pdf/diyopamp.pdf
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Old 12th September 2003, 01:56 PM   #5
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I would follow Per's advice and his (for example ) circuit solution.
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Old 12th September 2003, 01:59 PM   #6
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ACD,

that's just a basic, nothing more (DIYopamp). 34 years ago there was a better example of Sinclair Z-30 or Z-50.
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Old 12th September 2003, 02:08 PM   #7
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I know

It was only meant as a source of information
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Old 12th September 2003, 02:15 PM   #8
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ACD, I have guessed that .
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Old 12th September 2003, 02:22 PM   #9
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Why are you always on all???
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Old 12th September 2003, 02:28 PM   #10
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Well,an option on the pcb to add a 68 ohm resistor it's maybe a good idea.
But for Q1,Q2 do i need a pair of transistors in the same package like with an LM394?


The purpose of this is not for making an headphone amplifier.
The amplification factor is around 1000.
Maximum output around 1 volt.
Load: 10 to 100 kilo resistor.


The "idea" is to make a final stage with transistors which work "far away" from the cutoff current..
Forgive me for my English.

Regards,herve.
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