Complete Electronics Self-Teaching Guide with Projects

Introduction to Oscillators

In addition to their use in circuits used to filter input signals, capacitors and inductors are used in circuits called oscillators.

Oscillators are circuits that generate waveforms at particular frequencies. Many oscillators use a tuned parallel LC circuit to produce a sine wave output. This section is an introduction to the use of parallel capacitors and inductors in oscillators.

30 When the switch in the circuit shown in drawing (1) of Figure 7.44 is closed, current flows through both sides of the parallel LC circuit in the direction shown.

Figure 7.44

It is difficult for the current to flow through the inductor initially because the inductor opposes any changes in current flow. Conversely, it is easy for the current to flow into the capacitor initially because with no charge on the plates of the capacitor there is no opposition to the flow.

As the charge on the capacitor increases, the current flow in the capacitor side of the circuit decreases. However, more current flows through the inductor. Eventually, the capacitor is fully charged, so current stops flowing in the capacitor side of the circuit, and a steady current flows through the inductor.

Question

When you open the switch, what happens to the charge on the capacitor? _____

Answer

It discharges through the inductor. (Note the current direction, shown in drawing [2] of Figure 7.44.) ...