Environmental Engineering Reference
In-Depth Information
4.6 TWO-POLE FEEDBACK AMPLIFIERS WITH A ZERO
Often the loop gain of a feedback amplifier has a zero which can heavily
affect the transfer function of the closed-loop amplifier. Indeed, if the loop
gain is
the closed-loop transfer function exhibits the same zero and is given by
where in the denominator is still given by (4.7) and the damping factor is
modified with respect to (4.8) according to
The phase margin of the feedback amplifier, under the assumption of a
dominant-pole behaviour whose pole and zero is higher than the transition
frequency,
is given by
which shows that a negative zero helps stability, but a positive zero can
drastically reduce the phase margin. Therefore, during compensation
particular care must be taken to avoid or minimise the effect of positive
zeros.
Although it is seldom used, the step response of a feedback amplifier with
the closed-loop transfer function in (4.61) is
