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.
The Colpitts oscillator uses a parallel LC circuit. The capacitance of this tuned circuit is made up of two series connected capacitors which form a capacitive voltage divider. The LC tuned circuit is used as the input to an amplifier, and the output of the amplifier is fed to the capacitive tap.
The circuit below is a practical example of a Colpitts oscillator. The components L1, C1, C2 and VC1 form the LC tuned circuit. The capacitor C3 couples this to the amplifier formed by Q1. The output of the amplifier is taken from the top of R3 and fed to the tap formed between C1 and C2. Components R4, R5 and Q2 form an emitter follower. The emitter follower helps to ensure that connections to the output do not load the oscillator.
The Hartley oscillator uses a parallel LC circuit. The 'L' part of this tuned circuit is made up of a tapped inductor. The LC tuned circuit is used as the input to an amplifier, and the output of the amplifier is fed to the tap on the inductor.
The circuit below is a practical example of a Hartley oscillator. The components L1 and VC1 form the LC tuned circuit. The capacitor C1 couples this to the amplifier formed by Q1. The output of the amplifier is taken from the emitter of Q1 and is fed to the tap on L1. A follower stage would be necessary in practice to prevent loading on the 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.
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.
The 555 is an integrated circuit first introduced by Signetics, but now produced by many semiconductor manufacturers. The usual package is an 8-pin DIL. Internally the 555 is DC coupled. It can be used for monostable and astable oscillators. The internal construction of the 555 is as follows:
The 555 IC may be used to form an astable oscillator:
The 555 IC may also be used as a monostable:
