SBOA418 july   2023 OPA2197-Q1 , OPA392

 

  1.   1
  2.   Abstract
  3.   Trademarks
  4. 1Introduction: Open-Loop Output Impedance (RO)
  5. 2Overview: Closed-Loop Amplifier Stability
  6. 3Example, Calculate Optimal Isolation Resistance to Drive Large Load Capacitance
  7. 4RO, RISO, and Capacitive Load Drive
  8. 5Derive Equation for Optimal Isolation Resistance
  9. 6Summary
  10. 7References

2 Overview: Closed-Loop Amplifier Stability

Amplifier stability is a primary concern for designers, as amplifier circuits are expected to operate under a variety of output loading conditions. Capacitance directly at the output of an amplifier can cause time delays between the input and output pins. These time delays are often represented as phase shifts between the output and feedback nodes, which can result in oscillations at the output of the amplifier.

An amplifier’s closed-loop gain (Acl) over frequency is defined by Equation 1. The derivations for this equation can be found in Stability Analysis of Voltage-Feedback Op Amps application note.

Equation 1. Acl = AOL1+AOLβ

Where,

  • AOL is the amplifier's open-loop gain
  • β is the feedback factor

When the magnitude of AOLβ is 1 and the phase shift is 180°, AOLβ can be expressed in phasor notation as 1∠180° which is equal to -1. This -1 term in the denominator results in a division by zero, and the equation is undefined. Systems with AOLβ approaching 180° phase shift at unity gain begin to exhibit signs of instability in the form of oscillations or ringing at the output. The term phase margin is used to account for this, and is defined as how close a system is to the 180° total phase-inversion when the magnitude of AOLβ is 0dB. A phase margin of 45° is a common benchmark for assessing the stability of a system. Designers commonly target a minimum phase margin of 45° to avoid instability.

A common technique for analyzing amplifier stability, is simulating the open-loop gain (AOL) and feedback factor (β) of the amplifier circuit over frequency. A stable circuit will show 20-dB rate of closure between the AOL and 1/β curves and 45-90 degrees of phase margin. Rate of closure, phase margin, and other details related to stability analysis are explained in detail in the Texas Instruments Precision Labs (TIPL) video series on stability.