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A d v a n c e d P o w e r A m p l i f i e r D e s i g n Te c h n i q u e s
Unity-gain output stage e (x) In
–
+
S2
+
Σ
Out
Σ +
+
S1
–
Σ
Figure 12.1 Feed-forward error correction.
represented by the source ε(x). The error is subtracted from the unity-gain signal path to signify nominal gain less than unity. Summer S1 compares the input and output of the output stage and recovers ε(x). Summer S2 adds ε(x) back into the signal path to restore the signal to its original input value. The problem with this kind of error correction is the difficulty with which an additional error signal can be passively added in a precise way to the output signal after the output stage. Making such a passive adding circuit at high power levels is difficult. It is even more difficult to do it in such a way as to not seriously compromise the output impedance. Any departures from the ideal signal will not be corrected after the summing network.
Reduced Effectiveness at High Frequencies The cancellation of distortion with feed-forward requires that the error signal have just the right amplitude and phase. In principle, these can be adjusted with potentiometers, but this adds to cost and difficulty of setup. It is also generally the case that it becomes progressively more difficult to maintain the proper phase relationship as the frequency increases. Ultimately, this means that the amount of achievable distortion reduction through error correction decreases with increasing frequency, just as does the distortion reduction afforded by negative feedback.
12.2 Hawksford Error Correction In 1980 Hawksford introduced a form of error correction that did not depend on a passive addition of the error signal to the output signal after the output stage [1]. Instead, as illustrated below, it fed back the error signal in a particular way. This will be referred to as Hawksford error correction (HEC). This technique is illustrated in Figure 12.2. As in Figure 12.1, the output stage is modeled as having exactly unity gain with an error voltage ε(x) added. This error represents any departure from unity gain, whether it is a linear departure due to less than unity gain, a distortion due to transconductance nonlinearity, or injected errors like power supply ripple. A differential amplifier, represented by summer S1, subtracts the output from the input of the power stage to arrive