Harmonic Distortion

[lonemanoj]

What is harmonic distortion ? How it occurs in Power Amplifier? How we can reduce it?

[studiot]

Harmonic distortion occurs because the amplitude transfer function is (hopefully only slightly) non linear.

 

The standard design method of reduction is to attempt to make the basic amplifier as linear as possible and then apply negative feedback.

 

The user's method is to operate the amplifier with the volume at around the 45% to 75% of maximum.

 

You should be aware that there are other types of distortion that are may need other countermeasures.

[lonemanoj]

What other type of distortion's are present ?

[studiot]

 

What other type of distortion's are present ?

 

 

Note I didn't say they were always present.

 

Is this homework ie have you been asked to find out a list of distortions?

 

I would look at

Crossover distortion

Intermodulation distortion

Phase distortion

Transient intermodulation distortion

 

Other unwanted signals include

 

Noise

Hum

Interference

 

Other unwanted effects include

 

Pumping/ motorboating

Limiting/ clipping

Inaccurate pre-emphasis/de-emphasis

[lonemanoj]

Thanks. By the way this isn't a homework. My lecturer gave a us a lecture on Power Amplifiers, so i wanted to know more, you know "curiosity"

[studiot]

 

Thanks. By the way this isn't a homework. My lecturer gave a us a lecture on Power Amplifiers, so i wanted to know more, you know "curiosity"

 

 

Glad you have the interest.

You will certainly need to know all this before the end of an electrical engineering course.

 

The more information about where you are coming from or your interest in a question the better will be the forthcoming answer.

 

I was expecting further questions about my initial response.

[lonemanoj]

Yeah i would like to know more like what is crossover distortion and others. And yeah is there any electronic circuit which can overcome all the distortion all in one.

[studiot]

 

And yeah is there any electronic circuit which can overcome all the distortion all in one.

 

 

One manufacturer used to call the perfect amplifier as "A piece of straight wire with gain".

 

What did your lecturer say about distortion, of any type?

[lonemanoj]

My lecturer said, transistor is a non linear device. And Power amplifier handle large signal hence there is always distortion. Due to distortion the output power targeted on the basis of non distortion is not correct.

But i didn't agree with him.

How can amplifying large signal be only reason?

[studiot]

 

My lecturer said, transistor is a non linear device. And Power amplifier handle large signal hence there is always distortion. Due to distortion the output power targeted on the basis of non distortion is not correct.

But i didn't agree with him.

How can amplifying large signal be only reason?

 

 

Now we are getting somewhere. Your lecturer is correct.

 

 

Think about a sine wave.

This means that the instantaneous amplitude is continually varying and we have an instantaneous (voltage) amplitude V_inst = V_maxsin (wt+p)

I hope you understand this bit?

 

So when the amplitude is say 0.25 volts the amplification factor is say 100.

When the amplitude is say 0.5 volts the amplification factor drops to say 85

When the amplitude is say 0.75 volts the amplification factor drops even more to say 65.

 

This means that different parts of even a simple sine wave are amplified by different amounts.

So the result is not longer a true sine wave.

 

I don't know if you have come across Fourier or Harmonic analysis yet?

 

But essentially any continuous waveform can be expanded in a trigonometric series called a Fourier series.

 

The original sine wave had only one term in this series - the fundamental.

 

Harmonic distortion introduces additional terms which describe the distorted waveshape.

 

So another description of harmonic distortion is that it introduces frequencies not present in the original.

Edited September 17, 2014 by studiot

[lonemanoj]

V inst =V maxsin (wt+p) , i understand this but we haven't come across Fourier or Harmonic Analysis yet.

 

If say in simple word, "as soon as the amplitude of sine wave increases gradually, then there is a gradual drop in the amplification." Is this term logical and correct.

 

If the sine wave is no longer a true sine wave of input, then the distorted wave is know by what name. Or its simply known as Distorted Output.

 

What if we connect a filter in the output of any signal to decrease distortion somewhat or a rectifier.

[studiot]

 

What if we connect a filter in the output of any signal to decrease distortion somewhat or a rectifier

 

Rectifiers will simply loose you half the signal

 

 

Distortion can be filtered to some extent, but you would not want to do this at the power stage, further filters will also reduce wanted output.

 

OK so you haven't yet done Fourier series.

Do you understand feedback?

 

I have roughed out some quick sketches to help.

 

The first is a voltage transfer function, linear and non linear like a transistor or most amps.

 

The second is what happens when you feed a pure sine wave into an amp with the non linear transfer function.

You get a flattened sine wave out.

 

You should plot for yourself a few points to see what happens.

This is the best teaching aid.

 

[lonemanoj]

It really helped to after drawing sketch.

 

What are the components Harmonic Distortion brings in a pure wave form what's it's time period to last. ?

[studiot]

 

It really helped to after drawing sketch.

 

What are the components Harmonic Distortion brings in a pure wave form what's it's time period to last.

 

I'm happy to do some (easy) maths with you as a reasonable presentation for distortion in a transistor can be made without Fourier series.

 

However I would appreciate more full answers as this means quite a bit of work and I need to get the level correct.

 

Here is a start, tell me if we can continue.

 

First I will simplify my original equation by dropping the phase part. Also since I am considering specifically transistors my transfer function will be written in terms of an input voltage and a resultant output (collector) current. That is at any instant the collector current will be I_c = f(V_i)

 

So consider a signal V_i= V_msin(wt) , input to the transistor.

 

If the transistor were linear then the output would be a first order or linear function of the input

 

That is I_c = I_q + aV_i

 

Where the instantaneous output (collector current) equals a constant current (I_q) plus a variable current proportional to the instantaneous input voltage.

(The constant current is called the bias or quiescent currrent of the transistor)

a is the constant of proportionality called the transfer coefficient.

 

So I_c = I_q + aV_msin(wt)

 

So for linear operation the output is the input multiplied by a constant of proportionality.

 

So there is no distortion.

 

Now I showed the transfer curve curling over for non linear operation.

 

The simplest such curve is second order or a parabola so let us assume that

 

I_c = I_q + aV_i + b(V_s)² = I_q + aV_msin(wt) + b(V_msin(wt))²

 

Where b is a second constant to be evaluated.

 

= I_q + aV_msin(wt) + b(V_m)²sin²(wt)

 

Since sin²(wt) = 1/2(1-cos2wt)

 

I_c = (I_q + b(V_m)²/2) + aV_msin(wt) - b(V_m)²/2 (cos2wt)

 

So the output collector current consists of the following three components

1. A DC component equal to (I_q + b(V_m)²/2)
2. An alternating fundamental componenet of frequency equal to that of the driving frequency with a peak value equal to

   aV_msin(wt)

3. An alternating second harmonic component of a frequency equal to twice that of the driving frequency and of a peak value equal to b(V_m)²/2

If you can follow this so far we can continue this simple analysis of the harmonic distortion.

[lonemanoj]

Glad you helped me. I know about a distortion now.

 

Does distortion occur in every output or is there any specific conditions for that

[studiot]

Edit

 

Sorry put this into the wrong thread.

 

Moderator please delete this post.

Edited September 23, 2014 by studiot