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Power supply

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patroclus

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I own a variable power supply, 0-30V, with fixed outputs 5 and 12 V.

Can I use this power supply for op amp circuits?
I studied op amp for a while, and now I have to start practicing.
I usually see a dual voltage polarization (+12 -12V, for example). If only positive voltage is used you just cannot use inverting circuits (such as simple inverting amplifier), as it would saturate in 0V. Am I wrong?

Could I just use 0V for -V, 24V for V+ and the fixed 12V output as GND to work this out?

Thank you!
 
If you bias the input of an opamp at half the supply voltage then it can be inverting or non-inverting and use a single supply voltage like this:
 

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Yes provided you do not exceed the maximum rated voltage of the op amp.

If the negative terminals of your supply are not connected together internally (measure with a DVM), you can make a +/- 5V supply by setting the variable supply to 5V (for the +5V) and connecting the positive of the 5V fixed to the neg of the varaible supply so that the neg of the 5V fixed becomes -5V.
Ground then becomes the fixed +5V pos/variable neg connection.

However you can still use a single supply for op amps which accept them (eg LM324) by biasing it so the output sits at a dc level of about half the supply voltage when there is no input signal present.

However, getting the bias right can be tricky. A really useful equation for op amps under any situation is

Vo = V1(1 + R2/R1) - V2R2/R1

V1 = Voltage at + pin
V2 = Voltage at neg pin input resistor
R1 = neg pin input resistor
R2 = feedback resistor to neg pin
 
Are all the outputs bonded together at 0V?

If not you could connect the fixed 12V in series with the variable 0-30V output tuned to 12V and use it as a +/-12V supply.
 
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