I have been using TL084cn IC which is basically an amplifier IC.

I was giving the input to it of amplitude around 450mv.

This IC is capable of providing a slew rate of 13.

The Vcc given to it was 5V and -Vcc was GND also the inverting terminal was grounded so basically the opamp was being used as comparator.

Theoretically the output should have been 5v for positive half cycle of the input however the output was only 3.6 V.

Why is this so?

Datasheet of TL084CN

  • 1
    \$\begingroup\$ The link you give for the datasheet is terrible...barely readable. Please provide a link to the actual manufacturer's site. \$\endgroup\$ Jan 23, 2019 at 17:36
  • \$\begingroup\$ I have updated the link \$\endgroup\$
    – user183710
    Jan 23, 2019 at 17:48
  • \$\begingroup\$ As the answers have indicated, it's not the small input that is the problem - it's the small power supply voltage. \$\endgroup\$ Jan 23, 2019 at 21:40

2 Answers 2

  • if it doesn't say Rail to Rail (RR) output, read datasheet for Vopp Output swing.

In order to have active current limiters, these devices do not swing to Vcc or Vee

Comparators are usually open collector so you can get perfect logic levels with a pull-up R of 10K

Some are RR out, others RR in & out (RRIO)

Yet, 3.6V is a valid logic 1

enter image description here

Always start with a Design Spec in point form like a datasheet with tolerances on input and outs V,I R,P etc, then add slew rate , f or BW , temp range as required.

  • \$\begingroup\$ 3.6V might work for logic high on the output, but logic low won't likely make it close enough to 0V for a TL084. Try a LM339 if you want a quad comparator. \$\endgroup\$
    – glen_geek
    Jan 23, 2019 at 17:49
  • \$\begingroup\$ In the datasheet the output swing is given as 10v or 12 v depending upon Rl value. It doesn't say anything about railing \$\endgroup\$
    – user183710
    Jan 23, 2019 at 17:58
  • \$\begingroup\$ I would like to discuss some more details of my project your experience could help me a lot I think. I have so many doubts and questions. Can I contact you? If so how maybe Facebook or some other platform? \$\endgroup\$
    – user183710
    Jan 23, 2019 at 18:03
  • \$\begingroup\$ THe output swing reduces with load R , so not a RRO !! \$\endgroup\$ Jan 23, 2019 at 18:03
  • \$\begingroup\$ Always start with a Design Spec like a datasheet with tolerances on input and outs V,I R,P etc \$\endgroup\$ Jan 23, 2019 at 18:04

The short answer is that the TL084 is not suitable for your application. It is not intended for single-supply operation at 5V, and the common mode input range doesn't go down to the negative supply voltage.


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