22.1: Theory Overview

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As the signal current changes in a transistor, the total current flowing through the emitter changes along with it. As a result, these changes produce small changes in \(r’_e\) which in turn changes the voltage gain. In other words, the gain changes throughout the signal producing slightly more or less gain at some points along the signal than others. These changes show up as a squashing or elongating of the positive and negative peaks of the output signal. Generally, these forms of waveform distortion are to be avoided. Also, they tend to worsen as the output signal amplitude increases. A method of mitigating this distortion is to add AC resistance to the emitter portion of the circuit. This added resistance tends to buffer or “swamp out” the changes in \(r’_e\) and therefore reduces the distortion. A side bonus is that \(Z_{in(base)}\) will also be increased which will result in an increased \(Z_{in}\) to the circuit. On the downside, the added resistance will lower the voltage gain. Consequently the swamped amplifier exhibits a lower gain but one of higher quality. In general, the larger the swamping resistance is compared to \(r’_e\), the greater the effects on distortion, gain and input impedance.