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## Electronic Oscillation 

Oscillation in electronic circuits is accomplished by combining a transistor and a tank circuit. A tank circuit is comprised of a capacitor and coil parallel to each other. [Figure 11-58]  When attached to a power source by closing switch A, the capacitor charges to a voltage equal to the battery voltage. It stays charged, even when the circuit to the battery is open (switch in position B). When the switch is put in position C, the capacitor and coil are in a closed circuit. The capacitor discharges through the coil. While 

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receiving the energy from the capacitor, the coil stores it by building up an electromagnetic field. When the capacitor is fully discharged, the coil stops conducting. The magnetic field collapses, which induces current flow. The current charges the opposite plate of the capacitor. When completely charged, the capacitor discharges into the coil again. The magnetic field builds again and stops when the capacitor is fully discharged. The magnetic field collapses again, which induces current that charges the original plate of the capacitor and the cycle repeats. 

This oscillation of charging and discharging the capacitor through the coil would continue indefinitely if a circuit could be built with no resistance. This is not possible. However, a circuit can be built using a transistor that restores losses due to resistance. There are various ways to accomplish this. The Hartley oscillator circuit in Figure 11-59  is one. The circuit can oscillate indefinitely as long as it is connected to power. 

When the switch is closed, current begins to flow in the oscillator circuit. The transistor base is supplied with biasing current through the voltage divider R$_{A}$ and R$_{B}$. This allows current to flow through the transistor from the collector to the emitter, through R$_{E}$ and through the lower portion of the center tapped coil that is labeled L$_{1}$. The current increasing through this coil builds a magnetic field that induces current in the upper half of the coil labeled L$_{2}$. The current from L$_{2}$ charges capacitor C$_{2}$, which increases the forward bias of the transistor. This allows an increasing flow of current through the transistor, R$_{E}$, and L$_{1}$ until the transistor is saturated and capacitor C$_{1}$ is fully charged. Without force to add electrons to capacitor C$_{1}$, it discharges and begins the oscillation in the tank circuit described in the previous section. As C$_{1}$ becomes fully charged, current to charge C$_{2 }$ reduces and C$_{2}$ also discharges. This adds the energy needed 

