Systematic Design of Linear Feedback Amplifiers

Sammanfattning: This work adds a quantitative description of distortion to the structured design method of negative-feedback amplifiers. It constitutes a power-series representation of the feedback amplifier, which may include dynamic (memory) effects, where the design parameters appear explicitly. These results have been incorporated into the systematic design method. It now includes design steps for calculating the required number of amplifying stages, and the optimal bias current allocation. The power-series representation of negative-feedback amplifiers models several properties of active devices. The effects of collector-base current tracking (current drive), high-level injection of BJTs, and the presence of base resistance influences on the nonlinear distortion performance are described. Furthermore, mobility reduction, and channel length modulation of FETs are modeled. Moreover, the power-series representation also allows the mismatch of balanced stages to be considered. This power-series representation explicitly states the conditions for achieving the desired nonlinear distortion performance of negative-feedback amplifiers. One of the main findings is that the current/voltage gain of an amplifying stage is more important than its intrinsic nonlinearity in this regard. The intrinsic nonlinearity of an amplifying stage is represented by nonlinearity coefficients. The coefficients are, in general, of the same order of magnitude for all technologies and configurations. Therefore, the ability of amplifying stages to contribute to the loop gain is more important. What is more, in contrast to common belief, applying local feedback does not improve the nonlinear distortion performance of negative-feedback amplifiers. The linearity loss can be slight, assuming that local feedback is applied to the amplifying stage that dominates the linearity performance. The least harmful place for local feedback is, therefore, often the output stage.