A Tickler Circuit Oscillator

Author: J.B. Hoag

In the chapter on amplifiers we have seen that the amplified current in the plate circuit of a three-electrode tube can be fed back to the grid circuit by means of a tickler coil, with resultant increase in the amplification of the signal. If the tickler coil is moved closer and closer to the grid coil, the amplification increases until, at a critical value of " coupling " between the two coils, the tube will produce oscillations within itself, without the aid of an input signal. In order that a tube shall be self-oscillating, two main conditions must be satisfied. First, it is necessary that the power transferred from the plate circuit to the grid circuit shall be equal to or greater than the circuit losses. Second, it is necessary that the feedback be positive or regenerative.

Fig. 14 A. A tickler-circuit oscillator

Figure 14 A shows a simple tickler circuit. When the B-battery is first connected, small random variations — no matter how minute — are rapidly amplified to such a point as to start the tube in oscillation.

External excitation is unnecessary. It will be noted that the B-battery is in series with the feedback coil, L₁, (the tickler). This is called a series-fed oscillator. Condenser C₁ is used to provide a low impedance path for the alternating current around the B-battery. In this and, in fact, in practically all oscillators, the grid becomes positive during part of the cycle, with the result that a flow of electrons occurs through the resistance R_g. Condenser C_g forces this current through R_g, preventing it from short circuiting through coil L. The grid current flow through R_g results in a negative bias voltage on the grid. Practically all oscillators use grid-leak bias.

The frequency of the oscillations will be nearly equal to the resonant frequency of the LC or tank circuit, as given by the equation

In general, the oscillation frequency will be governed by that circuit in which the losses are least, i.e., by the LC circuit which has the highest Q.