Page 154 - Op Amps Design, Applications, and Troubleshooting

Zero-Crossing Detector 137

Minimum Input Impedance. The input impedance is established by the dif-
ferential input resistance of the op amp. The input resistance of the 741 (R D) is
listed in the manufacturer's data sheet (Appendix 1) as at least 300 kilohms. The
minimum input resistance, then, is computed as

In this particular case,

It is important to note that without feedback the (-) input does not behave as a vir-
tual ground.

Maximum Input Current. The maximum input current can be calculated by
application of Ohm's Law:

Since this is a sinusoidal waveform, we can convert it to a peak value if desired:

Output Voltage. The limits of the output voltage in the circuit shown in Fig-
ure 3.2 are simply the values of ±V SAT. For ±15-volt power supplies and a load
resistance of greater than 10 kilohms, the manufacturer's data sheet in Appendix 1
lists a minimum output swing of £12 volts. If we loaded the circuit with a resis-
tance of less than 10 kilohms, we could expect the output levels to decrease.

3.2.3 Practical Design Techniques

For low-frequency noncritical applications, the simple circuit shown in Figure 3.2
can be very useful. For purposes of a design example, let us develop a circuit to
satisfy the following requirements:

1. Input voltage 500 millivolts peak
2. Input frequency 1 to 10 kilohertz