Third-order distortion and cross modulation in a grounded emitter transistor amplifier

The nonlinear modulation characteristic of a transistor is approached by the first terms of a Taylor-series formula and the output voltage is calculated for a suitably-chosen ac-input voltage. For higher frequencies and different working points, the influence of the diffusion capacity and the emitter admittance are illustrated by experimental results. It is shown that--as in electronic tubes-the third-order distortions (harmonic distortion, distortion of modulation, alteration of the degree of amplitude modulation, degree of cross modulation) are closely connected, but unlikethe situation in electronic tubes, the distortions disappear at a special working point. The influence of the base resistance and the internal resistance of the signal source are discussed in relation to the appearance of this zero point and the frequency dependence. In an RF amplifier a transistor is modulated, for example, only by a part of the resonant circuit voltage because of the low input resistance in a grounded emitter circuit. Although this voltage transformation is accounted for, the transistor is inferior to the electronic tube with respect to cross modulation. To reduce the cross modulation in a transistor RF stage, two possibilities are described for correcting the distortions with a fixed or controlled working point using a predistortion or a push-pull modulation. In this case the different influences of a resistance in the base lead, or of a resistance blocked for HF but not for LF in the emitter lead, are discussed.