# Why Harmonic Appear in Output of Amplifier When Pure Sine Wa - RF Cafe Forums

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jkhoo2989
Post subject: Why harmonic appear in output of amplifier when pure sine wa
Unread postPosted: Tue Mar 22, 2005 5:34 pm
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Joined: Tue Mar 22, 2005 5:29 pm
Posts: 3
Pure sine wave does not have harmonics. If this pure sine wave is input to a power amplifier, why does harmonic start to appear on the output. The reason behind my question is if a pure sine wave go into the power Amplifier, the output should still be a pure sine wave with higher amplitude. Hope someone can help to shed some light on this.

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Guest
Post subject: Harmonics
Unread postPosted: Tue Mar 22, 2005 9:59 pm

The output of the power amplifier is not exactly a constant times the input. As the output departs from strict linearity, other frequencies are generated. If there is only a single sine wave present at the input, only harmonics are generated. The other signals are generated in and by the amplifier.

For example, suppose the actual output of the amplifier is
5Vin + 0.01 Vin^2. If the input is sin(wt), then the output is
5 sin(wt) + .01 sin^2(wt). But sin^2(x) = 1/2[cos(0)+cos(2x)], so we now have a 2x or 2wt or second harmonic present, where none was present at the input.

I hope this helps.

Good luck!

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j_almira
Post subject:
Unread postPosted: Wed Mar 23, 2005 5:27 am

Hi,

I agree with the posting from another fellow. I'd like to add regarding this harmonics signals. One of the main reason is because active devices in power amplifier design pratically is non-linear. As we know the active devices such as BJT, MESFET, HBT,etc all are non-linear devices.

That's why it produces output signals with frequency different with input signal frequency, which are known as harmonic signals.

Hope it helps..

Regards,
Jean

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Guest
Post subject:
Unread postPosted: Wed Mar 23, 2005 3:09 pm

The other two posts are correct , that what you are seeing is the non-linear behavior of the amplifier. If you look at the the output in the time domain the sahpe of the signal will not be a perfect sine wave, but slightly distorted. Simple test for you, back the input power of the sine wave into the amplifier off, you will see that the lower in power the lower the harmonics , until they dissapear. This is because you are away from the compression poin tof the amplifier and are operating in the linear region.

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jkhoo2989
Post subject:
Unread postPosted: Thu Mar 24, 2005 5:39 pm
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Joined: Tue Mar 22, 2005 5:29 pm
Posts: 3
Thankyou to both of you for your input to help me to see the light.

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