Internal And External Compensation Of An Operational AmplifierÂÂ

Question: Discuss the internal and external compensation of an operational amplifier? Answer: Introduction Operational amplifier has the ideal frequency response of gain for a set of pre-defined frequencies. Since op amps work with the feedback, there will be certain problems associated with the feedback, especially positive feedback that result in instability and noise and they can be compensated with the compensation techniques. Ideal response The ideal response of the operational amplifier is given as below. Figure: Frequency response of ideal op-amp differentiator(blue), integrator(red) and amplifier(gold). Gain In single pole systems, gain keeps on increasing and it results in decreasing the bandwidth proportionally with the same factor. When it comes to the two pole or double pole, the gain exceeds the gain of the single-pole op-amps. The difference in the gain is seen for all the frequencies or wider range. Roll off Op amp has the frequency response, where the signals are amplified for certain frequencies and attenuated for the rest of frequencies. The point at which the signal is started attenuating and where the response falls slowly to zero, is called roll off. The roll off point will be reduced with compensation for the op amps. Compensation is one process, which is applied to make an op amp that is close to perfect in operation. It is done my making use of RC network, as a judicious patch to the circuit. There is multiple compensation schemes developed and used in the industry. The objective of compensation of an op amp is to gain stability. Compensation can roll off the high frequency of the op amps. Internal Compensation Miller Effect Compensation Circuit: Miller effect compensation circuit Internal compensation can be obtained in many ways and one of the ways is done using a capacitor. For a voltage gain transistor, a capacitor is connected across a collector base junction. The capacitor value is multiplied by Miller effect to stage gain. The closed loop gain obtained is unity or higher unity. Usage The reason internal compensation is done to the op amps are for saving the external components. It allows less knowledgeable users or novice users to use the op amps with fewer complications. However, there are certain applications instructions are to be used to make the op amps to be internally compensated and become stable. Though these are multiple pole systems, they appear to be single pole system with internal compensation, for wider range of frequency. We should not use the op amps with the internal compensation in certain circuits. These should not be used with the some of the negative feedback circuits, as it would result in high frequency instability. Problems The internally compensated op amps are stable, but conditionally. Internal compensation minimizes the closed-loop bandwidth to a best possible extent for the op amps. This is the main feedback of the internal compensation of the op amps. External Compensation External compensation is implemented with more op amps, where the closed loop gain achieved is much higher than unity. Why we use it The primary reason for implementing the external compensation is to gain the stability of the op amp circuit by preventing the oscillations. The other reasons, why we tend to use the external compensated op amps are noise reduction, highest possible bandwidth, flat amplitude response. Types of Compensation Techniques Gain Compensation Lead Compensation In-the-loop compensation Out-of-the-loop compensation Lead compensation Lead-lag compensation Feed-forward compensation Problems Compensation can provide fair level of guarantee of stability of the output. However, any kind of technique used in compensation, especially with the external components, there is quite possibility of the instability at certain higher frequencies, as the gain is increased beyond unity. The same problem can also lead to certain noise level, especially, at higher frequencies. Conclusion Op-amps need compensation for optimum operation, in order to give closer frequency response, in ideal condition. The gain varies from single-pole and two-pole systems. Roll off is an important point, where the bandwidth is calculated, accordingly. Internal compensation is implemented rarely during the design for ready to use and external compensation techniques are often used for optimum operation of the op-amps. Internal compensation is discussed in terms of when to use and when not to use. Later external compensation is discussed with the techniques and the associated problems with the same. References Jacob Millman,Microelectronics: Digital and Analog Circuits and Systems, McGraw-Hill, 1979 Lee, Thomas H. (November 18, 2002)."IC Op-Amps Through the Ages"(PDF). Stanford UniversityHandout ung, Walter G. (2004). "Chapter 8: Op Amp History".Op Amp Applications Handbook. Newnes Sergio Franco, 2014, Design with Operational Amplifiers and Analog Integrated Circuits; 4th Ed;; McGraw Hill