24.1: Theory Overview

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The function of a voltage follower is to present a high input impedance and a low output impedance with a non-inverting gain of one. This allows the load voltage to accurately track or follow the source voltage in spite of a large source/load impedance mismatch. Ordinarily this mismatch would result in a large voltage divider loss. Consequently, followers are often used to drive a low impedance load or to match a high impedance source. While typical laboratory sources exhibit low internal impedances, some circuits and passive transducers can exhibit quite high internal impedances. For example, electric guitar pickups can exhibit in excess of 10 k \(\Omega\) at certain frequencies. Although the voltage gain may be approximately one, current gain and power gain can be quite high, especially if a Darlington pair is used. Besides unity voltage gain and a high \(Z_{in}\) and low \(Z_{out}\), followers also tend to exhibit low levels of distortion.

The Darlington pair effectively produces a “beta times beta” effect by feeding the emitter current of one device into the base of a second transistor. This also produces the effect of doubling both the effective \(V_{BE}\) and \(r’_e\).