How it works
In the Colpitts oscillator, two capacitors C1 and C2 in series form the bottom half of the tank, with inductor L in parallel. The junction between C1 and C2 provides the feedback signal to the transistor base. The effective tank capacitance is C_eff = C1·C2/(C1+C2). Feedback fraction β = C1/(C1+C2) (assuming C1 is the capacitor connected from emitter to ground). Barkhausen: the transistor gain must exceed C2/C1. Phase: the emitter voltage (feedback point) leads the collector by 180° through the tank, and the common-emitter stage inverts by 180°, satisfying 360° total loop phase shift.
Key points to remember
Oscillation frequency f_0 = 1/(2π√(L·C_eff)) where C_eff = C1C2/(C1+C2). The condition for oscillation is g_m·r_e ≥ C2/C1 (or equivalently, transistor current gain > C2/C1). Colpitts oscillators operate reliably at higher frequencies than Hartley because stray inductance is less problematic than stray capacitance at RF — capacitors have more predictable parasitics. The JFET Colpitts oscillator is popular for very-high-impedance applications because gate loading does not damp the tank. Practical frequency stability is 0.01–0.1%, much poorer than crystal but much better than RC oscillators.
Exam tip
Anna University analog electronics papers frequently ask you to compare Hartley and Colpitts oscillators — the one-line answer is that Hartley uses a tapped inductor for feedback while Colpitts uses a capacitive voltage divider, and Colpitts is preferred at higher frequencies because capacitor parasitics are easier to control.