A Colpitts Oscillator

Author: J.B. Hoag

The feedback of energy from the plate circuit to the grid circuit can be obtained by means of electrostatic coupling as well as by magnetic coupling. Electrostatic coupling is accomplished through a condenser. Such circuits, as, for example, the parallel-fed Colpitts circuit of Fig. 14 C, are said to have capacity feedback. In this circuit, the tank voltage is divided into two parts by means of condensers C₁ and C₂.

Fig. 14 C. A Colpitts oscillator

In order to understand the operation of the circuit, imagine that at a given instant the grid becomes less negative than its static value, causing an increase in the plate current. This raises the potential across condenser C₁ i.e., stores more energy in this condenser. This increase of energy is transferred via the coil L to the condenser C₂, making its grid side more negative, the opposite of that across C_1. The increase of negative potential on C₂ feeding through C_g makes the grid more negative, which is the reverse phase of that which we assumed at the start. When this feedback voltage is sufficiently large, the plate current decreases, less energy is stored in C_1, and the feedback is again reversed through the coupling of the tank circuit. This process continues. The energy for the oscillations in both the tank circuit (and for the grid bias) comes from the B-voltage source.