LM741 Noise Figure Improvement

[just0m3]

Hi! I would like to improve the noise figure of the LM741 equivalent discrete design. What are the key areas that i need to work on?It seems to me that the input differential pair contributes the most in noise. Thanks! I am using 3904 and 3906 transistors.

 

[basreflex]

it all depends on the circuit, the impedance of the source etc. some opamps are made for low voltage noise, some for low current noise. pick one.

 

[as_audio]

I think we need more information about your efforts to replicate a 741 topology.

 

[just0m3]

Thanks for your replies!
I want to use it as a gain stage in a non inverting configuration with a fixed gain of 20dB . It will be just an audio guitar booster to experiment with and to fine tune noise. So mostly i care about voltage noise i guess

 

[basreflex]

 

[davidsrsb]

Back when that IC was designed, the PNPs Q3 & Q4 and Q8 & Q9 were laterals and truly dreadful performance.

 

[basreflex]

as you can see , not all transistors have the same size and will scale differently. for example C1, the compensation cap is the large area in the middle. enjoy!

 

[as_audio]

I think for the simple purpose of post 4 and battery power a better solution can be found.

 

[basreflex]

A jfet opamp will do better like the TL062 or TL072, there are many better ones than that too.

 

[just0m3]

The thing is that i just want to find out if there is a way to change the topology in order to gain better noise figure. I dont want to use a dedicated opamp
but instead i am trying to implement a discrete LM741 topology in order to find out ways of designing with low noise in mind.

 

[as_audio]

No idea of your knowledge, but low noise design is not exactly a beginners topic.

 

[just0m3]

Indeed it is very advanced and at this time i'm trying to find material in order to expand my knowledge on low noise design.
I am an electrical engineer and i have basic knowledge/education in these terms.

 

[maylar]

Hi! I would like to improve the noise figure of the LM741 equivalent discrete design. What are the key areas that i need to work on? It seems to me that the input differential pair contributes the most in noise. Thanks! I am using 3904 and 3906 transistors.

Search the forum for threads on low noise transistors. Like this one -
https://www.diyaudio.com/community/threads/lowest-noise-bjt-transistor.222485/

 

[basreflex]

https://www.analog.com/media/en/technical-documentation/technical-articles/s64_en-op_amps.pdf

 

[as_audio]

It is instructive to have a look at op amp internal schematics in original data sheets,
especially those of the "complementary bipolar" technology. The details of low noise
design are however not covered in these data sheets I think.

The book Designing Analog Chips written by Peter Camenzind can be found online and
probably has a chapter about this.

 

[just0m3]

Thank you all for your time. I will have a look at your references.

 

[Mark Tillotson]

For high impedance sources like a guitar use low-noise JFET input devices, current noise will be dominant and FETs basically don't have any current noise because they take no current (well, extremely little). Good examples of low-noise JFET opamps are OPA1656, ADA4625, OPA1652.

 

[rayma]

The thing is that i just want to find out if there is a way to change the topology in order to gain better noise figure. I dont want to use a dedicated opamp but instead i am trying to implement a discrete LM741 topology in order to find out ways of designing with low noise in mind.

Integrated circuits cannot be directly ported to discrete circuits, as their design relies on ratios, which are precise.
Discrete components cannot achieve such precision, even with manual matching. Different approaches are needed.

 

[Mark Tillotson]

But noise isn't to do with component matching, it can't be laser-trimmed away!

 

[rayma]

The topology comes first. Then noise optimization.
The best monolithic topology won't be the same as the best discrete topology.
He's trying to build a discrete 741. Not even a reasonable goal.