How Hartley Oscillator Circuit works?

In Hartley Oscillator, the combination of L-C is used to produce high output frequency. It consists of one transistor, a tank circuit and the positive feedback to produce oscillations. Its circuit diagram is given below.

Connection Details: As shown above, resistors R₁, R₂ and R_e provide self bias to transistor T₁. The capacitor C_e acts as bypass capacitor.

The tank circuit consists of two coils L₁ and L₂ with capacitor C. They decide the output frequency of the circuit. The capacitor C₁ is decoupling capacitor which block DC component and allows only AC to pass through it.

The RF choke is used to limit the collector current of T₁ to a safe value at high output frequencies. The capacitor C₂ and resistor R₂ provide self biasing to T₁.

Working: When supply is connected to the circuit, a small transient current is produced in tank circuit. So damped oscillations start producing in it. So AC voltage is produced across L₁ and L₂. But the center point of L₁ and L₂ is grounded.

So phase shift of 180° is produced between AC voltage of L₁ and L₂. Another phase shift of 180° is produced by transistor T₁ (used in CE mode). So the feedback signal is in-phase with input signal. The voltage across L₂ is the feedback to base of T₁ and voltage across L₁ is given at the output.

The output frequency of the circuit is given by:

Where,
L = L₁ + L₂
C = capacitor in parallel with L₁ and L₂

Advantages

1. The output frequency of the circuit can be controlled easily with capacitor C.
2. It provides good frequency stability in radio frequency range.
3. Single coil at the output can be tapped at any point to adjust output voltage.