The Hartley Oscillator

  
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The Hartley Oscillator is a particularly useful circuit for producing good quality sine wave signals in the RF range, (30kHz to 30MHz) although at the higher limits of this range and above, The Colpitts oscillator is usually preferred. Although both these oscillators oscillator use an LC tuned (tank) circuit to control the oscillator frequency, Th
The Hartley Oscillator - schematic

e Hartley design can be recognised by its use of a tapped inductor (L1 and L2 in Fig. 2. 1. 1). Fig. 2. 1. 1 shows a typical Hartley oscillator. The frequency determining resonant tuned circuit is formed by L1/L2 and C3 and is used as the load impedance of the amplifier. This gives the amplifier a high gain only at the resonant frequency (Method 2 in Introduction to Oscillators). This particular version of the Hartley circuit uses a common base amplifier, the base of TR1 being connected directly to 0V (as far as AC the signal is concerned) by C1. In this mode the output voltage waveform at the collector, and the input signal at the emitter are in phase. This ensures that the fraction of the output signal fed back from the tuned circuit collector load to the emitter via the capacitor C2 provides the necessary positive feedback. C2 also forms a long time constant with the emitter resistor R3 to provide an average DC voltage level proportional to the amplitude of the feedback signal at the emitter of Tr1. This is used to automatically control the gain of the amplifier to give the necessary closed loop gain of 1. The emitter resistor R3 is not decoupled because the emitter terminal is used as the amplifier input. The base being connected to ground via C1, which will have a very low reactance at the oscillator frequency. The LC circuit that controls the frequency of oscillation is often called the "TANK CIRCUIT" because it...



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