Figure 136. Shunt-fed Hartley oscillator.
(1) Buffer amplifier. If the output of an oscillator is coupled directly to a power amplifier, undesirable loading effects occur. There can be distortion of the output waveform or even a stopping of oscillation. In addition, the frequency of the oscillation will not be stable. A buffer amplifier, therefore, is used to couple the oscillations to the power amplifier. An ordinary triode voltage amplifier can serve as a buffer, since it draws little power from the oscillator.
(a) By using a multielectrode tube, the oscillator and buffer stages can be replaced by one circuit which performs both functions. Such a circuit is called an electron-coupled oscillator. Figure 137 is a typical circuit arrangement, using a 6F6 pentode.
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