phase shift and feedback

[Kevin Weddle]

The way I understand it, phase shift is detrimental to the effects of feedback. We know that the phase shift caused by a transistor of 180 degrees is different from the phase shifts caused by reactive components. In fact, while the transistor phase shift produces an exact replica of the input, only opposite, reactive components produce an output that is strictly out of time. Therefore, you are producing an output of a time that does not directly correlate to the time of the input. So in effect, we are only complicating the signal with unknowns. One of the things we try to do in electronics is to determine what the signal is telling us. If we run the signal through the circuitry, and we can account as to what has been done to it, we know what the signal was trying to tell us. Without knowing exactly what was done, we could not determine what the signal was originally.

[Guest Alun]

Are you talking about phase shift in amplifiers?

Phase shifts caused by parasitic capacitances in amplifiers can lead to oscillation when a large amount of negitive feedback is used, here's an extract from a college assignment I did a couple of years ago that covers this and here's the datasheet for the 741 op-amp discussed  in the assignment.