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I am using this Chip: LM833N to essentially trying to Half Wave rectify my low AC signal input ~1.36Vp.

Using it in this configuration for now to get things started (Ignore the part number).

enter image description here

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CH1 is the input and CH2 is the output, noted this is while the OP AMP is turned off. (Image Above).

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Once the OP-AMP is turned on this is the result, CH2 is a flat Rectified line?

Here's the schematic

schematic

simulate this circuit – Schematic created using CircuitLab

I used the Green output of my single power supply as the ground, and connected the black output to -VEE.

Picture for reference: enter image description here

Any light/clue onto the situation would be greatly appericiated, I am sure this isn't working as indeed.

Leoc
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    An LM338 is not an opamp. it's a 5A Adjustable Output Linear Regulator. Your 1st schematic then calls it an MCP601, and your second schematic calls it a TL081. Please make up your mind, and fix your question. – brhans Jun 26 '18 at 18:50
  • Sorry about that, got mixed up. The first schematic is just for the configuration and not the part number, as the first sentence says (corrected now). – Leoc Jun 26 '18 at 18:56
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    An LM833 won't work with a single 5V supply. It needs at least 10V (single) or +/-5V (dual). – brhans Jun 26 '18 at 18:59
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    Most of us are going to read the schematic and miss your note. Your CircuitLab schematic shows that 0V and -Vee are connected to circuit ground (as indicated by the same symbol on both. Your PSU is a single-rail type with an earth connection that can be connected to black or red depending on whether you want negative ground or positive ground. It is not a split-rail type. – Transistor Jun 26 '18 at 19:00
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    It also doesn't work with an input referenced to the negative supply. VCM is specced at +-12V for a +-14V supply, so you need to keep the inputs at least 2V away from either rail. – brhans Jun 26 '18 at 19:03
  • @brhans What should I try first then? Higher input voltage as suggested (10V)? – Leoc Jun 26 '18 at 19:05
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    I'm confused. Do you want it to work as a rectifier? If so, google "precision rectifier." If you don't want it to rectify, set your positive input halfway between VCC and VEE. (Note that your second schematic has -VEE and OV, but they're both still connected to GND.) – Cristobol Polychronopolis Jun 26 '18 at 19:06
  • @CristobolPolychronopolis I have it to be rectified, but just taking it as baby steps. The configuration initially suggested should provide a half wave rectification – Leoc Jun 26 '18 at 19:07
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    Supply voltage, not input voltage. And even then it probably won't work properly because your input is referenced to the negative rail. You're using the wrong opamp. The MCP601 referred to in your schematic would probably work as intended. – brhans Jun 26 '18 at 19:07
  • Thats what I meant sorry. What makes the MCP601 work over the LM833? – Leoc Jun 26 '18 at 19:10
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    Get a Rail to Rail OA and diode half wave cct or dual OA for full wave precision rectifier (search) – Tony Stewart EE75 Jun 26 '18 at 19:15
  • @TonyStewartolderthandirt I agree with you on the Rail-to-Rail. I would imagine I need a fast one as well as I will rectifying audio signals correct? Should I also be looking for High Slew Rate op amps? – Leoc Jun 26 '18 at 19:19
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    Your title says LM833, but the schematic shows a MCP601. Huh? What? Closing as unclear. – Olin Lathrop Jun 26 '18 at 19:30
  • I believe I said ignore the model number lol... I just used to show you the configuration I am trying to replicate? If you looked down further and the first sentence of the topic is says LM833. – Leoc Jun 26 '18 at 19:31
  • Precision Rectifier designs can be low BW OA’s since you have a large signal small gain – Tony Stewart EE75 Jun 26 '18 at 19:48
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    "I believe I said ignore the model number lol...". It doesn't work that way. The first thing I saw was the title, and the second the schematic. With that discrepancy, the rest was only going to get skimmed at best. In any case, the schematic is obviously hand-draw, so you could have easily written the correct opamp. Even if you got the schematic image from elsewhere, you could have simply made your own schematic. It's got only 3 parts. What you did makes no sense at all, and is just abusing the volunteers you are asking a favor from. – Olin Lathrop Jun 27 '18 at 10:40

3 Answers3

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Well, your "off" picture is explained by the input voltage passing unimpeded through the two 10K resistors. It doesn't go below GND due to the protection diodes, but if your VCC is floating, that'll float enough to let positive excursions through. Not sure about your DC output, though...I can't troubleshoot it without some probing, but it's probably because the 833 doesn't support rail-to-rail input, so you have to keep the inputs within the common mode voltage range. At +/-15V supply that's +/-12V worst case. You'll have to use a rail-to-rail input op amp (or at least one that can take 0V relative to VEE) or switch to a more complex circuit.

Cristobol Polychronopolis
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  • I understand it isn't a Rail-to-Rail OP, however the fundamental theory should be there (Having it be rectified still). I know it may not be pulled down to exactly 0V, but close-ish to it? As you should see the general shape of a half wave rectification still, no? It doesn't explain why there's a flat line ? – Leoc Jun 26 '18 at 19:37
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    No, the fundamental theory is not there. With a single supply, that opamp cannot handle an input signal less than 3V. It's not a rail-to-rail input opamp. – brhans Jun 26 '18 at 19:48
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    The input common voltage range specifies the input voltages at which the op amp will work as specified. Outside that range, there are no guarantees. Also, I noticed your description of 0V as green...if you mean the power supply banana jacks, that's not quite true. Green is earth ground, and is not referenced to your DC output unless you do it yourself. You'll still need a divider to bias it. – Cristobol Polychronopolis Jun 26 '18 at 19:53
  • @brhans Wait where does it say cant work with input signal less than 3V? – Leoc Jun 27 '18 at 01:45
  • @Pllsz - read Cristobol's answer again. And then do the math on (15 - 12). – brhans Jun 27 '18 at 11:23
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It seems your basic problem is putting a positive-only signal into a inverting amplifier that can't go below ground.

Olin Lathrop
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  • But isnt that how it works? Since its "Capped" at ground it will produce 0V at the output when the input signal is High? and when the input signal is low below 0V it will produce the negative wave but flipped. – Leoc Jun 26 '18 at 19:33
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    @Pllsz: If the input signal is symmetric about ground, the opamp is rail to rail on input, rail to rail on output, and can tolerate the negative input being substantially higher than the positive one, then at best it sorta works. It's also not clear what you want out. "Rectify" is no spec at all. Is that full wave, half wave, something else? In any case, there is too much confusion and the question is way too long to make it worth spending time on. – Olin Lathrop Jun 26 '18 at 19:41
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The green terminal of your power supply will be case and earth ground, and probably not connected to either output terminal. The actual output terminals will be Black (negative) and Red (positive). (At least, that is how any supply I've used is wired).

Peter Bennett
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  • I see, however I did try just tie-ing the ground on the OP AMP PIN 3 and PIN 4 just to the black (negative) rails of the supply, still the same effect. – Leoc Jun 26 '18 at 19:39