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In amplifiers having emitter degeneration, an impedance is sometimes used between base and emitter. A common case occurs when several emitter followers are used in cascade. The resistors become necessary in order to provide some measure of stability. In an audio amplifier of similar design it became necessary to know what effect this shunt resistor would have upon the input impedance of the stage. As a result, the analysis given in this paper was performed. Experimental measurements which support the resulting equation are given. This analysis and these measurements led to the discovery of a circuit which would exhibit a high ac input impedance, yet the resistors in the dc base circuit could be kept low to provide good stability. Although it can be shown that circuit power gain is the same regardless of whether a resistor is used in series with the emitter or the base to obtain a high input impedance, considerations of dc stability and distortion demand a more thorough investigation of this problem. Design principles are outlined for this high input impedance stage and an example is worked out in detail. Supporting measurements are given.
Date of Publication: November-December 1959