Transistor Basics is a free introductory textbook on transistors and their basic applications. See the editorial for more information.... 
Home Transistor Oscillators NegativeResistance Oscillators Conditions for Oscillation  
See also: Negative Resistance and Transistor Stability  
Search the VIAS Library  Index  
Conditions for OscillationAuthor: Leonard Krugman Fig. 65. Basic resistance controlled negative resistance circuit. The preceding paragraphs indicate that transistor oscillators can be designed as equivalents for all the known types of vacuumtube oscillators that use an external feedback path. In addition, the unique property of a transistor that furnishes current gain can also be used to design many other novel types of oscillators.
In general, external resistance can be added to any of the three electrode leads, as illustrated in Fig. 65. Substituting the transistor parameter values into equation 61 results in: (r_{e} + r_{b} + R_{B} + R_{g}) (R_{L} + r_{b} + R_{B} + r_{c})  (r_{b} + R_{B})(r_{b} + R_{B} + r_{m)} < 0 Neglecting r_{e} and r_{b} as compared to R_{B}, r_{c}, and r_{m}, this becomes: (R_{B} + R_{g}) (R_{L} + R_{B} + r_{c})  R_{B} (R_{B} + r_{m})< 0 and multiplying out R_{B}R_{L} + R_{B}^{2} + R_{B}r_{c} + Rg^{R}L + R_{g}R_{B} + R_{g}r_{c}  R_{B}^{2}  R_{B}r_{m} < 0 which becomes: R_{g}(R_{L} + r_{c}) + R_{B}(R_{L} + R_{g})  R_{B} (r_{m}  r_{c}) < 0 Notice that when r_{m} is less than r_{c} (as in the case of the junction transistor) , the condition for oscillation cannot be satisfied. This reemphasizes the fact that negativeresistance oscillators can only be designed using the pointcontact transistor. Notice also in this equation that if both R_{L} and r_{g} are small compared to the value of (r_{m}  r_{c}), the conditional equation is primarily controlled by the value of R_{B}. The higher the value of R_{B}, the more definite the instability. Furthermore, as the external collector and emitter resistances are increased in value, a higher resistance of R_{B} is required to assure circuit oscillation. The control of oscillation in negativeresistance transistor oscillators, then, is determined by the following three factors, either separately or in combination: the external resistance of the emitter lead (a low value favors oscillation), the external resistance of the base lead (a high value favors oscillation), and the external resistance of the collector lead (a low value favors oscillation).


Home Transistor Oscillators NegativeResistance Oscillators Conditions for Oscillation 