circuit-designduty cycleoperational-amplifierpulsepwm
I want to design pulse generator with specific period and duty cycle. I found a pulse generator with single op amp and diode. I couldn't find information about how to calculate period and duty cycle. If you have any suggestion about pulse generator design with single op amp (not comparator or 555 timer). You can find my schematic in figure below.
Take a look at these slides (starting about slide 13). It details creating a PWM generator of a fixed frequency (which you can choose) using only a 555 and a comparator. The level shift and amplify portion of the circuit isn't required; it is only to compensate for a slow input AC waveform of about 100mVpp.
Overall, the circuit looks like this:
All in all, it's a relatively simple circuit. The 555 creates the sawtooth waveform for you, and the comparator compares your input voltage to this waveform. Depending on the level of the input as compared to the level of VCC on the 555, the duty cycle will be altered. The equations for the frequency and Ton are even included, so no need to derive.
There is quite a lot of information available on the web, however knowing good search terms is an obstacle.
You need a constant current source to reliably drive a LASER.
I recommend looking at Sam's LASER FAQ and specifically Communications Systems Using Diode Lasers which, I think may provide solutions to your problem.
Current control is very important to ensure a LASER works properly.
Comment:
I don't think current modulation is easy. If it is to ensure the LASER operates correctly then it is addressed at that web site.
I'll quote a useful part:
Modulating a laser to complete cutoff is a very, very bad game for all sorts of reasons:
- The light versus current behavior is hideously non-linear below about 10% of full output, so you really need dynamic feedback control - but the photodiodes tend to be slow, so that's not on.
- Spectral quality, and beam shape, go to blazes at low power levels
- lasers don't turn on from fully-off anywhere near as fast as they can vary intensity around the 50% level.
- It's pig-difficult to design the modulation circuit so it is guaranteed never to overshoot the current that gives 100% full light output (which is essential, because even very brief over-power transients dramatically shorten the laser's life).
That link seems like a good place to start. At best it solves the problem, at worst it gives you enough understanding to be able to write good search queries.
Best Answer
simulate this circuit – Schematic created using CircuitLab
Figure 1. The basic relaxation oscillator.
How it works:
Figure 2. The waveforms obtained from Figure 1.
The simplest way to control the duty cycle is to control the charge and discharge times independently with diodes as shown in Figure 3.
simulate this circuit
Figure 3. Duty-cycle control with diodes.
Figure 4. Resultant simulation.
simulate this circuit
Figure 5. By adding a potentiometer the duty cycle can be adjusted while maintaining the frequency reasonably stable.
simulate this circuit
Figure 6. You can also leave out one of the diodes. In this case discharging C1 is done through R5 only while charging is done through R5 and R4.
There's a useful calculator at Must Calculate that will work out the time constants for you. For the timing of my Figure 1 it worked out 6.93 ms.
You can play with the CircuitLab simulations by "editing" my question and editing the schematic. Just don't save them on your way out!
Note that I haven't included the voltage drop of the diodes in my calculations. It will affect the results a bit but the simulator should pick that up for you.