Environmental Engineering Reference
In-Depth Information
indicating that the second pole and the first zero remain very close provided
that In this case, the second (RHP) zero tends to and must
be higher than the unity-gain frequency given in (5.74). to ensure stability.
Setting the inner phase margin greater than 64° yields The above
relation establishes a lower limit for the ratio between and Under
this condition, any value of ideally ensures the stability of the
inner amplifier. A minimum usable value for exists in reality.
Compensation capacitors must be greater than the parasitic capacitances at
the high-impedance nodes to be valid for development. Besides, and usually
more importantly, slew-rate considerations posit the fundamental limit for
the minimum value of
[PP02], [PPP01].
5.5 REVERSED NESTED MILLER COMPENSATION
When the amplifier is made up of three gain stages and the inner stage is
the only inverting one, reversed nested Miller compensation (RNMC)
becomes the most suitable technique [EH95].
5.5.1 General Features
Figure 5.12 shows a three-stage amplifier small-signal circuit including
reversed
capacitors and
As usual, parameters and are the
i
-th stage transconductance and
output resistance, respectively. Capacitors represent the equivalent
capacitance at the output of each stage, while is the equivalent load
capacitor. Since capacitor has no connection with the load capacitor (but
only with parasitic capacitor inner loop stability is virtually achieved for
all practical values, and will not be examined. For the same reason, this
technique has an inherent bandwidth advantage over other multistage
compensation approaches based on the Miller effect.
nested
Miller compensation performed
by
