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

7.3 Decompensated Op Amps and Stability

Some operational amplifiers are not stable in unity gain. That is, the op amps oscillate if configured in a buffer configuration (G = 1V/V). The reason for this instability at low gains is that there is a second pole in AOL below the unity-gain bandwidth. This second-pole is internal and behaves the same way as the second pole introduced from a capacitive load (see Isolation Resistor (RISO) Method). These types of amplifiers are called decompensated amplifiers, because these amplifiers lack the internal compensation required for unity gain stability. The reason that decompensated op amps are developed is because these op amps have higher gain bandwidth products than comparable compensated amplifiers with equivalent power consumption. For this reason, decompensated amplifiers are generally high-speed amplifiers with bandwidths beyond 50MHz.

Figure 7-8 illustrates the open-loop response for a typical decompensated amplifier (OPA892). The secondary pole is located at approximately 200MHz. The OPA892 has a minimum gain requirement of 10V/V (20dB). The stability requirement can be understood by inspecting the AOL curve. In this example, the rate-of-closure for a gain of 1V/V is 40dB/decade and for 10V/V is 20dB/decade.

OPA187 OPA202 OPA320 OPA192 Open-Loop Response for Decompensated Op Amp Example (OPA892) Figure 7-8 Open-Loop Response for Decompensated Op Amp Example (OPA892)