Page 195 - Op Amps Design, Applications, and Troubleshooting

178 OSCILLATORS

Compute Rectifier and Filter Components. The R GC G network is a Biter for
the half-wave rectifier circuit. As with any basic rectifier circuit, the time constant
of the filter should be long relative to the period of the input signal. In the case of
the circuit shown in Figure 4.2, the input signal is the basic oscillator frequency
(fo). Thus, the R GC G time constant is computed as

In this particular case, the required RC time constant is computed as

Let us select a value for R G and compute the associated value for C G. The value for
R G is not critical, but it is generally in the range of 10 kilohms to 1 megohm. Let us
select 270 kilohms as that value. Capacitor C G can now be computed using Equa-

We will choose a standard value of 0.039 jjf.
The rectifier diode can be any general-purpose diode capable of withstand-
ing the currents and voltages present in this application. Although the actual volt-
ages and currents will be less, the following provides an easy and conservative
computation for diode selection:

where V PW is the minimum reverse voltage breakdown rating for the diode. Also,

where l f is the maximum average forward current rating for the diode. Let us
select a 1N914A for this example. The data for this diode is listed in Appendix 6.