Environmental Engineering Reference
In-Depth Information
Starting from their low-frequency values, second- and third-order harmonic
distortion factors linearly increase at a frequency equal to and
respectively. Compared to dominant-pole compensation, we
see that the frequency band where distortion factors remain equal to their
low-frequency values is greater in the Miller-compensated amplifier by a
factor equal to
Equations (7.70) and (7.71) also predict that and become
constant at frequencies equal to and respectively. At
they begin to decrease. This behaviour was already found appropriate
in two-stage amplifiers compensated with a dominant pole. In contrast, when
using Miller compensation it is unrealistic. Indeed, the local feedback
operated by the Miller capacitor causes coefficients to decrease with
frequency. At high frequencies, distortion of the first stage becomes
dominant and a nonlinear model of the first stage should then be included to
accurately predict harmonic distortion.
The use of nonlinear models for both the first and second stage
considerably complicates distortion evaluation. However, since the two
distortion mechanisms are dominant over different frequency ranges
(distortion due to the input stage is effective at high frequencies, whilst
distortion due to the output stage is dominant at low frequencies) we can
separately study the two cases with our distortion models
4
. We shall not use
this approach now, because it can be shown that fairly good approximation
for distortion factors valid up to the gain-bandwidth product is found simply
4
An example of how to treat distortion coming from two cascaded stages is
described in the next section, 7.3.2.
