5 Stabilizing the OPA593 and Darlington Current Booster for 1μF Capacitive Load

The design approach using the OPA593 with a Darlington current booster topology for driving 1µF capacitive load follows the dual feedback compensation (DFC) process designed for the emulated power amplifier, shown in Figure 4-5. A key difference is that the current booster effectively acts as the isolation resistor (R_iso) in the designed emulated PA example. While the OPA593 maintains an output impedance of approximately 228Ω from 1kHz to 1MHz, this is not designed for driving large capacitive loads using the compensation technique outlined in Table 4-4, where alternative DFC compensation techniques are more appropriate.

The design requirements for the OPA593 and the current booster configuration are detailed in Table 2-2 at the beginning of the article. Integrating the OPA593 with a Darlington current booster creates a composite amplifier with low open-loop output impedance. This composite amplifier benefits from the OPA593’s performance attributes—such as high input voltage handling, high slew rate, current limiting, and Enable or Disable functionality. These features make the OPA593 capable of driving large capacitive loads and meeting high current demands in ATE applications, provided that the feedback network is properly compensated.

In the current booster configuration shown in Figure 5-1, simulations reveal that the unity gain bandwidth, f_unity of the OPA593 combined with the Darlington current booster remains consistent, with a measured f_unity of approximately 1.4MHz and a phase margin of 79°. When the gain bandwidth product is applied, the simulation results closely match those of the emulated power amplifier depicted in Figure 4-1. This suggests that the closed-loop of the composite amplifier needs to exhibit similar behaviors as the emulated power amplifier.

Figure 5-1 OPA593 + Current Booster Driving Resistive Load with V_REF = 1V_dc