Transistor Hartley Oscillator

Posted on Feb 7, 2014

Any vacuum-tube oscillator configuration has an equivalent transistor circuit. For example, consider the vacuum-tube oscillator, illustrated in Fig. 6-1 (A), which represents one form of Hartley oscillator. Positive feedback is accomplished by arranging the resonant tank E to be common to both the input grid and output plate circuits. The equivalent transistor circuit using a grounded emitter

Transistor Hartley Oscillator
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connection is illustrated in Fig. 6-1 (B). Again, positive feedback is provided by placing the resonant tank so that it is common to both the input base and output collector circuits. If ground is removed from the emitter lead, and placed at the bottom of the tank circuit, the electrical operation of the oscillator is unchanged. Notice that when this circuit is rearranged as illustrated in Fig. 6-1 (C), it is now in the grounded-base connection. While the grid bias of the vacuum-tube oscillator in Fig. 6-1 (A) is regulated by the grid leak resistor RG, the equivalent transistor base in Fig. 6-1 (C) is self-biased through resistor RB. In all three circuits, the battery supply is decoupled by an R-F choke. The major difference between the operation of the vacuum-tube Hartley oscillator and that employing a transistor lies in the loading effect of the emitter resistance on the tank coil. This resistance is reflected into the tank circuit and acts as an equivalent shunting resistance. The tank is also shunted by the collector resistance, and the equivalent shunt resistance of the resonant circuit becomes. Oscillation starts when the equivalent shunt resistance of the tank is counterbalanced by the reflected negative-resistance of the emitter. The optimum tap point of the coil (as determined both mathematically and, where T is the ratio of the feedback turns included in the emitter circuit to the total number of tank coil...

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