I am working on a project where i am amplifying a fire alarm signal using a microphone and an inverting amplifier. Instead of a dual supply of +/-12V I am trying to use 24V with a 12V reference to achieve +12V and -12V for my lm2902 op-amp Vcc and Vee respectively. I will post a photo of my schematic and of my circuit. My confusion is how to connect the Vcc and Vee to my capacitor circuit to get -12V.

 

You need a virtual-ground circuit to generate the 12V common point.
The simplest is to connect a 1kΩ resistor across each filter capacitor.
The amp plus (+) input then goes to this junction (and anything else you want to be biased at the half-way point).
The op amp power goes to the 24V plus and minus terminals.

What is the op amp output load and what voltage is it referenced to?

 

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You need a virtual-ground circuit to generate the 12V common point.
The simplest is to connect a 1kΩ resistor across each filter capacitor.
The amp plus (+) input then goes to this junction (and anything else you want to be biased at the half-way point).
The op amp power goes to the 24V plus and minus terminals.

What is the op amp output load and what voltage is it referenced to?

Thank you. I connected the 12v reference (between the capacitors) to the non inverting terminal and connected 24V to Vcc and 0V to Vee and I get a pretty decent square wave like expected. What is the purpose of connecting the resistors across each capacitor? And when you say across each cap do you mean in parallel?

 

You need a virtual-ground circuit to generate the 12V common point.
The simplest is to connect a 1kΩ resistor across each filter capacitor.
The amp plus (+) input then goes to this junction (and anything else you want to be biased at the half-way point).
The op amp power goes to the 24V plus and minus terminals.

What is the op amp output load and what voltage is it referenced to?

Thank you. I connected the 12v reference (between the capacitors) to the non inverting terminal and connected 24V to Vcc and 0V to Vee and I get a pretty decent square wave like expected. What is the purpose of connecting the resistors across each capacitor? And when you say across each cap do you mean in parallel?

 

That is not going to work. You can't create a dual +/- supply from a single ended supply using capacitors. Your microphone is giving you an AC signal that is going above and below 0 Volts. I suggest you give this a read:
Offset Adjustment in Op-Amp Circuits
Additionally, try a few Googles of the above phrase.

Nevermind, I see Carl has addressed your problem.

Ron

 

I am working on a project where i am amplifying a fire alarm signal using a microphone and an inverting amplifier. Instead of a dual supply of +/-12V I am trying to use 24V with a 12V reference to achieve +12V and -12V for my lm2902 op-amp Vcc and Vee respectively. I will post a photo of my schematic and of my circuit. My confusion is how to connect the Vcc and Vee to my capacitor circuit to get -12V.

Only one capacitor is needed. The op amp creates the ground for you. If you need more current just add buffer transistors on the output.

 

Having the two capacitors in series act as a voltage divider works great in theory, but not in practice. The fault, dear Brutus, is not in our stars, but in our electrolytes. Large electrolytic capacitors can have tolerances like +80%/-20% or worse, and that's just for their initial value. The operating value varies with temperature, applied voltage, ripple current, and load current. For a low power circuit you can get good-enough stability by placing a resistor in parallel with each cap. The resistors stabilize the DC value of the center point independent of the capacitor values (since they are theoretically open circuits at DC). As long as the circuit does not draw more than a few mA, this point will be stable enough.

Or, a pseudo-ground as in post #7 is so common that TI makes a specialized chip for this.

If it were me, I'd put a 1 K 1/4 W resistor across each 2200 uF cap, declare the center point my ground, connect the rest of the circuit as indicated, and try it. If you can't get the big caps within 1 inch of the opamp power pins, connect a ceramic capacitor from the opamp Vcc to Vee right at the chip. 0.1 uf to 1.0 uF should work.

Next up - you are asking a single opamp stage to make over 50 dB of gain at audio frequencies. That's a lot. What is the opamp part number?
Also, 8 V is higher than usual for electret microphone bias. If you are staying with 8 V, I'd increase the 1 K boas resistor to 4.7 K.
And, you have the values of the 1 K resistor and 4.7 uF capacitor in the wrong places.

ak

 

What is the purpose of connecting the resistors across each capacitor? And when you say across each cap do you mean in parallel?

You need resistors to carry any unbalanced DC current between the power supply and the 12V virtual ground point.
For example, any DC current through your mic circuit, which is connected to the virtual ground point, has to have a path to the negative side of the power supply and that would be through that resistor.
Capacitors block DC so it can't go there.
The circuit may work awhile without those resistors, but eventually it will fail.

Yes, you place one resistor in parallel with each capacitor.