Specifications

levels are available but large noise signals on ground are
also present. To take full advantage of the common-mode
rejection of the input differential stage, a balanced bias con-
figuration must be provided. One such circuit is illustrated in
Figure 5(e). With this arrangement, the effective common-
mode rejection may be virtually infinite, owing to the input
hysteresis.
Output Configurations
LM2907, LM2917 series devices incorporate an unusually
flexible op amp/comparator device on-chip for interfacing
with a wide variety of loads. This flexibility results from the
availability of both the collector and emitter of the output
transistor which is capable of driving up to 50 mA of load
current. When the non-inverting input is higher than the in-
verting input, this output transistor is turned ‘‘ON’’. It may be
used to drive loads to either the positive or the negative
supply with the emitter or collector respectively connected
to the other supply. For example,
Figure 6(a), a simple
speed switch can be constructed in which the speed signal
derived from the frequency to voltage converter is com-
pared to a reference derived simply by a resistive divider
from the power supply. When the speed signal exceeds the
reference, the output transistor turns on the light emitting
diode in the load. A small current limiting resistor should be
placed in series with the output to protect the LED and the
output transistor.
This circuit has no hysteresis in it, i.e., the turn ‘‘ON’’ and
turn ‘‘OFF’’ speed voltages are essentially equal. In cases
where speed may be fluctuating at a high rate and a flashing
LED would be objectionable, it is possible to incorporate
hysteresis so that the switch-on speed is above the switch-
off speed by a controlled amount. Such a configuration is
illustrated in
Figure 6(b). Figure 6(c) shows how a grounded
load can also be switched by the circuit. In this case, the
current limiting resistor is placed in the collector of the pow-
er transistor. The base current of the output transistor (Q45)
is limited bya5kXbase resistor (see
Figure 2 ). This raises
the output resistance so that the output swing will be re-
duced at full load.
The op amp/comparator is internally compensated for unity
gain feedback configurations as in
Figure 6(d). By directly
connecting the emitter output to the non-inverting input, the
op amp may be operated as a voltage follower. Note that a
load resistor is required externally. The op amp can also be
operated, of course, as an amplifier, integrator, active filter,
or in any other normal operational amplifier configuration.
One unique configuration which is not available with stan-
dard operational amplifiers, is shown in
Figure 6(e). Here
the collector of the output transistor is used to drive a load
TL/H/7451–13
(a) Switching an LED
TL/H/7451–14
(b) Adding Hysteresis
to LED Switch
TL/H/7451–15
(c) Switching a Grounded Load
TL/H/7451–16
(d) Voltage Follower
TL/H/7451–17
(e) Voltage to Current Converter
TL/H/7451–18
(f) Integrator
FIGURE 6. Output Configurations
6