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Negative Resistance and Transistor StabilityAuthor: Leonard Krugman Consider the general expression for input resistance
Expanding equation 342, r_{c}r_{e} + r_{c}r_{b} + r_{b}r_{e} + r_{b}^{2} > r_{h}r_{m} + r_{b}^{2 } Dividing through by r_{b}
This equation emphasizes the importance of the backward transfer resistance r_{b}, since when r_{b} = 0, the transistor must have a positive input resistance. On the other hand, if the value of r_{b} is increased by adding external resistance, it is possible to reach a condition where a normally positive input resistance becomes negative. Notice, however, that increasing the total base resistance eventually causes the input resistance to become negative only if r_{e} + r_{c} is less than r_{m}. In the case of the junction transistor, r_{c} is always greater than r_{m}, and increasing the base resistance cannot produce a negative input resistance. The conditions for negative output resistance are obtained similarly. In the general output resistance equation,
the output resistance r_{o} is positive provided that r_{22} is greater than This condition for stability is most difficult to meet when the generator resistance is equal to zero. For the transistor to be stable under this condition, r_{n}r_{22} again must be greater than r_{12}r_{21}. The same stability factor and equations then exist for both the input and output resistances. It is evident, then, that one method of fabricating a transistor oscillator is by adding sufficient resistance to the base arm. Typical circuits incorporating this principle will be considered in Chapter 6.


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