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

4.2 Op-Amp Driving Capacitive Load and Challenges

In automated test equipment (ATE) applications, the output stage of a power amplifier frequently interfaces with substantial capacitive loads. Driving large capacitive loads (for example, 1μF) presents challenges due to an additional pole introduced within the op-amp’s unity-gain bandwidth (UGBW). This extra pole (fp2), described in Equation 4 can destabilize the op-amp’s loop gain and reduce the phase margin in the closed loop within the UGBW.

No op-amp can drive large capacitive loads while making sure of stability without appropriate feedback loop compensation. The term "large capacitive load" is relative and varies based on several factors, including the op-amp's open-loop output impedance, load resistance, load capacitance, and unity-gain bandwidth (funity). Typically, op-amps can drive capacitive loads ranging from 10pF to 100pF without requiring external compensation. Op-amps with low open-loop output impedance can drive higher capacitive loads, up to 1nF, while still maintaining adequate phase margin without additional compensation. However, capacitive loads exceeding 1nF are generally considered "large" and can lead to issues such as oscillation, making compensation essential for preserving loop stability.

Equation 2. fp2= 12πZo∥RLCL

While some op-amps can drive capacitive loads up to 1nF, others can become unstable. This distinction depends on the op-amp’s open-loop output impedance, the interaction with the capacitive load (CL), and the location of the extra pole (fp2 ) relative to unity-gain bandwidth (UGBW). These factors collectively determine an op-amp's ability to drive capacitive loads effectively. Table 4-4 summarizes the stability of an op-amp when driving capacitive loads under various scenarios.

Table 4-3 Op-Amp Closed-Loop Behaviors Related to an Additional Pole (fp2) and UGBW
Op-Amp Closed-Loop Stability AssessmentCL Load Example
Stablefp2 is more than 2 octaves beyond the UGBW of the op-amp10pF to 100pF load (typical)
Unstablefp2 is within the UGBW of the op-amp, leading to instabilityLarge capacitive load (CL> 1nF)
Unity Gain Unstablefp2 coincides exactly with the UGBWfp2 = UGBW
Conditionally Stablefp2 is within 1 octave beyond the UGBWUncertain stability behaviors

In summary, the stability of an op-amp driving capacitive loads is influenced by the relationship between the second pole (fp2) and the UGBW. If fp2 is far beyond the UGBW, the system remains stable. However, if fp2 is within or near the UGBW, the op-amp can become unstable or conditionally stable.

The phase margin, obtained from open-loop AC analysis, is a quantitative measure to evaluate the stability of an op-amp in a closed-loop feedback configuration. Phase margin predicts whether the system can remain stable, oscillate, or exhibit uncertain behavior, especially when driving capacitive loads. A phase margin of at least 45° is generally required to make sure of stable operation according to open-loop AC stability analysis.