I am using an amplifier to amplify the signal of a microphone that s sensing a fire alarm audible output. My amplifier Vee is connected to 0V so i only get the positive side of the waveform out of my amplifier. Therefore, I am trying to set a DC bias for my inverting amplifier so I can see both the positive and negative side of my output. My circuit schematic is shown below. I am using an Lm2902N op-amp. I want to know if this schematic will work. Also, in my schematic i have a capacitor added to the voltage divider with a dotted line. I am wondering if this is necessary and if so what capacitance to choose.

 

1. Yes, it will work
2. Yes, the divider needs a decoupling capacitor.
3. Add a coupling capacitor to Vin.

The divider and decoupling capacitor form a lowpass network. You want that corner frequency to be at least 10 times lower than your lowest frequency of interest. The effective resistance of the two resistors is their value in parallel, and the equation for the corner frequency is

f - 1 / (2 x pi x R x C)

ak

 

If I am not mistaken you already have a thread on this subject? You tried to use capacitors as a voltage divider.

This is your original thread.

How much gain were you looking for? Yes, it will work.

Analog was way ahead of me.

Ron

 

1. Yes, it will work
2. Yes, the divider needs a decoupling capacitor.
3. Add a coupling capacitor to Vin.

The divider and decoupling capacitor form a lowpass network. You want that corner frequency to be at least 10 times lower than your lowest frequency of interest. The effective resistance of the two resistors is their value in parallel, and the equation for the corner frequency is

f - 1 / (2 x pi x R x C)

ak

Ok so i built this circuit:

My input from my microphone happens to be ~3.3kHz due to the fire alarm characteristics. I am using a function generator for now to simulate this and the voltage is amplified and i get the full waveform, however, now I have another issue because my output is 60Hz with a 3000Hz input. Any idea why and how to get around this? The frequency of the output is important as I will be filtering this output

 

If I am not mistaken you already have a thread on this subject? You tried to use capacitors as a voltage divider.

This is your original thread.

How much gain were you looking for? Yes, it will work.

Analog was way ahead of me.

Ron

I need to filter the output of the amplifier circuit so i need the gain to be high enough for my output to be filtered. not sure what this needs to be honestly. Ill post my oscilloscope waveform. I should also note that instead of 5V i am using 12V. My multimeter reads 387mV at the output

 

now I have another issue because my output is 60Hz with a 3000Hz input.

You are picking up 60 Hz somewhere. Have you looked at your power supply voltage and made sure it is clean. Since I don't know your location is your mains power 50 or 60 Hz? That is a large 60 Hz signal.

Ron

 

You are picking up 60 Hz somewhere. Have you looked at your power supply voltage and made sure it is clean. Since I don't know your location is your mains power 50 or 60 Hz? That is a large 60 Hz signal.

Ron

It's 60Hz. All I'm using is a DC power supply at 12V and a function generator outputting ~100mV @ 3300Hz

 

Have you checked your DC 12 Volt supply for DC Ripple? Using your scope, AC couple your vert input and look for any AC ripple on your power supply. The AC is getting in there from somewhere. Possibly an unshielded cable.

Ron

 

Something isn't grounded. Check *all* connections.

ak