Page 203 - Op Amps Design, Applications, and Troubleshooting

186 OSCILLATORS

for the -1-volt input case. The input current for the -5-volt case is

j = -T = 500 jjA
* 4 10 k&

Let us consider the electron flow in the case of a 1-volt input. Recall that Rj
will have a 50-microampere current flowing in a right-to-left direction. R 4, as com-
puted, will have a current of 100 microamperes flowing from right to left. When
the latter current gets to the summing point of A lf it splits. One part, 50 microam-
peres, goes through RI and satisfies the requirements of +V IN and R 5. Kirchhoffs
Current Law tells us that the remaining 50 microamperes must flow into Q in a
left-to-right direction. It is very important to note that the magnitude of this charg-
ing current is identical to that which flowed on the previous alternation, but it is
flowing in the opposite direction. Therefore, Q will charge at the same rate but in
the opposite polarity. Because the charging currents are equal and the required
voltage change (AFci) is the same, the amount of time for this alternation will be
the same as the first. Given this observation, we can now compute the frequency
of oscillation for a given input voltage (+V IN):

This equation will be valid as long as the slew rates of the op amps do not interfere
with circuit operation. As the oscillator frequency increases, the slew rate limita-
tions of the op amp tend to reduce the actual frequency from the value computed.

4.3.3 Practical Design Techniques
We will now design a voltage-controlled oscillator that meets the following design
criteria:
1. Input voltage range 0 to 6 volts DC
2. Ramp output voltage ±4 volts (±3 volts minimum)
3. Frequency range 0 to 5.0 kHz

The configuration and gain values for A 3 should stay the same as that shown in
Figure 4.5. Therefore, the following components will be considered as "previously
computed": D 3, R 3/ R 5, D 5, D 6, R^ and D 4. If, because of availability, you elect to
change any of these resistors, be sure to keep their ratios such that the voltage gain