# Where does my negative input voltage come from? [closed]

I'm a bit confused about the concept of an input ground for an op Amp. Let's say I'm amplifying the output from a 555 timer ic. I assume the output pin from the 555 is my positive input but what is a negative input? Is it just a reference voltage that can wire to a common ground?

Sorry for a very basic question but I appreciate any input, or simply being pointed in the right direction.

Thanks!

• Possible duplicate of Negative Voltage – winny Jun 25 '17 at 6:42
• The linked dup doesn't answer this question. The OP is asking about the "positive" (non-inverting) and "negative" (inverting) inputs, not power supplies. Although clearly hasn't done any research, that's an entirely different 'close' reason. – brhans Jun 25 '17 at 10:01

Do you want the output of the op-amp to exactly follow the output of the 555 timer?

Either way, I think you might be confused about how an op amp works. They have extremely high gain (think 10,000 or greater), where the output is that gain times the difference between the "inverting input" (marked with a "-" in diagrams) and the non-inverting input ("+").

I don't think you want to simply connect the "-" to ground; it would act like a comparator, such that when "+" is taken above ground, the output would saturate high, and when taken below ground would saturate low.

Instead, connect the "-" to the op-amp's output. This creates a unity gain (gain = 1) amplifier. The output will then follow the non-inverting ("+") input.

If you don't want a unity gain follower, you can use resistors to set up different gains, subtraction/addition, and more.

One final thought - what are you connecting to the output of the op-amp? What rails will your system use? (Eg Ground and +5v). An op-amp might not be the ideal choice. Their output current is very limited (less then a 555 timer usually) and their outputs often don't saturate rail to rail (although many do - but I'm just letting you know to avoid a "gotcha")

• Thanks for the response...sorry if my question was poorly worded. I'm using the 555 as an oscillator/tone generator. The 386 is simply to amplify the volume that the speaker in my circuit outputs. My confusion is that the 386 has a + and a - input and the 555 only has one output which I would assume goes to the + input. Also want to clarify that I'm an idiot (ie a musician not an electrical engineer trying to use things he knows little about to find new ways to make noise) not worthy of the guidance from those as well versed as yourselves but deeply appreciate your guidance nonetheless. Thanks – Elliott Samuel Lemberger Jun 25 '17 at 23:08
• Assuming your 555 tone generator circuit is not loud enough as-is, then you will want an amplifier stage "beefier" than an op-amp provides. So skip the op-amp and use a circuit ( made-for-purpose audio amplifier IC if you don't need it very loud) or otherwise module or off-the-shelf consumer audio amplifier fed directly by the 555 output. As this one's on hold, I would ask a different more focused question, asking for an audio amp circuit, stating: 1. 555 output requiring amplification. 2. volume requirements & your speaker's specs 3. Power supply you'd use (battery? 12v wall wart? Etc) – CL22 Jun 26 '17 at 4:05