17.1: Theory Overview

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One of the problems with simpler biasing schemes such as the base bias is that the Q point (\(I_C\) and \(V_{CE}\)) will fluctuate with changes in beta. This will result in inconsistent circuit performance. If a fixed voltage can be developed across an emitter resistor, a stable Q point will result. To obtain this fixed emitter resistor voltage, a negative voltage supply may be connected to the low side of the emitter resistor instead of connecting that resistor to ground. The transistor's base is then simply connected back to ground via a single resistor. If this base resistance is relatively small, the base voltage will be close to zero as only the base current flows through it. Consequently, almost all of the negative emitter supply will drop across the emitter resistor, with the exception of the single base-emitter potential. This will result in a stable emitter current, and by extension, stable collector current and collector-emitter voltage. As beta varies, this change will be reflected in a change in base current. This can result in large percentage changes in base voltage; however, the magnitude of the base potential will remain small, and thus, inconsequential.