# Help powering op amp

Discussion in 'General Electronics Chat' started by philwalker, Aug 15, 2010.

1. ### philwalker Thread Starter Member

Aug 15, 2010
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High there. I was hoping someone may be able to help me out a little.

I need to power an op amp from a +9v battery. Regulating this voltage down to + 5v would be straight forward enough, But I need to be able to supply +/-5V to theop amp.

A little reading rings up something called a charge pump, however ive never heard of, or used one of the devices, would someone be able to please point me in the correct direction.

the op amp will be amplifying an input from 0.2v to about 2 volts for audio perposes. The output will be a small pair of in ear headphones so the power cunsumption will be minimal.

I have a full schematic from my circuit however im sure we all know what an op amp looks like.

Phil.

2. ### Ghar Active Member

Mar 8, 2010
655
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You can do this type of thing using single supply methods, which includes generating a reference potential to use instead of 0V.
Basically, you bias the op-amp at 4.5V. That way 0 is actually -4.5 and 9 is +4.5

You do run into issues of but many applications get by with AC coupling (i.e. through a capacitor) making it a non issue.

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

3. ### marshallf3 Well-Known Member

Jul 26, 2010
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More than half the op amps out thre will run as single supply now, and you're dealing with audio, not instrumentation amplification, so I'd just run it off the +9V and ground the Vdd pin if it employs capacitors on the inut and output or get an op amp designed for single supplies.

What part # is it calling for?

4. ### philwalker Thread Starter Member

Aug 15, 2010
41
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I was thinking about setting up two equal valued resistors across the battery, therefore setting up a potential devider circuit, then I would take Ground from inbetween the two resistors. Was just a thaught.

the circuit comprises of a variable bandpass filter, feeding into the non inverting input of an op amp, the then be outputted to some headphones.

I havent looked at a part number for the op amp yet, any recomendations? What sort of voltages do small miniture microphones output (Eg a couple of millivolts?) And what sort of voltage can the headphones handle before they saturate?

Thanks allot.

by the way I do appriciate the help, Im not just handing my problems over.

5. ### Audioguru AAC Fanatic!

Dec 20, 2007
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Headphones have an impedance of from 8 ohms to 600 ohms per ear.
Many opamps cannot drive an impedance that is lower than 2k ohms.

So we need to know the impedance of your headphones and the opamp part number that you will use.

Now you mention using microphones. If the microphones can "hear" the headphones then you will have very loud acoustical feedback howwwwling.

6. ### marshallf3 Well-Known Member

Jul 26, 2010
2,358
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A typical microphone puts out around 5 mV with a 47K ohm impedance. Some (such as electrets) have an internal amplifier/buffer circuit so the output impedance is lower but the voltage will remain around the standard level it's been for ages. The reduction in output impedance greatly reduces stray noise, especially in longer cable runs.

Headphones do vary quite a bit, instead of an op amp I'd use one of the multitude of ICs designed specifically as headphone drivers. Most will be stereo and also be single ended supply to begin with.

You can Google "headphone amp" and come up with thousands of designs as well but I'd far more trust a sample illustration from the data sheet of one.

For instance:
http://www.national.com/ds/LM/LM4880.pdf

Minimal external comonent requirements, good to 8 ohms, only needs a few volts to operate.

7. ### philwalker Thread Starter Member

Aug 15, 2010
41
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ahh right, I didnt know you could by specific headphone amps. Any you could recommend? Quality need to be good to excelent. Must be able to sweep the full audiable range (20Hz-20000Hz).

Can you controll amplitude (volume) with these ic's? If so, how?

Thanks again.

Phil.

8. ### Audioguru AAC Fanatic!

Dec 20, 2007
9,450
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You simply add a volume control to the input of the amplifier circuit shown on the datasheet.

9. ### marshallf3 Well-Known Member

Jul 26, 2010
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Somewhere around a 100K - 1M audio taper stereo pot. Input cap goes to the upper leg of the pot, wiper leg goes on to the amp circuitry and the remaining pot leg to ground.

I see nothing wrong with the one I posted but feel free to look around, headphones are very sensitive devices that don't need much power to drive and that circuit probably has more than full audio response and very low distortion when not driven to maximum output.

10. ### philwalker Thread Starter Member

Aug 15, 2010
41
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Is this the sort of layout you described? The input/output will be mono.

Thanks, phil.

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11. ### Audioguru AAC Fanatic!

Dec 20, 2007
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No.
Your circuit is missing the very important Ci and Ri which are part of the circuit shown in the datasheet.

The value of the volume control should be a little lower (10k) or the same as Ri.

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12. ### marshallf3 Well-Known Member

Jul 26, 2010
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That's the idea, although the value of the pot isn't as critical as you'd think if you're feeding a relatively high-Z input amp circuit.

13. ### Potato Pudding Well-Known Member

Jun 11, 2010
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A charge pump would have been a reasonable way to power a standard Op Amps Negative Rail but if you want a Headphone Amp you might not need a negative rail.

I suggest you look at Data Sheets and see if you can find one with reasonable application notes.

14. ### DangerousBill Active Member

Jul 21, 2010
30
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I use this voltage splitter circuit as part of one of my products. I use a fast op amp with high current output. The current output has to be greater than the maximum difference in current draw from the positive and negative supplies. My op amp of choice is the LM7171, which satisfies the 'fast' and 'high current' requirements. I've been building this circuit for over ten years.

Dangerous Bill

15. ### DangerousBill Active Member

Jul 21, 2010
30
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I use this voltage splitter circuit as part of one of my products. I use a fast op amp with high current output. The current output has to be greater than the maximum difference in current draw from the positive and negative supplies. My op amp of choice is the LM7171, which satisfies the 'fast' and 'high current' requirements. I've been building this circuit for over ten years.

Dangerous Bill

16. ### Audioguru AAC Fanatic!

Dec 20, 2007
9,450
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The amplifier circuit is inverting so its input impedance is only 20k ohms.

17. ### marshallf3 Well-Known Member

Jul 26, 2010
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In looking at the circuit again I'd probably put the pot directly on the inputs so the caps would remain going directly into the amp thus as not to upset the DC bias characterisitics. In this case the value of the pot once again becomes somewhat non-critical, more determined by the impedance of the source driving it.

Another thing to consider is that if you do go with a split rail design the input and output capacitors will all need to be non-polarized.

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19. ### marshallf3 Well-Known Member

Jul 26, 2010
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With a 100 mA output current limit the LM7171 is still only capable of 80 mW into an 8 ohm load.

Why complicate matters? Just use a headphone amp IC that was designed for this exact function using minimal external components.