Linear and Non Linear Circuits

Thread Starter

Sparky49

Joined Jul 16, 2011
833
Hi all.:)

I've been doing some reading on linear and non-linear circuits, however I've come across something which I need some more help with.

As I understand it, if I were to construct the black circuit below and pass a sine wave through it, I would get the spectrum shown (both the red a blue lines). Lots of harmonics.

However, if I add the green diode, the red lines will disappear leaving fewer harmonics.

Why is this?

I'm guessing that it's something to do with 'balancing' the circuit, but what exactly is going on? I realise that the diodes will 'clip' the tops and bottoms of the sine waves, but I don't see how this will affect the harmonics.

Many thanks for your time,

Sparky
 

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Papabravo

Joined Feb 24, 2006
21,158
It is hard to tell from the drawing without knowing about the input signal. If it is a sinewave then the single diode will clip the positive peaks and the other diode will clip the negative peaks. The output starts to look like a square wave with only even(odd) harmonics.

BTW - what does balancing the circuit mean?!!?
 

Ron H

Joined Apr 14, 2005
7,063
It is hard to tell from the drawing without knowing about the input signal. If it is a sinewave then the single diode will clip the positive peaks and the other diode will clip the negative peaks. The output starts to look like a square wave with only even(odd) harmonics.

BTW - what does balancing the circuit mean?!!?
The harmonics of a square wave are all odd.
 
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Thread Starter

Sparky49

Joined Jul 16, 2011
833
Thanks guys, that makes sense.

So if one were to keep 'squaring' the sine wave, the odd harmonics would become greater and greater?

Apologies for improper terminology, 'balancing' was kind of describing what was happening to the sine wave, with one diode, only the bottoms are clipped, but adding the second clips both. I sometimes use a poor choice of word.

Sparky.
 

Ron H

Joined Apr 14, 2005
7,063
Thanks guys, that makes sense.

So if one were to keep 'squaring' the sine wave, the odd harmonics would become greater and greater?

Apologies for improper terminology, 'balancing' was kind of describing what was happening to the sine wave, with one diode, only the bottoms are clipped, but adding the second clips both. I sometimes use a poor choice of word.

Sparky.
Yep. Here's a simulation with odd harmonics 3,5,7, and 9, scaled and summed. If you took a square wave and applied a brick wall filter that eliminated all the higher harmonics, it would look like this.
 

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Papabravo

Joined Feb 24, 2006
21,158
It gets my vote for answering numerous question without running the risk of letting the magic smoke out. It is well know that electronics depends on magic smoke for proper operation.
 

Wendy

Joined Mar 24, 2008
23,415
Ahh, there is nothing to wake you up like a mirror image brand of a part number on a finger tip. No permanent scars yet, but I'm working on it.
 

Jony130

Joined Feb 17, 2009
5,487
What I know is that :

- Even harmonics (2nd, 4th, etc.) result from asymmetrical non-linearities of a transfer function.
- Odd harmonics (3rd, 5th, etc.) result from symmetrical nonlinearities of a transfer function

But I also don't understand why.
 

Ron H

Joined Apr 14, 2005
7,063
What I know is that :

- Even harmonics (2nd, 4th, etc.) result from asymmetrical non-linearities of a transfer function.
- Odd harmonics (3rd, 5th, etc.) result from symmetrical nonlinearities of a transfer function

But I also don't understand why.
One way of understanding it is to work backwards. Sum a fundamental with an attenuated second harmonic, and observe the asymmetry (in the Y axis) of the result.
 
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