Internal Compensation Technique used in operational amplifiers

Internal Compensation Technique

Recently in op-amps like IC 741, the compensation is provided internally, which is generally built in lag compensation.

A capacitor ranging from 10 to 30 pF is fabricated between input and output stage to achieve the necessary compensation. This type of compensation is called Miller effect compensation. Such op-amps are called compensated op-amps.

1. Miller Effect Compensation

The main drawback of dominant pole compensation is the reduction in the bandwidth. Similarly the value of capacitance required in this method is very large and fabricating such large capacitance on the chip is not practicable.

These drawbacks are avoided by using Miller effect compensation, using the Miller effect. In dominant pole compensation, a capacitor is connected to ground from the output terminal but in Miller effect compensation it is connected in the feedback path of the Darlington pair used in the output stage of op-amp. This is shown in the Fig. 2.34.1.

The C_c is the compensating capacitor, R_i is the input resistance and R_o is the output resistance of the Darlington stage. The gain of the Darlington stage is given by,

a₂ = - G_mc R_o

Where G_mc = Transconductance of the stage 

Looking through the input terminal C_c appears as the Miller capacitance C_M and from the results of Miller effect we can write,

Thus effectively C_c gets multiplied by (1 + a₂) where a₂ is the gain of the stage which is large, as viewed through the input terminals. Thus practically small C_c values can be used, which is helpful from monolithic fabrication point of view.

This Miller equivalent capacitance C_M forms a low pass RC section with input resistance R_i whose comer frequency is given by,

f_d = 1/ 2πC_MR_i  ... (2.34.1)

The uncompensated and compensated magnitude plots are shown in the Fig. 2.34.2.

In addition to the multiplying the capacitance, Miller effect has another advantage. It causes rearrangement of original poles and cause Pole splitting. This means due to Miller effect compensation, f1 gets lowered while f2 gets raised. Thus poles get diverged. This increases the bandwidth compared to dominant pole compensation. 

Key Point The Miller effect compensation is simplest and most commonly used for internally compensated op-amps.

Such compensated op-amps usually have single break frequency and are inherently stable in nature irrespective of value of closed loop gain. External compensating network is not required for such op-amps.

Some internally compensated op-amps are Fairchild's µA 741, National semiconductor's LM 107, LM 741, LM 112 and Motorola's MC 1858.

Review Question

1. Write a note on Miller effect compensation.