OSCILLATOR CIRCUITS

Oscillators may be classified by name, such as Armstrong, Hartley, Colpitts, or by the manner in which dc power is applied. An oscillator in which dc power is supplied to the transistor through the tank circuit, or a portion of the tank circuit, is said to be SERIES FED. An oscillator which receives its dc power for the transistor through a path separate and parallel to the tank circuit is said to be PARALLEL FED OR SHUNT FED. All the oscillators in this chapter can be constructed either way, series or shunt fed. The construction depends on the characteristics of the oscillator circuit the designer is interested in.

A SERIES-FED, TUNED-COLLECTOR ARMSTRONG OSCILLATOR is illustrated in figure 2-9 , view (A). The dc path is from the negative side (ground) of V_CC through R_E, Q1, T1, and back to the positive side of V_CC. The figure clearly illustrates that both the ac and dc components flow through the tank circuit.

Figure 2-9A. - Series- and shunt-fed, tuned-collector Armstrong oscillators. SERIES-FED

By modifying the circuit slightly, it becomes a SHUNT-FED, TUNED-COLLECTOR ARMSTRONG OSCILLATOR as shown in view (B). The dc component flows from ground through R_E to Q1 to positive V_CC. The dc is blocked from the tank circuit by capacitor C2. Only the ac component flows in the tank circuit.

Figure 2-9B. - Series- and shunt-fed, tuned-collector Armstrong oscillators. SHUNT-FED

The function of an oscillator is to produce a sinusoidal waveshape of a specific frequency and amplitude. In doing so, the stability of an oscillator is very important. Depending on its application, an oscillator may be required to have either good frequency stability or amplitude stability; in many circumstances, both are required. Of the two, good frequency stability is usually considered more important.