9.5 Limitations On The Use of Negative Feedback

From the fore-going discussion, you may well think that negative feedback can do just about anything, short of curing a rainy day. Such is not the case. Yes, negative feedback can drastically lower distortion and increase bandwidth. Yes, it can have a very profound effect on input and output impedance. And yes, it certainly does stabilize our gains. What then, is the problem? Like all things, negative feedback has specific limitations. The first thing you should note is that 𝑆 is a function of frequency. This was graphically depicted back in Figure 3.3.2. The amount of change seen in impedances and distortion is a function of 𝑆 , it follows that these changes must be a function of frequency. Because 𝑆 drops as the frequency increases, the effects of negative feedback diminish as well. For example, if an SP amplifier has an open-loop 𝑍𝑖𝑛 of 200 k Ω and the low frequency 𝑆 is 500, the resulting 𝑍𝑖𝑛 with feedback is 100 M Ω . If we increase the input frequency past the open-loop 𝑓₂ , the open-loop gain drops and thus, 𝑆 drops. One decade up, 𝑆 will only be 50, so the 𝑍𝑖𝑛 with feedback will only be 10 M Ω . If this amplifier has a lower break frequency ( 𝑓₁ ), 𝑆 will drop as the frequency is reduced (below 𝑓₁ ). The same sort of thing occurs with distortion, however, the harmonics each see a different 𝑆 , so the calculation is a bit more involved. Along with the reduction in gain, there is also a change in phase. If the phase around the feedback loop varies from -180 ∘ , incomplete cancellation takes place, and thus, the effects of feedback are lessened. The bottom line is that the effects of negative feedback weaken as we approach the frequency extremes.

The other item that must be kept in mind is the fact that negative feedback does not change specific fundamental characteristics of the amplifier. Negative feedback cannot get a circuit to do something beyond its operational parameters. For example, feedback has no effect on clipping level (saturation point). Further, feedback has no effect on slew rate (the maximum rate of output signal change, and an item that we will examine in a later chapter). Actually, when an amplifier slews, feedback is effectively blocked. An accurate output signal is no longer sent back to the input, but the amplifier can’t correct for the errors any faster than it already is. In a similar manner, even though feedback can be used to lower the output impedance of a system, this does not imply that the system can produce more output current.

One possible “problem” with negative feedback is really the fault of the designer. It can be very tempting to sloppily design an amplifier with poor characteristics and then correct for them with large amounts of feedback. No matter how much feedback you use, the result will never be as good as a system that was designed carefully from the start. An example of this effect can be seen with TIMD (Transient Inter-Modulation Distortion). TIMD is a function of non-linearities in the first stages of an amplifier, and the excessive application of negative feedback will not remove it. On the other hand, if the initial stages of the system are properly designed, TIMD is not likely to be a problem.^[1]