Environmental Engineering Reference
In-Depth Information
affords a deep understanding of the most significant elements associated
wit2h the compensation step. More accurate results involving second-order
effects that are
required to
further refine the compensation
process are
achieved by using circuit simulators like SPICE [VNP80].
6.6.1 Frequency Compensation of the Series-Shunt Amplifier
The schematic shown in Fig. 6.30 is the high-frequency small-signal
equivalent circuit of the series-shunt amplifier in Fig. 6.1. In the scheme, the
load capacitance
includes the output capacitance,
of transistor T2.
To evaluate the return ratio we must set voltage source to zero. By
initial circuit inspection we find that the loop gain is made up of the gain-
stage provided by transistor T2, closed in loop through the feedback resistors
and and transistor T1. In particular, transistor T1 is in common Y
configuration and works as a current buffer.
Since inside the loop we have only one inverting gain stage with both
input and output high-resistance nodes, to compensate the circuit we can
profitably exploit the
Miller
approach by
connecting the
compensation
capacitor, across terminals Y and Z of transistor T2.
This generates a dominant pole at node Y2. The second pole is due to the
output capacitance, and we neglect the other high-frequency poles within the
loop. Now, to perform compensation we have to simply follow the procedure
developed in Chapter 5, on the equivalent circuit in Fig. 6.31, also equivalent
to the one in Fig. 5.2 except for the additional capacitor
Note however,
that since
is a large capacitor, the parasitic capacitor
can be neglected.
Search WWH ::




Custom Search