Page 182 - Op Amps Design, Applications, and Troubleshooting

Voltage Comparator with Output Limiting 165

Output Voltage Limits. The output voltage swing was essentially determined
in a prior step. The upper excursion is established by the zener voltage of D 2 plus
the forward voltage drop of D 3. In our case,

+
Vo = V& + 0.7 V = 3.3 V + 0.7 V = 4,0 V
The lower limit of v 0 is computed as

Vo- = V D^ -0.7 V = -5.6 V - 0.7 V = -6.3 V

3.6.3 Practical Design Techniques
We will now design a voltage comparator with output limiting that satisfies the
following specifications:

+
1. Upper output voltage (v 0 ) +5.0 volts
2. Lower output voltage (v 0~) -4.0 volts
3. Upper threshold voltage +2.0 volts
4. Lower threshold voltage +0.8 volts
5. Power supply ±15 volts
6. Op amp 741

These specifications (i.e., input and output requirements) would normally be dic-
tated by the application.

Compute Hysteresis Voltage. Our first step will be to calculate the required
hysteresis voltage. This is, very simply, the difference between the two threshold
voltages from Equation (3.6).

Compute R f and ftf. The ratio of R F to R/ is determined by the ratio of the out-
put voltage swing to the hysteresis voltage. That is,

(3.28)

In our design example, the required R F/Rj ratio is computed as shown: