In this note, we are discuss about the Hartley Oscillator and Its Circuit Diagram and Operation. Welcome to Poly Notes Hub, a leading destination for Polytechnic Notes.
Author Name: Arun Paul.
About Hartley Oscillator
The Hartley Oscillator is similar to the Colpitt’s Oscillator with minor modifications. Instead of using tapped capacitor, two inductors L1 and L2 are placed across a common capacitor C and the centre of the inductor is tapped as shown in the figure below. The tank circuit is made up of L1, L2 and C. The frequency of oscillation is determined by the values of L1, L2 and C and is given by:
f = 1⁄(2π√CLᵪ) ——– (i)
where, Lᵪ = L1 + L2 + 2M
Here, M = Mutual inductance between L1 and L2
Note that L1 – L2 – C is also the feedback network that produces a phase shift of 180°
Circuit Operation
When the circuit is turned on, the capacitor is charged. When this capacitor is fully charged, it discharges through coils L1 and L2 setting up oscillations of frequency determined by exp (i). The output voltage of the amplifier appears across L1 and the feedback voltage across L2. The voltage across L2 is 180° out of phase with the voltage developed across L1 (V out) as shown in Fig.
It is easy to see that voltage feedback (i.e., voltage across L2) to the transistor provides positive feedback. The transistor produces a phase shift of 180° and a further phase shift of 180° is produced by the L1 – L2 voltage divider. In this way, feedback is properly phased to produce continuous undamped oscillations.
Feedback Fraction
In Hartley Oscillator, the feedback voltage is across L2 and output voltage is across L1.
Therefore, Feedback Fraction, mᵥ = L2/L1
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