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Which one of the following statements is correct regarding lead or lag compensation?

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  1. The lag compensator improves the steady-state performance of the system.
  2. The lead compensation becomes effective when the phase angle of uncompensated system decreases rapidly near the gain crossover frequency.
  3. Choose the lead compensator when reduced noise level is required.
  4. The combination of decreased open-loop gain and lead compensator improves steady-state error and phase margin.

Answer (Detailed Solution Below)

Option 1 : The lag compensator improves the steady-state performance of the system.
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Lead compensator:

Transfer function:

If it is in the form of , then a > 1

If it is in the form of , then a < b

Maximum phase lead frequency: 

Maximum phase lead: 

ϕm is positive

Pole zero plot:

The zero is nearer to the origin.

Filter: It is a high pass filter (HPF)

Effect on the system:

  • Rise time and settling time decreases and Bandwidth increases
  • The transient response becomes faster
  • The steady-state response is not affected
  • Improves the stability


Lag compensator:

Transfer function:

If it is in the form of , then a < 1

If it is in the form of , then a > b

Maximum phase lag frequency: 

Maximum phase lag: 

ϕm is negative

Pole zero plot:

The pole is nearer to the origin.

Filter: It is a low pass filter (LPF)

Effect on the system:

  • Rise time and settling time increases and Bandwidth decreases
  • The transient response becomes slower
  • The steady-state response is improved
  • Stability decreases

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