Fully balanced Opamp

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R

rkn

Member
Joined 2008
Hello,

I'm not sure where to post this, but this project is inspired by Nelson Pass' article about Opamps so I guess it is the right place here. ;)
My idea is to build a discrete opamp with balanced inputs and outputs. My design follows mainly Nelson Pass' article but without the feedback loop. This can be attached externally if wanted. I'm going to make a seperate PCB for the discrete Opamp. Below you can see my design. I used JFETs inputs and a bipolar stage on every side.
Maybe the values I've selected are not good so comments regarding the design are welcome!
one point where I'm not sure is wether I should use two resistors in series with the jeft inputs or not. In Nelson Pass article you have these resistors but if you look on other discrete Opamp designs you don't have them.
I connected the three constant current sources to one reference. i hope it is allowed to do that. I've then that in some designs of power amplifiers but only for two current sources not for three like in his design.

An externally hosted image should be here but it was not working when we last tested it.


Greetings,

Raphael
 
G

GRollins

Member
Joined 2001
Before worrying about parts values, you're going to need to make at least one change. Each output will need a resistor to ground, feedback to the front end, or both. As it stands now, you'll have uncontrollable DC offset and horrendous amounts of gain (on the order of 60-80 dB).

Grey
 
R

rkn

Member
Joined 2008
Well, the idea was to place these feedback resisistors on the mainboard where I want to plug in my OPamp. But yesterday I found out that I have some space left on the OPamp PCB so that there is no big problem to place two additional feedback resistors.

Raphael
 
jam

jam

Member
Joined 2001
Hi Uli,

Long time no see................you are right the current mirror has to be replaved with active loads or resistors. John Lindsey Hood has a design for a symetrical currernt mirror that might work in this application. It was publisrd in Wireless World years ago.

Regards,

Jam
 
R

rkn

Member
Joined 2008
OK, bad idea with current mirror. With a little bit thinking about it I night find it out myself... I'll return bac to the first verison with the resistors.

David, I'm using gEDA: www.geda.seul.org Originally it was written for Linux but after MacOSX arrived it is also possible to run it on a Mac. You'll find a MacOSX-bundle on the website.
I stopped working with Windows and changed to Linux two or three years ago. And I don't want Windows back. But after some bad experiences with HP I switched to Apple hoping that they sell a more reliable hardware. I like Apple. I've a Mac mini and a MacBook. Now I'm using both in parallel: Linux and MacOSX and cannot say which system is better, both have pros and cons. Ok that's offtopic.

Raphael
 
forr

forr

Member
Joined 2004
jam said:
Hi Uli,

Long time no see................you are right the current mirror has to be replaved with active loads or resistors. John Lindsey Hood has a design for a symetrical currernt mirror that might work in this application. It was publisrd in Wireless World years ago.

Regards,

Jam
Click to expand...

Hi Jam and Rkn,

See
http://www.diyaudio.com/forums/showthread.php?postid=760090#post760090
schematics :
http://www.diyaudio.com/forums/showthread.php?postid=760486#post760486
Also see following comments.

Regards.
 
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