AN1512 Philips, AN1512 Datasheet - Page 3

• Manufacturer Part Number: AN1512
• Description: All in one NE5230
• Manufacturer: Philips

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Philips Semiconductors
Another classic oscillator uses a comparator and an integrator. The
output of the comparator is fed back to the input of the integrator.
The output of the integrator is connected to the input of the
comparator. Upon application of power, the comparator goes into
one state or the other. This comparator output voltage is fed back
into the input of the integrator which begins ramping up or down,
depending on the polarity of the first pulse from the comparator.
1991 Dec
All in one: NE5230
–
+
Figure 3. Rail-to-Rail Instrumentation Amplifier
V
IN
Figure 2. Voltage-to-Current Converter
Figure 4. Input and Output Waveforms
V
IN
–
+
+
–
NE5230
NE5230
A
+V
-V
V
+V
CC
CC
CC
-V
NE5230
CC
R 2
R 1
+V
PHOTO
CC
( 1
R
R
R
R
6
5
7
R
2R 6
1
3
R 5
–
+
)
I
R
O
-V
NE5230
R
R
R
L
CC
1
2
6
R
R
= R
= R
= R
4
2
V IN
R L
3
4
7
+
–
V
SL00906
SL00905
SL00907
OUT
3
When the voltage threshold of the comparator is reached, the output
changes state. The cycle then repeats.
VCO
If an inversion in the feedback loop can be introduced and external
energy applied at the right time, some interesting modifications of
the previously described circuit will result – namely, a
voltage-controlled oscillator. It works like this: the transistor inverts
the output of the comparator. This voltage is presented to the
inverting input of the integrator to begin the ramp cycle. When the
comparator threshold is reached, the comparator changes state as
before. This time, however, because the external applied voltage to
the same inverting input is present, the amount of current available
to the input is controlled by the external voltage and not by the
feedback voltage. Once the component values are selected, the
applied voltage, V
determines the charging time constant and, therefore, the frequency.
The more positive sets the frequency of oscillation because the the
V
oscillation. The converse is also true with minor differences. It is
interesting to note here that other low voltage op amps are not able
to perform as well as the NE5230 in this circuit. This is because the
NE5230 input voltage swing is able to exceed the rails by 250mV
and still operate within its linear region. For a given set of conditions,
then, the frequency range of the NE5230 is wider than conventional
low voltage amplifiers. The frequency range of this circuit can also
be changed by changing the value of the integrating capacitor. The
smaller the value, the higher the frequency of oscillation for a given
set of conditions.
The circuit in Figure 5 is the complete function generator. The
measurements were taken at room temperature with only two AA
batteries supplying the power. The outputs were loaded with 200
for the sine and triangular wave outputs and 50 for the square
wave output. The output voltage for the sine wave was 1V. The
square wave output swung from rail-to-rail while the output voltage
of the triangular wave varied with the input voltage, V
due, of course, to the voltage requirements of the transistor.
PERFORMANCE
The distortion of the Wien Bridge output was 0.015% at the lowest
frequency and 0.09% at the highest. Using the different capacitor
values, the frequency was varied from minimum to maximum using
the ganged 10k pot. The frequencies could be changed from 20Hz
to 2.5kHz. It was necessary to include a 500 resistor in the positive
feedback loop to prevent the complete saturation of the amplifier
when the potentiometer was in one extreme of its travel. In addition,
a small adjustment resistor was used in the negative feedback loop
to adjust the gain and to compensate for the slow response of the
lamp.
The maximum frequency obtained by the VCO was 9.7kHz with a
V
to 1.6kHz with 1V (V
CONCLUSIONS: FUNCTION GENERATOR
The thermal time constant of the lamp was an inhibiting factor in the
low frequency operation of the Wien Bridge. A diode or a FET will
work better here. Extreme ambient temperature will change the
operating point of the lamp and, therefore, the output amplitude.
Some non-symmetrical output was seen when operating the VCO at
C
C
, the more current is available and the higher the frequency of
of 1.65V with +1.4V batteries. The frequency varied from 8.4kHz
C
, current available to the integrating capacitor
C
) applied with 0.001 F integrating capacitor.
AN1512
Application note
C
. This was