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Why changes the behaviour of a circuit when I touch it?

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Mike98

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Hi to everyone, I have a question that suppose it must have a simple answer:

I have an audio circuit with operational amplifiers, capacitors, codecs and other components. The circuit doesn´t work until I touch some pins of the capacitors with an oscilloscope probe, when I touch them the circuit starts to work properly and sometimes it continues working without touching it, sometimes leaves working.

Do you know why? Ground problem, etc...?

Thank you very much!!!
 

Hi,
Most likely an oscillating amplifier, try putting a small capacititance across its feed back. Use tantalum electrlytic capacitor to bypass the power supply pins. Separate the ground return of the opamp and low input returns from power stages, connect them together and then connect to power supply negative point directly. Connect the return points of the power stage separately to the negative point of the supply.

Regards,
Laktronics
 

Thank you very much Laktronics, I will try it.

Otherwise, I´m seeing that my amplifier is not working properly. I´m using a Max4470 operational to amplify a signal that comes from a microphone. This signal is around 20 mV and when I try to amplify it in the non-inverting mode, the output has a continue signal that increases while I decrease the resistance that goes to ground, until reachs the saturation level in a way that it doesn´t leave to amplify the signal itself.

https://es.wikipedia.org/wiki/Imagen:Opampnoninverting.png

Do you know what I´m doing wrong? I have a capacitor for the input signal so I don´t think I´m introducing a continuous signal.

Thanks a lot, again.
 

Hi,
What type of microphone and how it is connected? You need to connect a resistance from +ve input to ground to provide a DC path for the bias current to flow. What are the values of Rf and R1?

Regards,
Laktronics
 

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