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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 poleChanging the dominant poleMiller compensationModifying the feedback path

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Frequency Compensation (internal)

Frequency response of internally compensated op-amp 741C

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Frequency Non-compensation

Frequency response of non-compensated op-amp A709

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Compensation circuit

Frequency compensation circuit of op-amp A709

50

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Addition of dominant pole

fD = 1/2RCx

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Compensation network

A(j)R R

Cx

CxVoVi

- Adding capacitor Cx ---- adds break frequency

- Decrease the BW

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Changing the dominant pole

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Compensation network

Adding a capacitor to internal amplifier circuit

2Cx

-Adding a capacitor, Cx at differential stage of an op-amp

-Value of Cx too large, typical > 1000 pF

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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

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Equivalent Circuit of Miller Compensation

Cx

Is CoCi

RoRi

Vi

gmVi

- Without Cx , fp1 =1/2CiRi and fp2 =1/2CoRo

-New poles : p1 =1/gmCxRoRi and p2

- =gmCx/CiCo+Cx(Ci+Co)

- If Cx >> Co : p2 =gm/(Ci+Co)

- As Cx , p1 and p2 ---- widen BW

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Modification of feedback path

Usually used in fixed-gain amplifier where large BW is important

RF

CF

RE

if

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