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Op amp power supplies

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blabley

New Member
I am working on a project that involves using various op amps and multiplier chips. The different opamps require different rail supplies (ex. one opamp requires +/- 10V, another requires +/- 5V, and another requires -2/+4V). What is the best way set this up. Can I use a +/- 15V supply and just use voltage dividers to supply the others or is there a better way? Thanks for any info.
 

crust

Member
I dont understand why you need these odd rails, perhaps you can enlighten us as to the purpose/reasoning. But you are not liikely to get very precise rails with voltage divider, particularly since the voltage will change as the load changes. You need to use regulators or switchers to generate all of your other voltages.
 

Russlk

New Member
You should be able to find one op amp that fills the requirements. The one using +- 10 V should handle the signals that the other two do. However, if you need an odd voltage that is lower than the supply, a unity gain op amp and voltage divider will do it.
 

blabley

New Member
Actually, i'm using the opamps as precise limiters in order to put together a piecewise transfer function. Also, i'm using other devices like V-F converters, multipliers, and function generators. Most of these devices have different power requirements. I think the best way to go would be to build a +/-15V supply and then parallel different regulators as needed.
 

ukeee

New Member
You are best of using linear voltage regulators, these only cost £0.50 each and provvide very stable volatge reagardless of the current drawn. The problem withh using a potential divider to get different output voltages is that it won't be very stable. By this I mean that if your load current changes so will your supple voltage. Remember that whatever you are running from the voltage supply will have an equivalent resistance that will appear in parallel to the lower resistor in th volatege divider, if this cahenges so will the output voltage. Voltage regulators are so cheap that for most circuits using a potential divider to get different voltages is a very bad practice. An alternative to a voltage divider is using a zener diode, this provides a stablle(ish) voltage. But I wouldn't reccomend using it for a power supply, stick with the linear regulators they're cheap and easy to use.
 

Optikon

New Member
blabley said:
I am working on a project that involves using various op amps and multiplier chips. The different opamps require different rail supplies (ex. one opamp requires +/- 10V, another requires +/- 5V, and another requires -2/+4V). What is the best way set this up. Can I use a +/- 15V supply and just use voltage dividers to supply the others or is there a better way? Thanks for any info.
What OP-amp _requires_ +/- 10V? I don't believe it.

Why is it that using multipliers & precision limiters need -2, +4V supplies?
Just use +/-5V for those as well. As long as the output of the op amp doesnt have to come close(or exceed) the max power supply voltage of the part, you will probably be ok.

I really think that you can simplify your power supply requirements.
I've never seen any multiplier or limiter circuits requiring so many odd supply rails.

If you must use such an arrangements, voltage regulators are ok (resistive dividers are out of the question) and zeners are good.

Voltage regulators will not give you as good of a transient response (change in load current through the amp supply pins) as a zener will but you should provide adequate bypassing for either of these methods or else your transient response will be poor. Also be sure to pick an op amp with good PSRR.
 

barry995

New Member
How do I power my opamps with only a 24V dc supply

Hi I have built plenty of circuits in school but I always had a +/- 15V supply. Now that I want to build circuits at home I do not have a supply that goes negative (I do have a 24V DC supply) Can I use it to power an OPAMP that wants +/-15V? by giving it +24 Vcc and 0V Vee or must I come up with a -12V supply. How can I get the -12V from my current supply?

any advise will be appreciated
 

audioguru

Well-Known Member
Most Helpful Member
Opamps don't need a positive and a negative supply. They just need to have their input voltages set to within the power supply voltages. A single power supply voltage can be used if the input, output and feedback divider resistor are capacitive-coupled like this:
 

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Analog

New Member
Working closely with ADI, it turns out that op amp specs on the data sheet only apply at the working voltages listed. For example, the op27's noise and gain figures are guaranteed only at +/-15VDC. Go figure.
 

audioguru

Well-Known Member
Most Helpful Member
A 15V positive and a negative supply for opamps is a standard since vacuum tubes were buried.

The noise from an opamp doesn't change with the supply voltage. They don't spec it because they are too lazy to measure noise at more than a single supply voltage.

They have a graph that shows how little the gain changes when the supply voltage is changed. An open-loop gain of 880,000 or 860,000. The next opamp has a gain of 1,200,000. Does it matter?
 

Hero999

Banned
If you really need a cheap +-15V supply, buy an AC mains adaptor and add a two diode voltage multiplier and LM7815/7915 to the output.
 
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