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Why harmonic appear in output of amplifier when pure sine wa - RF
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Below are the old forum threads, including responses to the original posts.
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Post subject: Why harmonic appear in output of amplifier
when pure sine wa
Unread postPosted: Tue Mar 22, 2005 5:34 pm
Joined: Tue Mar 22, 2005 5:29 pm
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.
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
+ 0.01 Vin^2. If the input is sin(wt), then the output is
+ .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
I hope this helps.
Unread postPosted: Wed Mar
23, 2005 5:27 am
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..
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.
Thu Mar 24, 2005 5:39 pm
Mar 22, 2005 5:29 pm
Thankyou to both of you for your
input to help me to see the light.