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The Projects Forum Working on an electronics project and would like some suggestions, help or critiques? If you would like to comment or assist others with their projects, this is the place to do it.

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  #1  
Old 12-26-2008, 08:13 PM
magikal magikal is offline
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Default Powering Op Amps

How do I generate the negative voltage reference required to power an op amp? I want to use the non-inverted op amp configuration to multiply. BTW I have the LM3900 quad amp chip and the LM1458 dual amp. Is there a simple circuit to generate the negative voltage source?

Thanks for the help.
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Old 12-26-2008, 09:02 PM
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You will need to make a power supply to do that correctly. Use a center tapped transformer and bridge rectifier to make the positive and negative voltages - http://www.allaboutcircuits.com/vol_3/chpt_3/4.html. A pair of three terminal regulators will complete the power supply.
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Old 12-26-2008, 09:23 PM
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Opamps do not need a negative supply if their inputs are biased at half the supply voltage. Then the input, output and feedback-common might need coupling capacitors.

An LM3900 is obsolete and had very unusual inputs.
The MC1458 is a dual lousy old 741 type. It has poor performance today.
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Old 12-26-2008, 09:28 PM
magikal magikal is offline
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what op amps are good and cheap?
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Old 12-26-2008, 09:37 PM
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I use TL071 single, TL072 dual and TL074 quad low noise, low distortion and fairly wideband opamps. They cost $.53US for one TL071, $.50US for one TL072 and $.48US for one TL074 at Digikey today. They have thousands in stock in many different packages.
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Old 12-27-2008, 04:26 AM
gee_emm gee_emm is offline
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here is a fairly simple way to get negative voltages. Not sure how stable it is, but it does work http://www.csgnetwork.com/ne555c1.html
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Old 12-27-2008, 05:00 AM
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Creating a vertual power supply ground...

http://forum.allaboutcircuits.com/sh...4026#post84026
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Old 12-27-2008, 01:21 PM
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For low currents look at the ICL7660. One IC (8 pin dip) + 2 caps to generate a negative supply (similar idea to the 555 schematic that was posted).

It is low current and unregulated but for a bias supply you may be OK. Some
versions of the chip add regulators. Intersil, Maxim, TI, Linear Technology and Analog
Devices all make various versions of these devices.

(* jcl *)
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Old 12-28-2008, 06:42 PM
magikal magikal is offline
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Thanks for the help, I figure it out. Does anybody know a good analog cheap squaring chip? The input range only has to be -1V to 1V or so.

I've looked at the AD532, but its a bit too expensive for me.
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