870 Chapter 25
voltage equal to the unbalanced input voltage appearing
between their outputs. (Two 6 dB quantities sum to
make zero gain.) Take the case of one output, the upper
one being shorted to ground. An error potential is
derived of such a phase and level on the error-sense line
that positive feedback increases the gain of the
unshorted amp by 6 dB, while matching on the positive
input of the shorted one the signal on the negative input,
canceling its amplification. Closing the shorted amp
down prevents ground-current problems; therefore, any
measure of output termination imbalance is reasonably
dealt with by this arrangement.
A major problem with any circuit depending on high
levels of positive feedback such as these is their poten-
tial instability. Both these circuits are right on the edge
of instability—they have to be in order to work accu-
rately; a measure of margin has to be given for peace of
mind and component tolerances. This backing-off
compromise affects primarily common-mode rejection
and output level against lopsided terminations. A loss of
about 0.5 dB in differential output level can be expected
when one side is shorted to ground, although tight
component tolerances can improve on this. Component
tolerance imbalance—even if constructed with 1%Figure 25-52. Electronic floating differential amplifier stage.B. Cross-coupled differential amplifier type.A. Inverter type.InputBipolarOutputInputBipolar