Need help amplyfing a small weak signal, to atleast 1-2 V

[integralx2]

So i have a heart signal, at around 300-400 mV. I need to amplify it to at least 1.3 V. How can I go about this without distorting the signal (i know that noise that is present will get amplified also)? I have a 9V battery that everything runs off.

 

[blueroomelectronics]

An OpAmp can do this.

 

[integralx2]

I wish it could, but it cant. For some reason when i try to amplify this signal, its gone.

 

[audioguru]

Where is your schematic? Post it here as a very clear PNG file type. The parts values should be shown.

 

[bychon]

Sorry if I sound disagreable, but op-amps are very good at this job. If yours isn't working, your circuit has something wrong. Please post a drawing of your circuit.

 

[integralx2]

There ya go. Let me know what I should fix, or maybe it is not needed

 

[audioguru]

Your opamp U4 has no DC input bias voltage. It is floating.
Your opamp with no number at the lower left has positive feedback instead of negative feedback so it is simply a latch.
None of your opamps have a power supply voltage or voltages shown.

 

[integralx2]

there all powered by +9 & -9V. What do you mean U4 is floating ? The last op amp is suppose to invert the signal. Crap i drew the last one wrong, ill fix that though.

 

[Externet]

Try feeding your signal to any audio amplifier first to confirm it is there. Your home stereo auxiliar input should suffice to amplify it considerably.

 

[audioguru]

Your opamp U4 has no input bias voltage. It should be connected to 0V somehow.

 

[integralx2]

what do you mean input bias voltage ? I dont get this. input offset voltage ? is that same thing ? I am just using that amplifier as a buffer.

 

[audioguru]

Input pin 3 of opamp U4 connects to capacitors but does not have anything giving it 0VDC. Therefore the input is floating and will not work.

 

[integralx2]

How can i fix this ? I still get the same results, without it.

 

[audioguru]

Try connecting a 330k resistor from pin 3 of opamp U4 to 0V.

 

[crutschow]

To elaborate on what Uncle $crooge said, is that with the input of U4 coupled by capacitors, there is no DC path to ground, thus the DC input level will float. The op amp input bias current will generally cause it to float to either the positive or negative rail. Thus you need to add the resistor to ground for a path for the bias current and to establish the input level at 0V with no signal. All op amp inputs must have a DC path for proper operation.

 

[Hero999]

What are you trying to do?

That circuit isn't very clear, can you zoom in a bit?

 

[Thunderchild]

I have to ask: if your circuit did not work how did you know about the signal in the first place, have you been measuring it with a scope or something ?