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I am trying to build an amplifier circuit for a showlaser that I am building. The amplifier circuit is used to convert the 3.3 V of the Teensy 4.1 to 5 V.

I previously build an working version, but I can't replicate it on new circuit board designs; all my attempts failed.

The op-amp output is always 9 V. The voltage always stays at 9 V, the input voltage doesn't matter, and the op-amp output voltage does not decrease when the input voltage decreases. I don't know how to fix this issue, I hope you can help me.

Below is the diagram. The op-amp is a TL084CN.

enter image description here

My attempt on a breadboard:

enter image description here

ocrdu
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Vincent Dw
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    Why would you use an op amp for level shifting, anyway? – Hearth Jan 02 '22 at 23:17
  • You can improve the schematic greatly by using op-amp symbol to show the *schema* rather than a wiring diagram. Then it should be obvious what the feedback arrangements are, etc. Also GND symbols should point to the ground. If you're interested in improving your schematics then have a read of [Rules and guidelines for drawing good schematics](https://electronics.stackexchange.com/questions/28251/rules-and-guidelines-for-drawing-good-schematics) on this site. – Transistor Jan 02 '22 at 23:22
  • Thank you for your input. I will use this as a guide for drawing schematics as i am still learning. I used an op amp since it worked for me in an previous circuit board. Also using an op amp as a level shifter is used in a lot of examples. What are your sugestions on an different approach? – Vincent Dw Jan 03 '22 at 08:19
  • @VincentDw A dedicated level shifter would be my choice. If you don't need high speed, there are designs with a single transistor that even work bidirectionally if you need that. If you do need high speed, there are dedicated ICs out there, and TTL logic levels are lenient enough that you could also just use a 74-series chip to do the level shift (unidirectionally of course). – Hearth Jan 03 '22 at 14:50

1 Answers1

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The TL084 is not a rail-to-rail op-amp.

  • The minimum output voltage is 2.5 to 5 V volts above ground. Check the datasheet under ±Vopp.
  • The minimumm input voltage is about 3 V above the negative supply. See under Vicm.

You need a rail-rail op-amp - if an op-amp is the right device for your application at all.

Transistor
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  • Thank you! This was indeed the problem. I used an LM321-TR in my first design which was an rail-to-rail op amp. But the version i was using is no longer available, so i went with the following op amp: https://www.lcsc.com/product-detail/Operational-Amplifier_COSINE-COS8551TRA_C695778.html – Vincent Dw Jan 04 '22 at 12:41