Optimal working condition for a bjt.

[ginetto61]

Good morning to everyone !

A friend of mine has given to me some 2sc2238 bjts.
A very good piece from what I understand.
I intend to use them for a very simple buffer using just one of them per channel.
I am learning here so I decided to start from some very basic schemas.
Also because I have the strong feeling that they could give very nice sound.
Now I have to chose the Vcb and I bias, the working conditions.
I suppose that for this scope the datasheet should be the tool.
But I do not know how.
Anyone willing to give me some hint ?
Thank you very much indeed.

Kind regards,
ginetto

[peranders]

This is rather simple but first:
* What is your supply voltage?
* How does your load look like?
* How does your signal source look like?

[ginetto61]

[QUOTE=peranders;1931675]This is rather simple but first:
* What is your supply voltage?
* How does your load look like?
QUOTE]

Thank you very much indeed for your kind reply.
I will try to answer
* What is your supply voltage?
Not decided yet. I would like to use the optimum for the chosen bjt
* How does your load look like?
Let's say that the buffer should be able to drive "any" power amp.
My present one has 5K as input impedance
* How does your signal source look like?
It is a cheap dvd player, usually.
I think that the output impedance should be around 500 ohm maybe ?

Thanks a lot and kind regards,
ginetto

[ginetto61]

At present I have no power supply.
So I am open to all voltage supply choice.
I would like to use the optimum for the above mentioned bjts.

Thanks again.
Kind regards,
ginetto

[ginetto61]

I swear is the last one, for now
My point is, which of these following choices is better ?

Vcb [V] Ibias [mA]
60 10
40 15
30 20

I think same W but different combination.
Thanks again,
ginetto

[peranders]

The sad truth here is that you may no need a buffer at all. 5 kohm is not a heavy load unless you have long cables and a weak signal source but you can still make this buffer for educational purposes also.

What is your desired level, good or "Spitzenklasse"?

You transistor has no optimum working conditions. You must first decide what you want to achieve.

My suggestion is that you Google after "diamond buffer". The simplest form of diamond buffer is pretty easy but you will need both NPN and PNP transistors.

[ginetto61]

Originally Posted by peranders

The sad truth here is that you may no need a buffer at all. 5 kohm is not a heavy load unless you have long cables and a weak signal source but you can still make this buffer for educational purposes also..

Thank you very much again.
I experimented with passive pre between the dvd player and the power amp.
I found the sound sterile and unsatisfactory for my taste,
I like an expansive sound, with more dimension
Maybe it is the source the problem, the cables or both.
I would like to stay active.

What is your desired level, good or "Spitzenklasse"? .

Well let's say good.
I suppose that to get top performance should be expensive.

You transistor has no optimum working conditions .

Do you mean the Vcb and I bias do not influence distorsion, slew rate and bandwidth ? This is a very important issue. thank you very much.

You must first decide what you want to achieve.
My suggestion is that you Google after "diamond buffer".
The simplest form of diamond buffer is pretty easy but you will need both NPN and PNP transistors

I will follow you suggestion.
I heard about that topology and I like its simmetry quite a lot.
But it is just a feeling.

Thank you very much again and kind regards,
ginetto

[peranders]

Originally Posted by ginetto61

Do you mean the Vcb and I bias do not influence distorsion, slew rate and bandwidth ? This is a very important issue. thank you very much.

The design process is only a long row of compromizes so you can't say what is optimum if you don't have a circuit but you are right about that working conditions have influence on distortion, noise, speed etc.

Since you seem to know very little about the basics I must tell you that I can't explain this in a few words but it's pretty easy if you start with a well-known design like the diamond buffer. Check out Walt Jung's articles in the subject. He has written very good about it.

[ginetto61]

Thank you very much sincerely again for your always kind and very valuable reply.
On the basis of your words I will abandon my idea to stretch a single bjt to its limits for using it as a buffer.
Much reasonable to stick with a proven and reliable schema.
Thanks again and kind regards,
ginetto

[jerluwoo]

These transistors are medium power type which would be overkill as a line buffer. Another large downside is they will have a much higher junction cap. than a small signal type which would degrade the high freq performance. That being said they would make quite good headphone buffers. The datasheet shows a linear hfe from uA's out to 200mA's or so.