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Operational Amplifier Stability and Compensation

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3.
a) Explain what is meant by 'unconditional stability' in an op-amp.

b) Figure 2 gives the open-loop response of an uncompensated op-amp. Compensation is to be applied to the amplifier to make it unconditionally stable. Estimate the frequency at which the breakpoint of the compensated response must occur.

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a) Frequency response of the uncompensated operational amplifier (op amp) (e.g. the one shown in figure 2) commonly contains several relatively closely spaced poles located in the high frequency range. Closing a unity gain feedback loop around such op amp would cause oscillatory behavior. This type of behavior is a consequence of large phase shift which can reach the value of -180° for closed loop gains larger than 1. An op amp is called unconditionally stable if it can provide stable operation ...

Solution Summary

This response defines operational amplifier stability and explains in detail dominant pole compensation of an uncompensated operational amplifier. The solution estimates the frequency at which the breakpoint of the compensated response must occur.

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Op-Amp THS4021

FIGURE 2(a) shows the Bode plot of op-amp THS4021. The
manufacturer describes this device as a ââ?¬Ë?decompensated op-ampââ?¬â?¢, by
which is meant that the frequency compensation is partial as opposed to
ââ?¬Ë?uncompensatedââ?¬â?¢ where no compensation is used at all. The advantages
of ââ?¬Ë?decompensationââ?¬â?¢ over ââ?¬Ë?full compensationââ?¬â?¢ include a higher open-loop
gain and better slew rate. Greater care, however, is needed in the use of
such an op-amp as frequency stability cannot be assured.

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