2-5
For a given RC network, only one frequency of the initial noise signal will be shifted exactly 180
degrees. In other words, the network is frequency selective. Therefore, the RC network is the frequency-
determining device since the lengths of the vectors and their phase relationships depend on frequency.
The frequency of oscillations is governed by the values of resistance and capacitance in these sections.
Variable resistors and capacitors may be used to provide tuning in the feedback network to allow for
minor variations in phase shift. For an RC phase-shift oscillator, the amplifier is biased for class A
operation to minimize distortion of the wave or signal.
LC Network
Some sine-wave oscillators use resonant circuits consisting of inductance and capacitance. For
example, recall the tank circuit in which a resonant circuit stores energy alternately in the inductor and
capacitor, producing a sine wave. You studied this action of the tank circuit in chapter 1.
If there were absolutely no internal resistances in a tank circuit, oscillations would continue
indefinitely, as shown in figure 2-3, view (A). Each resonant circuit does, however, contain some
resistance which dissipates power. This power loss causes the amplitude to decrease, as shown in views
(B) and (C). The reduction of amplitude in an oscillator circuit is referred to as DAMPING. Damping is
caused by both tank and load resistances. The larger the tank resistance, the greater the amount of
damping. Loading the tank causes the same effect as increasing the internal resistance of the tank. The
effect of this damping can be overcome by applying regenerative feedback.
Figure 2-3A.—Effects of damping.
Figure 2-3B.—Effects of damping.
Figure 2-3C.—Effects of damping.
