Page 226 - Complete Wireless Design

Oscillator Design

Oscillator Design 225

and C block the DC, but couple the AC feedback, while L and C decouple the
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oscillator output, preventing it from being injected into the power supply. L also
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functions as the transistor’s collector load, and R and R supply the forward
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bias. R and C further increase the temperature stability of this circuit without
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allowing the AC gain to be decreased, as it would be if just R were used.
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The LC tank of the Hartley furnishes the required 180 degree phase shift for
regenerative feedback, thus allowing oscillations. This is because L ’s tapped
1
coil forces the signal between the center tap and the top of the coil (due to the
current flow with respect to the grounded tap; Fig. 4.12) to be opposite in
polarity to the center tap and the bottom of the coil. The location of the tap on
the inductor sets the amplitude of the positive feedback.

Figure 4.11 A type of LC Hartley oscillator.

Figure 4.12 A tapped coil showing the phase relationships.

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