SBOA602 November   2024 OPA593

 

  1.   1
  2.   Abstract
  3.   Trademarks
  4. 1Introduction
  5. 2Current Booster, Push-Pull Topology Output Characteristics
    1. 2.1 Open-Loop Output Impedance
    2. 2.2 Minimizing Zero Crossover Distortion
  6. 3Various Current Booster Configurations
    1. 3.1 Complementary MOSFET versus BJT Current Booster Comparisons
  7. 4Stabilizing a Design for Power Amplifier Driving 1μF Capacitive Load (CL)
    1. 4.1 Op-Amp Driving Resistive Load
    2. 4.2 Op-Amp Driving Capacitive Load and Challenges
    3. 4.3 Open-Loop AC Stability Analysis - Compensating CL Effects Using DFC
    4. 4.4 Closed-Loop Stability Response - Small Signal Step Transient Analysis
    5. 4.5 Effects of Riso on Frequency Response in Dual Feedback Compensation
    6. 4.6 Summary of the DFC Technique
  8. 5Stabilizing the OPA593 and Darlington Current Booster for 1μF Capacitive Load
    1. 5.1 Open-Loop AC Stability Analysis - Composite Op-Amp Driving 1μF CL
    2. 5.2 Closed-Loop Stability Response - Composite Op-Amp's Step Transient Analysis
  9. 6Composite Amplifier's Effective BW and Step Time Response
  10. 7Test Bench Validation
  11. 8Summary
  12. 9References

2.1 Open-Loop Output Impedance

In this design, the current booster is configured as a complementary push-pull Darlington topology with unity gain buffering. Figure 2-1 and Figure 2-2 demonstrate that the open-loop output impedance is regulated by a small bias voltage applied to the bases of the transistors. Forward biasing the base-emitter junction of the NPN transistor (T1) allows the booster to source positive voltage and current, while forward biasing the PNP transistor (T2) enables it to sink negative voltage and current. The bias voltage directly affects the open-loop output impedance; higher bias levels result in lower output impedance, as shown in Equation 1.

Equation 1. ro≈ VAIC  →  ZCE= rop  × ronrop + ron   → ZCBO= ZCE∥RL

Where,

  • ro represents the BJT’s output impedance
  • VA refers to the Early voltage
  • IC denotes the bipolar collector current
  • rop represents the NPN’s open-loop output impedance
  • ron represents the PNP’s open-loop output impedance
  • ZCE represents the parallel output impedance of the complementary Darlington pair
  • ZCBO refers to the overall parallel open-loop output impedance

When the NPN transistor (T1) base-emitter junction is forward-biased, the current booster sources positive voltage and current at the output. Figure 2-1 illustrates the open-loop output impedance where the effects of ZCE ∥RL are shown to be less than 1Ω.

Conversely, when the emitter-base junction of the PNP transistor (T2) is forward-biased, the current booster sinks negative voltage and current at the output. Figure 2-2 demonstrates the open-loop output impedance, with the ZCE ∥RL effects yielding similar results. The combined open-loop output impedance remains consistent across the frequency range up to 1MHz.

Open-Loop Output Impedance (ZCBO) with T1 Forward BiasedFigure 2-1 Open-Loop Output Impedance (ZCBO) with T1 Forward Biased
Open-Loop Output Impedance (ZCBO) with T2 Forward BiasedFigure 2-2 Open-Loop Output Impedance (ZCBO) with T2 Forward Biased

The BJT transistor's forward biased voltage directly affects the open-loop output impedance; higher bias voltages lead to lower output impedance. The open-loop output impedance of the push-pull complementary BJT driver is primarily influenced by the output resistances (ro) of the NPN (rop) and PNP (ron) transistors, as well as the load resistance (RL). The open-loop output impedance of the Darlington current booster operates in parallel with RL, as illustrated in Equation 1.