Square and Triangle OpAmp Oscillator
Here is the circuit of a OpAmp based Square Wave
Generator. One of the main application of this is
in a Simple PWM circuit and Triangle Opamp
Oscillator. The Slope of the Triangle wave is
compared with a DC Level to derive a pulse width
or OnTime proportional to a Voltage.
The
Philbrick Archive and Operational Amplifier
This OnTime proportional to voltage is for a
fixed oscillation rate based on C2C3 and R15. The
C2C3 form a Unpolarized cap, This type is also
used in crossover networks in speaker boxes. This
is because, a plastic 4.7 uF (2 * 10uF series) is
big and costlier, but a plastic cap is very stable
and closer to an ideal cap.
First you need to know that most opamps in such
circuits can give a square wave, but very few can
swing to the rails (+/5). For designing take 75%
of Vcc. CA3130 is one that can swing to rails like
a CMOS gate.
Active
Filter Design with OpAmps
Assuming the swing of output is around +/ 4V,
the pin 10 + input will be at +/ 2V. When the cap
is discharged at 0 V, let us take that the output
is +, the cap charges to a little above +2, 
input becomes dominant, so output swings negative.
This discharges the cap to 0 and then charges it
negatively upto 2. This in turn flips the output
to + as pin 10 turns dominant. Hence this
continues as a oscillation, with a nice triangle
across the cap for the PWM comparator.
Dominant means more +, 0 is positive compared to
2V. Also 2.5 is negative compared to 2.2.
Generic Circuit Diagram of a OpAmp based Square
Wave Generator. Use it as a Simple PWM circuit or
Triangle Opamp Oscillator.Used in A to D Convertors,
Switchers and Communications .
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