Powering LM741 Op Amp Without -15V Supply

[Blenton]

I'm trying to build a circuit using a lm741 op amp, and normally the opamp requires +-15 V to run it. However currently I don't have a power supply that has a negative output.

Correct me if I'm wrong but isn't connecting the device from +15 and -15 the same as just connecting a +30 to ground supply?

If not how does one create a negative voltage short of buying a duel power supply? (I have a DC power supply that generates 5,7.5,9,12V)

 

[berkeman]

I'm trying to build a circuit using a lm741 op amp, and normally the opamp requires +-15 V to run it. However currently I don't have a power supply that has a negative output.

Correct me if I'm wrong but isn't connecting the device from +15 and -15 the same as just connecting a +30 to ground supply?

If not how does one create a negative voltage short of buying a duel power supply? (I have a DC power supply that generates 5,7.5,9,12V)

The difference between +/-15V and 30V is the ground in the middle. You need low impedance supply rails with respect to your ground/reference voltage. Most of your opamp circuits with split supplies will use ground as a reference, despite there being no ground connection to the opamp IC itself.

You can try to artificially make a middle reference rail in a 30V system, but forming some sort of voltage divider to make 15V, and then bypassing the heck out of it with caps. Depending on your citcuit, that can work okay.

A better way for you (and a good learning project) would be to make an inverting DC-DC circuit, followed by a negative linear regulator (to eliminate ripple). Check out the inverting Simple Switcher series from National Semiconductor, for example. They are pretty simple circuits to design and build.

 

[vk6kro]

There is a good article on using opamps with a single supply here:

http://courses.cit.cornell.edu/bionb440/datasheets/SingleSupply.pdf

In Figure 2 they use an opamp as a buffer to give a stable virtual ground. Often two resistors and a bypass capacitor are all that is needed.

The article also has an excellent summary of filter circuitry.If your signals are at or near ground level, and cannot be coupled via capacitors, you would still need a real split power supply. This would happen if the signals were DC or slowly varying AC.

If you have access to the inside of your 12 volt power source and feel comfortable with modifying it safely, it might be possible to use a half wave voltage doubler to produce a negative voltage.

 

[Blenton]

@vk6ro

Yes I've been reading the article, and it serves well to power op amps, however I also require negative volts to power a motor in reverse (the op amp to switch motor between forwards and reverse).

Do you think it would be possible/safe to use a powersupply from a PC for this?

 

[berkeman]

PC power supplies can be used for general power supplies, but there are two issues to keep in mind.

First, some PC power supplies have minimum output current requirements. If you draw less than the minimum (from whichever rail is spec'd), that rail can go out of regulation, usually too high. Second, since they are switching power supplies designed for digital logic, they tend to have pretty noisy ripple. Not good for analog applications, although motors don't care about their ripple.