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Old 19th March 2013, 09:31 PM   #1
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Default Some kind questions on linearity and distortion

Hello to Everyone !

i ask here because i do not know where to ask :
I have a big confusion in mind
1) linearity means a flat freq response at least in the audio band ?
2) i see sometimes those distortion spectra. Why if i send a monotonic signal to a transistor i get at the output also the harmonics ? why this happens ?
3) feedback has only influence on the point 2) ?
4) can i create feedback with only one active device ?

Thanks a lot indeed and sorry if these are silly questions
Kind regards,
gino

Last edited by ginetto61; 19th March 2013 at 09:35 PM.
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Old 19th March 2013, 09:57 PM   #2
jxdking is offline jxdking  China
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That's my answer.(Might not correct)
1) No. Linearity means Vout / Vin is Constant, at any time!
2) Transistor is not linear enough.
3) Feedback will correct some non-linearity. In some cases, it corrects almost all non-linearity, but can not eliminate all non-linearity.
4) Yes.
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Old 19th March 2013, 10:36 PM   #3
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Quote:
Originally Posted by jxdking View Post
That's my answer.(Might not correct)
1) No. Linearity means Vout / Vin is Constant, at any time!
Thank you very much indeed for your kind and valuable reply
I rephrase the question a little.
I am interested in a flat freq response in audio range
A single bjt circuit is capable of a flat freq response from 20 to 20kHz ?

Quote:
... 4) Yes
Click the image to open in full size.

How can i connect a single bjt circuit to have feedback ?
is it enough a resistor between the collector and the emitter ?
are there any examples in literature ?
I have seen circuit with feedback but at least with two bjts, never with just one and I am very curious about this. Really.
Thanks a lot and kind regards,
gino
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Old 19th March 2013, 11:00 PM   #4
dmills is offline dmills  United Kingdom
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There are a fair number of ways to arganise feedback around a single transistor, here are a few:

A resistor in the emitter lead will create feedback because the voltage across it will increase with increasing current flow. A very common trick for stabilising the dc operating point, and if you split the resistor and decouple the junction then you can set the DC operating point and the AC gain almost independently. Note all transistors have some intrinsic Re' so this mechanism always exists to some extent.

Connect the bias resistor to the collector, run common emitter and place a load resistor in the collector circuit. Not as stable as the first design, but it works and is often seen in simple minded mic preamps for things like baby monitors. You sometimes see this combined with the first idea.

Run the stage common base, more usually seen at RF this one, but if you need a low Z input and very good isolation it is not a bad approach at audio.

Then once you get away from audio, there are things like the norton noiseless feedback circuit wrapping a transformer around a common base stage to make a very broadband flat amplifier which is quiet but trades **VERY** poor reverse isolation for its other good bits (You never see this one at audio).

There are (probably) other circuits but I think the emitter degeneration and collector bias are the big ones at audio.

Regards, Dan.
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Old 20th March 2013, 12:16 AM   #5
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Distortion is any deviation in amplitude (or phase) from the input signal.

In order to get the distortion down, engineers use a variety of techniques and methods.

A simple single transistor circuit can sound good, but will not measure free of distortion. It can have a bandwidth far in excess of the audio band.

To keep this simple, making a circuit is not usually simple. First you have to determine what you are going to use it for. Next you need to figure out how much gain it needs, and then the input signal level and input impedance, and then the same for the output - level (gain) and impedance.

The next step is to decide how to accomplish that.

Opamps are the simple way to get good results fast with minimum design.

Discrete transistors (or jfets, mosfets) are more complicated, and often end up being rather similar to what is stuffed inside an opamp.

For a good overview of electronics engineering and design that you can read without much math at all, and is written very clearly, buy a copy of Horowitz and Hill "The Art Of Electronics". It is not inexpensive, but if you want to learn, this is a very very good book. If you can find the first edition, used, and save money that is fine.

Otherwise you have to study and read up on how transistors (or tubes) work, and figure out how circuits work with other books, or online.

It's at once simple on some levels and exceedingly complex and detailed on another...

Hope this helps some.
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Old 20th March 2013, 08:34 AM   #6
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Quote:
Originally Posted by dmills View Post
There are a fair number of ways to arganise feedback around a single transistor, here are a few:
A resistor in the emitter lead will create feedback because the voltage across it will increase with increasing current flow. A very common trick for stabilising the dc operating point, and if you split the resistor and decouple the junction then you can set the DC operating point and the AC gain almost independently. Note all transistors have some intrinsic Re' so this mechanism always exists to some extent.
Connect the bias resistor to the collector, run common emitter and place a load resistor in the collector circuit. Not as stable as the first design, but it works and is often seen in simple minded mic preamps for things like baby monitors. You sometimes see this combined with the first idea.
Run the stage common base, more usually seen at RF this one, but if you need a low Z input and very good isolation it is not a bad approach at audio.
Then once you get away from audio, there are things like the norton noiseless feedback circuit wrapping a transformer around a common base stage to make a very broadband flat amplifier which is quiet but trades **VERY** poor reverse isolation for its other good bits (You never see this one at audio).
There are (probably) other circuits but I think the emitter degeneration and collector bias are the big ones at audio.
Regards, Dan.
Good morning Dan and thank you very much for the reply
First, i have limited knowledge ... very limited
My question is very basic
Given this general schema

Click the image to open in full size.

can i connect with a resistor the output to the emitter of the NPN ?
will it work ? whit what results ?
I am obsessed with the idea of getting the most from the least
I understand that this is quite a technical challenge
But first I would like to understand what is possible with extreme simple typology
And only then, if the results are unsatisfactory, I would pass to more complex designs.
I have two "principles " stuck in mind
1) the KISS principle
2) act on parts selection and design fine-tuning to improve performance of very basic design
I have the feeling, it is clearly onlt a feeling, that very simple design can give astonishing results in terms of sound
Maybe it is just an illusion of a confused mind
Thanks a lot and kind regards,
gino
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Old 20th March 2013, 08:38 AM   #7
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Gino,

If you remove CE you introduce local feedback on the emitter (also sometimes called degeneration).

If you connect the top of R1 not to the supply but to the collector, you also introduce feedback, also called loop feedback.
Of course R1 should be changed in value because the DC conditions are now changed.
Does that help?

jan
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Old 20th March 2013, 08:49 AM   #8
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Quote:
Originally Posted by bear View Post
Distortion is any deviation in amplitude (or phase) from the input signal.
In order to get the distortion down, engineers use a variety of techniques and methods.
A simple single transistor circuit can sound good, but will not measure free of distortion. It can have a bandwidth far in excess of the audio band.
Thank you sincerely for your kind and valuable reply, but i do not understand.
you say

Quote:
A simple single transistor circuit can sound good, but will not measure free of distortion.
It can have a bandwidth far in excess of the audio band


Maybe i am missing something.
Should not this be the real aim of any audio design ?
Given that there are still problems to correlate sound and figures.
I would be a little trivial (it is my nature, sorry ) but for me if it sounds good is good.
Even if it is just a little bjt doing all the amplification work.

Quote:
To keep this simple, making a circuit is not usually simple.
First you have to determine what you are going to use it for.
Next you need to figure out how much gain it needs, and then the input signal level and input impedance, and then the same for the output - level (gain) and impedance.
The next step is to decide how to accomplish that.
Opamps are the simple way to get good results fast with minimum design.
Discrete transistors (or jfets, mosfets) are more complicated, and often end up being rather similar to what is stuffed inside an opamp.
For a good overview of electronics engineering and design that you can read without much math at all, and is written very clearly, buy a copy of Horowitz and Hill "The Art Of Electronics".
It is not inexpensive, but if you want to learn, this is a very very good book.
If you can find the first edition, used, and save money that is fine.
Otherwise you have to study and read up on how transistors (or tubes) work, and figure out how circuits work with other books, or online.
It's at once simple on some levels and exceedingly complex and detailed on another...
Hope this helps some
Thank you so much for your helpful advice
I am trying to study schematics of commercial amps buying service manuals
At least they are working and tested design
But apart all the added complexity for protections, input selection, tone control ...
For istance ... i do not understand why high end equipment do not have tone controls and cheap products have them
I understand saving in parts quality, but not bigger complexity in design
This is even perverse for me
I will look for the book you mention anyway
Thanks a lot.
Kind regards,
gino
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Old 20th March 2013, 08:57 AM   #9
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Quote:
Originally Posted by jan.didden View Post
Gino,
If you remove CE you introduce local feedback on the emitter (also sometimes called degeneration).
If you connect the top of R1 not to the supply but to the collector, you also introduce feedback, also called loop feedback.
Of course R1 should be changed in value because the DC conditions are now changed.
Does that help?
jan
Yes Sir ! it helps a lot indeed
It is what I wanted to know. How to connect a basic circuit
I need gain of 2 and no more than 2-300 ohm of output impedance
about 24V for the supply (two 12V batteries in series).
For a one npn line stage
If it will not sound good i will pass to other designs, more elaborated i mean
Thank you very much again
Kind regards,
gino
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Old 20th March 2013, 09:25 AM   #10
godfrey is offline godfrey  South Africa
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Quote:
Originally Posted by ginetto61 View Post
I am trying to study schematics of commercial amps buying service manuals
You can get a lot of schematics and service manuals for commercial amps on Jan Dupont's website for free.
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