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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:

Electric diagram

and here are the oscilloscope traces:

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

Triangle and adjustable input

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

Triangle input and output

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.

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  • \$\begingroup\$ What exactly is an AOP? \$\endgroup\$
    – Hearth
    Jan 24 at 20:14
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    \$\begingroup\$ TL082 is not a rail-to-rail opamp. You need a rail-to-rail opamp, or a dual supply if you must use this one. \$\endgroup\$ Jan 24 at 20:16
  • \$\begingroup\$ @Hearth: an AOP is a French op-amp. Edited it. \$\endgroup\$
    – ocrdu
    Jan 24 at 20:25
  • \$\begingroup\$ Your measurements appear to be taken with a Fluke/Philips combiscope, yes? (Just curious.) \$\endgroup\$ Jan 24 at 21:15
  • \$\begingroup\$ @Hearth Yes, it is an Op amp in french, sorry :) \$\endgroup\$
    – zepeu
    Jan 26 at 8:41

1 Answer 1

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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:

enter image description here

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.

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  • \$\begingroup\$ Thanks for the answer, is there a component you would recommand? \$\endgroup\$
    – zepeu
    Jan 24 at 20:09
  • \$\begingroup\$ @zepeu LM393. I'm finishing up an edit very soon. \$\endgroup\$
    – nanofarad
    Jan 24 at 20:10
  • \$\begingroup\$ Perfect, the LM393 seems to be the basic and very useful one, thank you a lot :) \$\endgroup\$
    – zepeu
    Jan 24 at 20:38
  • \$\begingroup\$ @zepeu Not a problem, happy to help. \$\endgroup\$
    – nanofarad
    Jan 24 at 21:09

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