Broadband transistor feedback amplifiers

The possibilities of negative-feedback amplifiers involving three cascaded common-emitter junction-transistor stages are investigated. Simple approximate expressions are given for the power gain, input impedance and output impedance of series, shunt and compound feedback amplifiers, for the case of large applied negative feedback. It is shown that with large negative feedback, dynamic stability considerations may be analysed with little error by considering the forward and return paths of the feedback loop as separable, in spite of the interaction effects between transistors. Owing to the restricted frequency range of application of the transistors employed, the maximum negative feedback possible without instability depends almost exclusively on the frequency characteristics of the current gain, ¿, of the transistor; it is found that the ¿-characteristics may be represented with adequate accuracy by a minimum-phase (RC) approximation, which leads to simple analysis both of stability conditions and of suitable phase and amplitude margins. Examples are given of feedback amplifiers having overall operating power gain and applied negative feedback of approximately 33dB and 30dB respectively, in which dynamic stability is secured by using phase-advancing networks in the return path of the feedback loop.