Frequency compensation

• Additional design step
  • to prevent oscillation in feedback amplifiers
  • to ensure a good transient response

• Modification of kA_v (s) that changes an unstable feedback system to a stable one
• Aim to realize a loop gain which contains exactly one pole smaller in magnitude than w₀
• Feedback gain k=1 (worst case-largest w₀ and smallest phase margin) is assumed for stability estimation
  (corresponds to a short circuit between the output and the inverting input of the amplifier)

Compensation by

• Pole-splitting capacitor

Shown as an example on the improved uncompensated CMOS operational amplifier:

 

 

• Compensation capacitor C_c connected between the high-impedance nodes A and B
• Good choice C_c ~ C_L
• Pole associated with node A is moved to a lower frequency
• Pole associated with node B is moved to a higher frequency
• Efficient for bipolar opamps, less favourable for MOS opamps

• Nulling resistor

 

• Compensation by pole cancellation ? two-pole response
• Condition: