Page 207 - Op Amps Design, Applications, and Troubleshooting

190 OSCILLATORS

TABLE 4.1

Design Goal Measured Results
Input voltage range 0-6 volts 0-6 volts
Frequency range 0-5 kilohertz 0-5.01 kilohertz
Ramp voltage ±4 volts -4, +3.75 volts
±3 volts (min)

This requirement exceeds the 0.5-volts-per-microsecond slew rate rating of the
standard 741, but falls within the capabilities of the MC741SC. We will utilize this
latter device in our design.
Figure 4.6 shows the schematic diagram of our design example, and the
oscilloscope displays in Figure 4.7 show the performance of the circuit. Addition-
ally, Table 4.1 contrasts the design goals with the actual measured performance of
the circuit.

4.4 VARIABLE-DUTY CYCLE

4.4.1 Operation

Figure 4.8 is the schematic of a very useful rectangular wave oscillator. What
makes the circuit particularly useful is that the duration of each alternation is
independently adjustable, which means that the duty cycle of the output can be
easily adjusted from a very small to a very high value.
To begin, let us assume that the output is at its +VSAT level. This voltage will
be regulated down to a lower value by D 3 and returned to the (+) input as a refer-
ence voltage. The positive output voltage is also regulated to a lower level by D 5.
This latter voltage provides a stable charging voltage for Q.
Q charges from ground through D 2, #3, and R 4 to the regulated positive volt-
age provided by D 5. Q will charge exponentially toward the D 5 voltage. As long as
the voltage on Q is less than the reference voltage on the (+) input of the op amp,
the circuit will be stable and Q will continue to charge.
Once the voltage of Q reaches the reference voltage on the (+) input, the out-
put of the op amp quickly switches to its -V SAT level. Diode D 4 regulates this neg-
ative voltage and provides a new reference voltage for the (+) input. Similarly, D 6
regulates the -V SAT voltage and provides a new charging source for Q. The new
source is negative, so Q will discharge and then recharge in the opposite polarity.
The charging path is from the negative source provided by D 6 through R 2, R lf D l7
and Q to ground. Again, this is an exponential charging action and will continue
as long as the voltage on Q remains more positive than the negative reference
voltage on the (+) input. Once the voltage on Q falls below the reference voltage
on the (+) input, the circuit quickly switches back to its original state and the cycle
repeats.
The time it takes Q to charge during the positive output alternation is deter-
mined by the values of Q, R 3, R 4, and D 5. Since R 3 is adjustable, it can be used to