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-   -   increase the bias current of an opamp final stage (http://www.diyaudio.com/forums/solid-state/20168-increase-bias-current-opamp-final-stage.html)

herve 12th September 2003 08:58 AM

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.

carlmart 12th September 2003 01:30 PM

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

peranders 12th September 2003 01:38 PM

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.

http://home5.swipnet.se/~w-50674/hif...r0_topview.jpg

Jan Dupont 12th September 2003 01:55 PM

If you want to make a discrete opamp, then take a look here:

http://www.passdiy.com/pdf/diyopamp.pdf

PMA 12th September 2003 01:56 PM

I would follow Per's advice and his (for example ;) ) circuit solution.

PMA 12th September 2003 01:59 PM

ACD,

that's just a basic, nothing more (DIYopamp). 34 years ago there was a better example of Sinclair Z-30 or Z-50.

Jan Dupont 12th September 2003 02:08 PM

PMA
 
I know;)

It was only meant as a source of information:nod:

PMA 12th September 2003 02:15 PM

ACD, I have guessed that ;) .

Jan Dupont 12th September 2003 02:22 PM

PMA
 
Why are you always:smash: on all???;)

herve 12th September 2003 02:28 PM

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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