# Linear and Non Linear Circuits

Discussion in 'General Electronics Chat' started by Sparky49, Nov 15, 2012.

1. ### Sparky49 Thread Starter Well-Known Member

Jul 16, 2011
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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.

Sparky

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2. ### Papabravo Expert

Feb 24, 2006
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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?!!?

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3. ### Ron H AAC Fanatic!

Apr 14, 2005
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The harmonics of a square wave are all odd.

Last edited: Nov 15, 2012
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4. ### Papabravo Expert

Feb 24, 2006
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Had to be one or the other and I couldn't remember off the top of my head.

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5. ### Sparky49 Thread Starter Well-Known Member

Jul 16, 2011
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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.

6. ### Ron H AAC Fanatic!

Apr 14, 2005
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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.

• ###### square wave from odd harmonics.PNG
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7. ### Sparky49 Thread Starter Well-Known Member

Jul 16, 2011
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Nice, thanks Ron!

What program is that?

8. ### Ron H AAC Fanatic!

Apr 14, 2005
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It's good ol' LTspice, known and loved by dozens(?) of AAC members.

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9. ### Papabravo Expert

Feb 24, 2006
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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.

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10. ### Sparky49 Thread Starter Well-Known Member

Jul 16, 2011
836
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I have found the best piece of test equipment I own, is my nose.

11. ### nigelwright7557 AAC Fanatic!

May 10, 2008
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That circuit is often used in guitar fuzz boxes to create lots of harmonics.

I found a better solution was a soft limiter which gave a rounded off square wave.

12. ### Wendy Moderator

Mar 24, 2008
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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.

13. ### Ron H AAC Fanatic!

Apr 14, 2005
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Yeah, I once had a nice red D on the tip of my index finger for a few hours.

14. ### Jony130 AAC Fanatic!

Feb 17, 2009
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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.

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15. ### Ron H AAC Fanatic!

Apr 14, 2005
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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.