deciding on single rail or dual rail op amps?

Thread Starter

rick5

Joined Jul 17, 2008
12
I'm designing an audio circuit and have to choose whether to go for a single rail or balanced supply IC.

For single rail op amps are there any available that internally create a negative rail with a kind of built in switching regulator, so even though the designer has only a 9V battery to play with you can still get an output that swings above and below the earth rail?

If not I guess for single rail op amps in audio circuits, you have to level shift the resulting output back to a zero offset - or you have a kind of level shift distortion.
 
In my experience, I have not come across a single rail opamp with a built in switching regulator. It might be out there.

Level shifting can be as simple as adding a capacitor in series with the output load. Be sure to select parts that give you the output frequency response that you desire.

Remember, you do not have a negative supply so you can not add an opamp or resistor network to shift the levels down to zero.
 

Thread Starter

rick5

Joined Jul 17, 2008
12
Come to think of it that's actually not a bad idea for an IC. An op amp with a built in inverting switching regulator, where the user only has to supply external capacitors for a charge pump.

I take your point about level shifting, that's what I was thinking, I don't want to unwittingly create a first order filter that messes up my frequency response. I'm not sure how else to do it though, unless I go the balanced supply route and use something like the MC34063A to get the -ve rail.
 
Look at the LM386, which is a low voltage audio amplifier that has been made by National Semiconductor for years. They used to be available even through Radio Shack. The data sheet, http://www.national.com/ds/LM/LM386.pdf, shows the output being driven through a 250 uF capacitor to an 8 ohm speaker. It is worth a look. It will run off of a 9 volt battery as well.

It might not be exactly what you are looking for but it may help you make your final decision.

Creating a dual supply is definitely another solution to the design; however, it involves a large number of parts and power consumption that you may be able to do without, especially for battery operation.
 

peranders

Joined May 21, 2007
88
Rick, if you want a simple solution you should choose two batteries but it's quite simple to use a single supply also. What is your application?
 

Thread Starter

rick5

Joined Jul 17, 2008
12
Well, actually I was reading that just before you made this reply!

You're right, a simply virtual power supply ground is a good idea and probably what the last poster was hinting at also instead of an inverting switching regulator. He also suggested using 2 batteries, that's actually quite a good idea . All my breadboarding has been with 2 batteries just for convenient split rail setup, but for some reason I didn't think of that for the production version due to brain fade. I've got the solution right infront of me already.

I like your split rail ideas though Bill, that thing with the complementary BJTs some diodes & resistors, that looks pretty good.
 
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