As said in the last article the class C amplifying device (tube/transistor) only conducts during a small portion of the cycle yet the output is a sign wave.  This accomplished by so called “flywheel effect” of a tuned circuit.


Most people understand that the flywheel is a heavy wheel that uses its centrifugal force to swing a piston driven engine around so it can return to the power stroke.  The tuned circuit responds in a similar manner electronically.


A coil will store power in its magnetic lines of force.  When the power is removed these lines of force will collapse back into the coil thus generating a voltage in the opposite direction as that which was applied.  A capacitor also stores energy and when the circuit is completed between the terminals it will produce a current opposite to that which flowed during its charging.   If a DC voltage is placed on a coil with zero resistance, no such coil exists, the instantaneous voltage will be equal to the source and the current flow will be zero.  As the magnetic lines build the voltage will drop and the current will increase until the voltage is zero and the current is maximum level.    As the capacitor charges the voltage will go up until it reaches the value of the source and the current will drop to zero.


When AC is applied to the capacitor it will start to charge during as the current starts to flow in one direction in one direction and then will start to discharge after the voltage peak starts back down towards zero.  It will do the same on the other side of the AC wave form.  This action will cause the current to be limited in a fashion similar to resistance except resistance results in power loss due to its generation of heat but this action of the capacitor, called capacitive reactance, does not produce heat.  The larger the capacitance of a capacitor the smaller the reactance will be.


Considering the reaction of the coil, also known as an inductor, it should be easy to see that there would be an opposite reaction to the AC and this is known as inductive reactance.


Reactance, like resistance, is measured in ohms.  When the inductive reactance and the capacitive reactance are equal the coil will discharge into the capacitor and when the clasping inductive field and the charging capacitor voltage are equal the current will cease to flow and the capacitor will discharge back through the coil.  If there was no resistance and if no energy was radiated this action would continue on indefinitely but there is resistance and energy is radiated so the process dies out.  If the output from the amplifying device continues to supply energy then the action can be sustained.  This action is called the flywheel effect and it is how a class C amplifier can put out a sign wave.


If the tuned circuit are resonate at a multiple (harmonic) of the frequency being injected into the amplifier the flywheel effect will continue between the pulses then will be reenergizes on the next pulse.  For example if the input circuit is tuned to 2 MHz the output circuit is tuned to 4 MHz  spikes from the amplifying device’s output will be every other cycle and the amplifier’s output will be 4 MHz.


If the output is fed back to the input when power is placed on the circuit the tuned circuit will start vibrating and if it is in phase then the output will support the input and the output will continue to keep the tuned circuit vibrating.  This results in the results being an oscillator.


My wife says when people ask me what time it is I have the bad habit explaining how to build a watch.   That is a little of what happened here.  My original intent was to explain what a linear amplifier is but I guess I got carried away.  I hope you enjoyed this explanation of how amplifiers work.  If you have questions or comments please email me at and be sure to put the words Ham’s Life in the subject line so I don’t think it is spam.  I need feedback so I know what people would like to see.


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