How Colpitt’s Oscillator circuit works?

In previous circuit of Hartley oscillator, we used two coils L₁ and L₂ and one capacitor C. But in this circuit of Colpitt’s oscillator, we use two capacitors C₁ and C₂ with a single coil L. 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 one coil L with two capacitors C₁ and C₂. Their values control the output frequency. The capacitor C₄ 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 C₃ and resistor R₂ provide self biasing to T₁.

Working: When power supply is switched on, a small transient current is produced in tank circuit. So damped oscillations start. A small input signal is connected to the base of T₁. This signal is amplified by T₁ to produce large collector current.

The phase shift of 180° is produced by L with C₁ and C₂. 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 output frequency of the circuit is given by:

Where,
L = inductor in parallel with two capacitors
C’ = series combination of C₁ and C₂ – check the article of electrostatics to understand series-parallel combination of capacitors.

Advantages

1. No variable capacitor in the circuit. So output frequency cannot be controlled easily.
2. It provides good frequency stability in radio frequency range.