Aspects of return-difference evaluation in transistor feedback amplifiers

The concept of return difference is basic to the proper study of stability in any linear feedback circuit. The paper considers the practical problems encountered in the evaluation of return difference in transistor feedback amplifiers. It describes three procedures for such an evaluation; each one, in its own way, takes account of the internal feedback associated with the transistor stage in question. The first method is a direct one, but requires the simulation of the controlled source responsible for the internal feedback. The second method, which is based on the return-difference matrix, overcomes this simulation problem at the expense of three measurements. The third method is derived from a generalisation of Blackman´s formula, and it requires two admittance measurements to be made at a convenient port in the feedback amplifier.