SBOA626 December   2025 OPA187 , OPA192 , OPA202 , OPA320

 

  1.   1
  2.   Abstract
  3.   Trademarks
  4. 1Introduction
    1. 1.1 Simple Analogy Explaining Instability
    2. 1.2 Circuits With Possible Stability Issues
    3. 1.3 Simple Stability Correction Based on Datasheet Plots
    4. 1.4 Introducing Lab Tools and Measurements
  5. 2Stability Theory for Operational Amplifiers
    1. 2.1 Poles and Zeros
    2. 2.2 Operational Amplifier Model Requirements for Stability Verification
    3. 2.3 Stability Definitions Based on Control Loop Model
    4. 2.4 Graphing Loop-Gain Based on AOL and 1/β
    5. 2.5 Rate of Closure Stability Test
    6. 2.6 Indirect (Non-Invasive) Stability Tests
  6. 3Simulating Open-Loop Stability Tests
    1. 3.1 Breaking the Loop the Wrong Way
    2. 3.2 Breaking the Loop With LC Test Circuit
    3. 3.3 Differential Loop Break Test
  7. 4Stability Correction for Capacitive Load
    1. 4.1 Isolation Resistor (RISO) Method
    2. 4.2 Dual Feedback Method
      1. 4.2.1 RISO-Dual-Feedback With RL
      2. 4.2.2 Dual Feedback With RFX Method
    3. 4.3 Snubber Circuit for Compensating Power Amplifiers and Reference Drive
    4. 4.4 Noise Gain for Stability Compensation
    5. 4.5 Feedback Capacitor (CF) Compensation for Capacitive Load
  8. 5Stability Corrections for Capacitance on the Inverting Node
    1. 5.1 Input Capacitance Instability Due to Zero in 1/β
    2. 5.2 Feedback Capacitor Solves Stability Issue for Capacitance on the Inverting Node
    3. 5.3 Minimum, Balanced, and Maximum Feedback Capacitance
    4. 5.4 Transimpedance Case
  9. 6Complex Open-Loop and Closed-Loop Output Impedance
    1. 6.1 Converting Open-Loop Output Impedance to Closed-Loop Output Impedance
    2. 6.2 Open-Loop and Closed-Loop Model Test
    3. 6.3 Instability Due to Resonance From Complex Output Impedance
    4. 6.4 Impact of Internal Op Amp Topology on Output Impedance Versus Frequency
    5. 6.5 Other Factors Effecting Output Impedance
  10. 7AOL Impact on Stability
    1. 7.1 AOL Secondary Poles and Zeros
    2. 7.2 Modeling the AOL Secondary Poles and Zeros and Input Capacitance
    3. 7.3 Decompensated Op Amps and Stability
    4. 7.4 The Impact of Closed-Loop Gain on Stability
  11. 8Common Problems in Stability Analysis
  12. 9References

3.2 Breaking the Loop With LC Test Circuit

Figure 3-3 shows an open-loop stability test configuration where the feedback loop is broken with a 1TH inductor, and the signal is injected using a 1TF capacitor. The very large values for the capacitor and inductor are not practical for real-world circuit implementations, but work well for most simulation cases. The inductor acts like a short at DC, but acts like an open for AC frequencies (recall Equation 18). Conversely, the capacitor acts like an open at DC, but acts like a short for AC frequencies (recall Equation 19 ). Thus, at DC the circuit is in a closed-loop configuration, and at AC the circuit is in an open-loop configuration (see Figure 3-4 and Figure 3-5). The reason the very large values are used for the inductors and capacitors is to allow for a very low-frequency open-loop operation. For example, in many cases, the simulation is run from 0.1Hz to see the dominant pole. The large LC values allow for the circuit to operate in open-loop even at this low frequency.

Equation 18. XL=2×π×f×L
Equation 19. XC=1/2×π×f×C
OPA187 OPA202 OPA320 OPA192 Open-Loop Test Configuration for Stability VerificationFigure 3-3 Open-Loop Test Configuration for Stability Verification
OPA187 OPA202 OPA320 OPA192 Open-loop Test Configuration for Stability Verification at DCFigure 3-4 Open-loop Test Configuration for Stability Verification at DC
OPA187 OPA202 OPA320 OPA192 Open-Loop Test Configuration for Stability Verification for AC FrequenciesFigure 3-5 Open-Loop Test Configuration for Stability Verification for AC Frequencies

Running an AC transfer characteristic for the test circuit in Figure 3-3, generates the graph shown in Figure 3-6. In TINA™SPICE you can identify the curves with the question mark button (see Figure 3-6). For stability analysis AOL, AOL × β, and 1/β magnitude curves are needed, and the AOL × β phase curve is needed. Deleting the unnecessary curves helps improve the readability of the graph. Also, adjusting the y-axis scaling to show magnitude in 20dB increments, and phase in 45° increments, can make interpreting the results easier. The last step is to add a legend to show the phase-margin. In TINA™ SPICE this can be done by placing a cursor on AOL × β and finding the frequency where AOL × β = 0dB. Once the cursor is in position, pressing the legend tool generates a legend on the graph that shows the gain and phase for all pertinent curves at the cursor frequency. In Figure 3-7 the legend indicates a phase margin of 65.7°.

OPA187 OPA202 OPA320 OPA192 Delete Unnecessary Curves and Adjust ScaleFigure 3-6 Delete Unnecessary Curves and Adjust Scale
OPA187 OPA202 OPA320 OPA192 Display Phase Margin With LegendFigure 3-7 Display Phase Margin With Legend