By deriving the complex-frequency expression for ?? from the BJT hybrid-???odel@ we can write the ??-dependent impedance transformations at base and emitter nodes in a general form. When these impedances are represented graphically on a reactance chart@ the effects of circuit element variations on circuit behavior@ especially HF resonances@ become evident. The frequency dependent ?? transform is also applicable to FETs and single-pole feedback amplifiers. They show the same impedance gyrations as the BJT@ so the BJT results can be easily extended to them also. Author D. Feucht