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<p>[QUOTE=&quot;IDIeselman, post: 1859690, member: 61409&quot;]MAT, This is a post from eagle1911 on another forum, I found it informative. Hopefully WA4GCH wil be along to add to it.</p><p>&nbsp;</p><p>&nbsp;</p><p>&nbsp;</p><p>&nbsp;</p><p>First off let me state that when an amplifier adds parts to or removes parts of the input signal, that is distortion. Since the transistors in class C amps conduct less than 180 deg. of the full waveform, the clipping of the waveform (missing parts) is a big part of what causes the distortion. If you take a sine wave, as an example, and clip better than half of it, you only get part of the original waveform back, (and a small part at that) but the points at which the wave is clipped form sub-waveforms that are at multiples of the original frequency. This is harmonic distortion. When you increase the bias current and/or voltage, the transistor begins to reproduce more and more of the waveform until, with class A amps, the full input waveform is reproduced exactly. This is the best possible design, since there are no missing parts of the waveform, but requires the most current.</p><p>&nbsp;</p><p>Typically, what a conscientious operator wants is full power, with the least amount of trash (spurious emissions like harmonics and splatter). This is easily obtained with properly driven class A circuitry. Even with class A though, if you overdrive the amp it will cause distortion just like class C amps. Class B amps are dirtier and more efficient than class A by a large margin, but they are cleaner than class C. Class AB is a compromise between class A and class B, resulting in a good balance between cleanliness and efficiency.</p><p>&nbsp;</p><p>Now my understanding of IMD is a bit limited, but as I understand it when a transistor passes the threshold between it&#039;s &quot;off&quot; state and it&#039;s &quot;on&quot; state (the point at which it starts conducting), at that instant the transistor&#039;s output is not always perfectly faithful to the input waveform. I think of it like what happens when you flip a switch. Often there is a spike of power at the instant the switch engages. This is more pronounced when there is a lot of current involved. When there is a large signal entering a transistor, every time it switches on (which happens every cycle) a tiny spike is generated. This train of spikes becomes it&#039;s own signal superimposed on the main signal, which is distortion. IMD is much less of a problem at low drive levels, and becomes a BIG problem when an amp is overdriven.</p><p>&nbsp;</p><p><a href="http://www.worldwidedx.com/members/eagle1911.html" target="_blank" class="externalLink ProxyLink" data-proxy-href="http://www.worldwidedx.com/members/eagle1911.html" rel="nofollow"><span style="color: black"><b><font size="5">eagle1911</font></b></span></a> <img src="http://www.worldwidedx.com/images/statusicon/user_offline.gif" class="bbCodeImage wysiwygImage" alt="" unselectable="on" />[/QUOTE]</p><p><br /></p>

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