Environmental Engineering Reference
In-Depth Information
In contrast to what a superficial analysis could show, the down resistance
is not simply equal to the resistance seen by looking into terminal Z of
transistor T2. The down resistance is affected by the feedback provided by
the current mirror which injects almost the same current flowing through
resistance into the output node Hence, a unitary loop gain (
T =
1) is
experienced by the down resistance which is reduced by a factor of two
compared to its open-loop value. We can calculate this open-loop value by
setting the controlled current generator to zero, and recognising that
transistor T2 is in common Y configuration with an equivalent degenerative
resistance
given by
which is much smaller than
Thus, the open-loop resistance is
Substituting (9.2) into (9.3) we obtain
and the close-loop value is
