Page 178 - Op Amps Design, Applications, and Troubleshooting

Voltage Comparator with Output Limiting 161

TABLE 3.3

Design Goal Measured Value
Upper window limit +10 volts +10 volts
Lower window limit +7.5 volts +7.5 volts

L6 VOLTAGE COMPARATOR WITH OUTPUT LIMITING

1*6.1 Operation
The voltage comparator shown in Figure 3.17 is configured to be noninverting.
Additionally, the output uses two zener diodes (D 2 and D 3) to limit the swing of
v 0. The zener pair, along with resistor Rj, acts like a bidirectional, biased limiter
circuit. DI and current limiting resistor R s establish the reference voltage. Feed-
back resistor R F, in conjunction with R It establishes hysteresis for the circuit.
For purposes of discussion, let us assume that the input is well below the
upper threshold voltage. Since this is a noninverting circuit, we know that the out-
put of the op amp will be driven to the -V SAT level. The zener pair in the output
circuit, along with RI, regulates the -VSAT voltage to a value established by D 3. This
reduced and regulated voltage appears as v 0.
Resistors R F and R/ form a voltage divider that appears between the regu-
lated v 0 and the changing input voltage. The circuit will remain in this stable con-
dition as long as the (+) input of the op amp remains lower than the reference
voltage on the (-) input.
As the input voltage increases, the voltage on the (+) input also increases.
Once the (+) input goes above the voltage on the (-) input, even momentarily, the
output of the op amp will go toward the + V SAT level. This rising potential, through
R F, further increases the potential on the (+) input pin. With the output of the op
amp at the +V SAT level, diode D 2 establishes the value of voltage at v 0. Again, the
circuit will remain in this state until the voltage on the (+) input falls below the

FIGURE 3.17 A voltage comparator i
with output limiting.