Stabilizing Circuitry

Author: Leonard Krugman

Figure 6-12 (A) illustrates one arrangement of a high-frequency base-controlled oscillator that incorporates the various stabilizing features discussed in the preceding paragraphs. C₁ and C₂ are phase compensating condensers. The base lead is connected to a tapped tank coil as a means of reducing the resonant impedance while maintaining a high Q tank. Bias stability is accomplished by using one common battery source. Notice also that positive emitter bias is supplied by the bypassed resistor R_B. Figure 6-12 (B) illustrates an alternate method of providing a constant collector-to-emitter bias ratio by means of a common battery supply. The advantage of this circuit is its design simplicity, since it is basically a voltage divider network. The values of C₁ and C₂ are not critical; they complete the a-c circuit between the collector, base, and emitter leads, and bypass the battery and bias divider network.

Fig. 6-12. (A) Stabilized base-controlled high-frequency oscillator. (B) Alternate method of providing common bias supply.

Except for the inductance and capacitance elements of the resonant network, the values of the external components in negative-resistance oscillators are not critical The values of R_E and R_c should be large enough to limit their respective currents to safe values, but not so large that they cause excessive degeneration. The value of the base resistance R_B must be large enough to provide sufficient regeneration for sustained oscillation. Typical values for these parameters are: R_E = 50 to 2,000 ohms; R_c = 2,000 to 10,000 ohms; R_B = 10,000 to 20,000 ohms.