This is a simple PWM modulator circuit. Comparing the message signal to a ramp or triangular waveform is the simplest way to produce a PWM signal. When the input is greater than the triangular waveform (the carrier), the resulting output is high, when the input is less than triangular waveform, the output is low. So, a comparator and triangular-waveform generator are required. Here is the circuit :
The triangular wave is generated by a MAX492 dual rail-to-rail op amp. The triangular waveform’s dynamic range must be larger than that of the sensor, so we need rail-to-rail capability. Cf and Rf determine the slope of the positive- or negative-going ramp that is produced by the integrator circuit (U1-A). To reverse the slope of the ramp, and the triangular-waveform frequency, this circuit use the comparator stage (U1-B). Following equation shows the triangular-waveform frequency :
F=R3/(4*Rf*Cf*R4)
Using component values shown in the figure, the ramp frequency and thus the PWM output is about 500Hz. The sensor’s analog  voltage and the ramp waveform are compared by a dual comparator (MAX942). The accuracy for the PWM output in the presence of rapidly changing sensor inputs is insured by R2 and R1 by providing hysteresis. U2-B provides additional buffering and isolation, the PWM output is provided by U2-A. [Source: maxim-ic.com]
Possibly Related Circuits
Self Powered Compressor
The most unique feature of this dynamic compressor circuit is that it needs no power supply, looks like  as passive connector or adapter cables. The attenuation is good enough, about 20dB, works for  input range of 100mV to 10V signal level. Here is the schematic diagram of the circuit: The...
Pulse Width Modulator Using 555 IC
A very simple pulse modulator circuit can be built using 555 IC. In 555 chip, a special modulator input is available at pin 5. Here is the schematic diagram of circuit: This pulse modulator circuit need an external clock signal to work. You can build an astable multivibrator using the...
Pulse Position Modulator Using 555 IC
Pulse position modulation is similar to pulse width modulation, but the frequency is not constant.  Like pulse width modulator circuit, pulse position modulator can be easily constructed using 555 IC.  Here is the schematic diagram of the circuit: If you look at the output signal plot in the above timing...
Gamma Correction Circuit
Gamma correction need to be applied in video processing since the transfer function of video acquisition and video display is non linear.  Here is the schematic diagram of a gamma circuit: If gamma value equal to 1, the the  transfer function is linear. Typical CRT usually have  gamma factor from...
Fast Pulse Detector
A schematic diagram of a fast pulse detector is show in the figure below.  Error detection rate under 10% can be expected for 60 ns pulse. To get error free performance, it is recommended to use this circuit for detecting 100 ns or wider pulse. [Schematic diagram source: Linear Technology...
Search Terms:
simple op amp pwm, integrator circuit using 741 op amp 555 ic, fm modulator circuit using 741, op amp pwm circuit, dual rail power supply using signal generator, 555 circuit with op amps, pwm driver circuit using lm741, pwm 20 amps, pwm using op amp, transfer functions for non-linear op amp circuits, pwm control with opamp, comparator pwm circuit, hysteresis pwm op amp, pwm mit opv, triangular generator using ic 741, dc cdi using 555, pwm integrator circuit, pwm fm modulation  
http://www.simplecircuitdiagram.com/2010/02/26/pwm-modulator-using-op-amp/