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The Secret for Designing the Operational Amplifier with High Stability (1)

The secret for designing the operational amplifier with high stability

During the process of designing the analog electronics, we will use the operational amplifier often, and we come across the degenerative feedback commonly: the degenerative feedback can restrain the instability of gain, reduce the nonlinear error introduced by the components, decrease the effects of driftage of temperature, impedance transformation and extended frequency band. However, even though the use of degenerative feedback will make the designed circuit become more stable at some extent, there will also be an unstable situation that makes the circuit change if the keys of design has been ignored.

Through listing the transfer function of operational amplifier, we can acquire the poles and zeros which will have a great impact on keeping the stability of operational amplifier: the poles can not only decrease the amplitude (for stability), but also reduce the phase margin (for stability); the zeros increases the amplitude (not for stability), but increase the phase margin (for stability); the zeros in the right half plane of complex plane, which increase the amplitude will reduce the phase margin.

During designing the circuit in reality, if we can find the principal and auxiliary poles, we have done half of the work, and the rest part is to eliminate the corresponding zeros and poles by using different compensation methods for different models, which is for making the system more stable.

The common methods for stabilizing the operational amplifier:

1. The series resistance is added at the positive and negative input end of the input terminal to reduce the loop gain and achieve the stability of the system.

2. There will be noise gain of high DC sometimes on the input terminal of the operational amplifier, so when it occurs, the positive and negative input terminal series RC network is needed to stabilize the system.

3. To add a capacitance to the feedback loop to give rise to the effect of phase advance compensation to achieve stability.
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