A simplified current booster without a preconfigured forward bias can be adequate for voltage regulators with discrete setpoints; however, a simplified current booster is not designed for applications that require minimal non-linearity and distortion. In a Darlington configuration, a dead zone of approximately 1.3V occurs when the transistors do not receive sufficient biased voltage, resulting in a lack of output current amplification and exhibiting a cutoff characteristic. This situation can delay the step response, as the booster must first overcome the V_BE threshold. Adding a zero-crossover resistor, R_ZRO (for example, 1-10kΩ), can alleviate this dead zone, as shown in Figure 2-3, allowing some output current to flow even when the transistors are non-conductive. This modification does not affect the open-loop output impedance, as shown in Figure 2-4.

Figure 2-3 Minimize Crossover Distortion with Zero-Crossover Resistor (R_ZCO)

By placing a 10kΩ zero-crossover resistor (R_ZRO) inside the common emitter current booster, as shown in Figure 2-4, the open-loop output impedance of the current booster driver remains unaffected.

Figure 2-4 Open-Loop Output Impedance Remains Unaffected by R_ZCO