Hello everybody,
I have very simple question, I suppose, but it still makes some difficulties to me.
I tried to find it in internet but it was really difficult to answer my questions.
I detect a signal from photodetector using a/d converter DAQ and then aquire it using computer. But the problem is that input resistance of converter is too small and signal, about 1,5V maximum, is significantly reduced. That is why I want to use voltage follower. I would like to use uA741, because I have two and changes in voltage are at frequency of about 10Hz, so problably this op-amp is enough. But I want to supply it just only with single supply - two or three batteries 9V. How can I do it to work properly. I want to link them in series since they are rechargeable and voltages can be not equal. The problem is that if simple voltage divider consist of two resistors is enough or some more sophisticated circuits for virtual ground are needed (for example with another op-amp to get Vcc/2). How to calculate resistances for these resistors and what are dis- and advantages of these approaches.
Thank You in advance for replying.

 

The input common mode range for an old 741 opamp is from +3V above the negative supply voltage to 3V less than the positive supply voltage. A few might work a little outside that range. The output can not go lower than about 1.2V above the negative supply voltage. So your +1.5V and less input voltage is too low for a single positive supply.

Half of an LM358 dual opamp will work because its inputs and its output can work fine down to 0V in your circuit with a single positive supply.

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Uncle $crooge

 

Thank You for reply.
That is right, that I couldn't use it so simple. But that's why I wanted to make a virtual (phantom) ground and then vary my input voltage around that point.
Thank You anyway. I have already idea how to do it and hope it will work. If it didn't, then I would ask You specialists on that forum.
Best wishes.