2-19
Figure 2-14.—Shunt-fed, tuned-base Hartley oscillator.
Q-12. What is the main difference between the Armstrong oscillator and the Hartley oscillator?
Q-13. What is the difference between the series-fed and the shunt-fed Hartley oscillator?
COLPITTS OSCILLATOR
Both the Armstrong and the Hartley oscillators have a tendency to be unstable in frequency because
of junction capacitance. In comparison, the COLPITTS OSCILLATOR has fairly good frequency
stability, is easy to tune, and can be used for a wide range of frequencies. The large value of split
capacitance is in parallel with the junctions and minimizes the effect on frequency stability.
The Colpitts oscillator is very similar to the shunt-fed Hartley oscillator, except that two capacitors
are used in the tank circuit instead of a tapped coil (figure 2-15). The Hartley oscillator has a tap between
two coils, while the Colpitts has a tap between two capacitors. You can change the frequency of the
Colpitts either by varying the inductance of the coil or by varying the capacitance of the two capacitors in
the tank circuit. Notice that no coupling capacitor is used between the tank circuit and the base of Q1.
Capacitors C1 and C2 of the tank circuit are in parallel with the input and the output interelement
capacitance (capacitance between emitter, base, and collector) of the transistor. Thus the input and the
output capacitive effect can be minimized on the tank circuit and better frequency stability can be
obtained than with the Armstrong or the Hartley oscillator.
