Op-amp comparator 0 V - +5 V outputting +1 V - +5 V PWM

Question

I am trying to make a PWM signal for controlling a motor.

The TL082 op-amp, connected between 0 and +5 V, takes

• a triangular input generated by an astable and filtered by a capacitor on the (-) input
• an adjustable voltage, made by a R/2R resistor network on the (+) input

All this works at about 50 kHz. In theory, the output should vary from 0 to +5 V, but in reality, it varies from +1.4 V to +5 V.

I tried to add a pull-down resistor (1 kΩ and 10 kΩ), I also tried a pull up (never know) without results. I also tried a feedback loop with a 1 kΩ resistor on the (-) which changed nothing. I lowered the frequency to 25 kHz, which changed nothing. I know that these were useless, but I was not able to come up with tomething else, this is a REALLY basic circuit.

Here is the diagram:

and here are the oscilloscope traces:

The triangle input and the adjustable one (random value):

The triangle input and the output depending on the adjustable input level:

I must be wrong somewhere, but I don't get where; normally in comparator mode, the outputs are between V- and V+ and that's it. Maybe it is the fact that I use it between 0 V and +5 V?

On the other hand, the PWM is working, it changes with the adjustable voltage increase.

Answer 1

A TL082 is an op amp, and is not designed to be used as a comparator. Its internal circuitry is optimized for linear operation in feedback, and isn't optimized for the kinds of saturation that occur with large-signal effects in a comparator.

Moreover, this is not a "rail to rail IO" op amp, and its output driver cannot reach all the way down to 0 V or all the way up to 5 V. In the datasheet you can see that the output stage has a fairly straightforward push-pull as its final stage:

This structure encounters Vbe drops on both its PNP and NPN output transistors, which means that the best output range you can expect is around 0.7 - 4.3 V (assuming the prior stage can reach a wide enough range).

Use a comparator instead (and if it has an open-collector/open-drain output, add a pullup resistor). The LM393 conveniently has the same pinout and an open collector output, requiring only the addition of the pull-up resistor. It's also faster - it slews from 0 to 5 V in 0.25 microseconds (20 V/μs), while the TL081 only slews at 13 V/μs. Aside from that, it's a fairly old and simple comparator, and you can get even faster/more advanced ones to meet specific needs.