The LC Resonant Circuit.

Many electronic oscillator circuits, especially radio frequency circuits, are based on the LC resonant circuit or 'tank' circuit. When a charged capacitor is connected across an inductor current can oscillate to and fro through the circuit formed. The oscillations are a consequence of resonance. The frequency of these oscillations (known as the resonant frequency) is dependent on the values of the inductor and capacitor. This type of circuit is known as a parallel LC circuit. If an A.C. voltage of the same frequency as the resonant frequency is applied to the circuit, then the circuit behaves like an open-circuit.

In contrast, if an inductor and capacitor are connected in series, a series LC circuit is formed which also has a resonant frequency. If an A.C. voltage of the same frequency as the resonant frequency is applied to the series LC circuit, then the circuit behaves like a short-circuit.

Wien Bridge Oscillator.

Wien Bridge Principle

Blocking Oscillator.

Blocking Oscillator

UJT Oscillator.

The Clapp Oscillator.

If a Colpitts oscillator must have an output frequency that is variable, there are two choices: vary the inductance in the LC circuit, or use twin-ganged variable capacitances of unequal value. In practice the easier option is to make the inductance variable. An alternative is to use the Clapp oscillator (sometimes called the modified Colpitts). A practical example based on an n-channel JFET is shown below.

Clapp Oscillator

The components L1, VC1, C1 and C2 form the LC resonant circuit. In practice, C1 and C2 are made much larger than VC1 so that it is VC1 alone that has the largest effect on the frequency of oscillation. This arrangement also has the effect of eliminating the contribution of the transistor's capacitances. L2 and L3 are high value low Q inductors (i.e. RFC's). A follower stage would be necessary in practice to prevent loading on the oscillator.