1 ANALOG ELECTRONICS II Frequency Compensation  the technique of modifying open-loop gain  the purpose is to ensure that op-amp circuits will be stable  An amplifier can be stabilized by:-  Adding a dominant pole  Changing the dominant pole  Miller compensation  Modifying the feedback path

2 Frequency Compensation (internal) Frequency response of internally compensated op-amp 741C

3 Frequency Non-compensation Frequency response of non-compensated op-amp A709

4 Compensation circuit Frequency compensation circuit of op-amp A 

5 Addition of dominant pole f D = 1/2  RC x

6 Compensation network A(j  ) RR CxCx CxCx VoVo ViVi - Adding capacitor C x ---- adds break frequency - Decrease the BW

7 Changing the dominant pole

8 Compensation network Adding a capacitor to internal amplifier circuit 2C x -Adding a capacitor, C x at differential stage of an op-amp -Value of C x too large, typical > 1000 pF

9 Small capacitance is connected between the input and output of a gain stage in a multi-stage amplifier Using Miller multiplication of the capacitance Miller compensation

10 Equivalent Circuit of Miller Compensation CxCx IsIs CoCo CiCi RoRo RiRi ViVi gmVigmVi - Without C x, f p1 =1/2C i R i and f p2 =1/2C o R o -New poles :  p1 =1/g m C x R o R i and  p2 - =g m C x /C i C o +C x (C i +C o ) - If C x >> C o :  p2 =g m /(C i +C o ) - As C x ,  p1  and  p2  ---- widen BW

11 Modification of feedback path Usually used in fixed-gain amplifier where large BW is important RFRF CFCF RERE ifif ioio