op-amp supply_AC vs DC general performance understanding

[alexgluc]

Hi, sorry in advance for the basic question but I've just started putting in practice my electronic hobby dreams. I've problem in understanding how to give power to the op-amps of the circuit I'm supposed to realize. I've no specifications about the op-amp to be into my circuit but my input signal (at V+ or V-) is an AC one with 5Hz or 10Hz frequency. I wonder if, in order to use the whole input signal (without loosing energy from it for what possible), I can use both single supply op-amps (+Vcc) or I necessarily have to use double supply ones (+Vcc/-Vcc). I've heard that in the first case there is no way to power my AC input oscillations, therefore for half of the input signal (negative part) my op-amps will not do their job.
Can someone make this point clear to me, please?
Thank you in advance.

 

[chuckey]

With an AC signal you can use a single supply op-amp, providing you use a capacitor to isolate the DC bias on the op-amps input from what ever is supplying the AC. With a double supply op-amp, the inputs work at 0V - no DC bias.
Frank

 

[alexgluc]

Many thanks Frank for your reply. Is somehow the amplitude dc bias related to the amplitude of my +Vcc voltage supply? And sorry about it but, since there is lack in my knolwedge of electronics, could you better explain me about dc bias? Why is it undesirable? Is there no energy associated to it or what? What if I connect a capacitor to my input as you have just suggested to me?

 

[barry]

What you have to do is bias the input at 1/2 VCC. This can be done by using a voltage divider. You then apply your signal to the input through a capacitor. Your input range is constrained to be + or - 1/2VCC (assuming your op-amp output can go all the way to VCC and ground). This assumes you're using unity gain, it gets a little more complicated if you have gain.

Barry