Inverting Op Amp

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Tykulcsar666

New Member

• Dec 2, 2019

• #1

This may be pretty simple- but I’m a electronics newbie. Trying to power a UA741 op amp with 3V as VCC+ and -3V as VCC-. But not entirely sure how to get a negative voltage without a voltage generator (just using batteries). Any help would be much appreciated.

 

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Pommie

Well-Known Member

Most Helpful Member

• Dec 2, 2019

• #2

I don't think the ua741 will operate from ±3V.

Mike.

 

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Tykulcsar666

New Member

• Dec 2, 2019

• #3

What does it require??

 

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Pommie

Well-Known Member

Most Helpful Member

• Dec 2, 2019

• #4

It's a very old chip and the newer ones will operate at lower voltages but I think the minimum is ±5V.

Mike.

 

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Tykulcsar666

New Member

• Dec 2, 2019

• #5

Awesome. And any knowledge how to get a negative voltage and positive from battery?

 

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Pommie

Well-Known Member

Most Helpful Member

• Dec 2, 2019

• #6

Just wire them in series and use the middle connection as ground (0V) and each end as the ± supply. Two 9V batteries will give you ±9V.

Mike.

 

audioguru

Well-Known Member

Most Helpful Member

• Dec 3, 2019

• #7

The 741 opamp was introduced 52 years ago. Its datasheet shows only a +15V/-15V power supply. Some of them might work poorly with a total of 10V. Any opamp can use a single positive supply if its + input is biased at about half the supply voltage and coupling capacitors are used. Kiss the old 741 opamp goodbye then bury it!

 

unclejed613

Well-Known Member

Most Helpful Member

• Dec 3, 2019

• #8

https://www.ti.com/lit/ds/symlink/ua741.pdf

minimum supply voltage is +/-5V. i've seen 741's do some squirrely things at such a low voltage because the current sources are barely working at such low voltages.

 

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Pommie

Well-Known Member

Most Helpful Member

• Dec 3, 2019

• #9

Got to agree, the 741 is a very bad choise for any current circuit.

Mike.

 

audioguru

Well-Known Member

Most Helpful Member

• Dec 3, 2019

• #10

I have never seen that new datasheet. All its written spec's and most graphs are with +15V/-15V. A graph of voltage gain vs supply voltage shows how bad it is at a low supply voltage and the graphs are only for a typical one, a minimum one is worse. Modern opamps are much better and many work perfectly with a single 3V supply.

 

ChrisP58

Well-Known Member

Most Helpful Member

• Dec 3, 2019

• #11

What are you actually doing with the op-amp?

Many textbook circuits (which are still often built around the 741) are shown using bipolar supplies to the op-amps. Many modern op-amps work fine with single rail supplies, but you may need to adjust your circuit to do that.

Please post your schematic and we can likely suggest a single supply alternate.

 

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Tykulcsar666

New Member

• Dec 3, 2019

• #12

First two are just voltage buffers, third one is amplifying the signal. Any suggestions on better op amp models would be appreciated.

 

audioguru

Well-Known Member

Most Helpful Member

• Dec 3, 2019

• #13

Your schematic shows "skin" and "Prof" so the circuit is an ECG or lie detector school circuit attempted 50 years ago. It needs a modern intrumentation amplifier IC. Use much higher supply voltages for the old 741 opamp.

 

ChrisP58

Well-Known Member

Most Helpful Member

• Dec 3, 2019

• #14

Thanks for the schematic.

I haven't modeled it, but it looks like all of your signals are positive, so you should not need to change the basic circuit for use with a single supply op-amp.

The op-amp you want would be have rail-to-rail input and output capability, and voltage operation below 3 volts. There are many parts that meet those needs. Here is one that is already in LT-Spice, ADA4505. Tie it's negative supply pin to GND.

PS. It would be useful to include your LT-Spice .asc file in your posts when asking questions. That way other forum members can model your circuit without having to rebuild it.

 

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Tykulcsar666

New Member

• Dec 3, 2019

• #15

Thank you for the input. Attached the .asc file for reference.

 

Attachments

• project schematic.asc

  3.1 KB · Views: 157

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Tykulcsar666

New Member

• Dec 3, 2019

• #16

Is there an equivalent of ADA4505 that can be used on breadboard?

 

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Pommie

Well-Known Member

Most Helpful Member

• Dec 3, 2019

• #17

Tykulcsar666 said:

Is there an equivalent of ADA4505 that can be used on breadboard?

Click to expand...

You could use an adapter.

Mike.

 

ChrisP58

Well-Known Member

Most Helpful Member

• Dec 3, 2019

• #18

I assume that you mean that you want a DIP package.

Look through this list to pick something out. My search criteria was: Rail to rail output, Vcc down to 1.8 Volts.

 

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MrAl

Well-Known Member

Most Helpful Member

• Dec 4, 2019

• #19

ChrisP58 said:

Thanks for the schematic.

I haven't modeled it, but it looks like all of your signals are positive, so you should not need to change the basic circuit for use with a single supply op-amp.

The op-amp you want would be have rail-to-rail input and output capability, and voltage operation below 3 volts. There are many parts that meet those needs. Here is one that is already in LT-Spice, ADA4505. Tie it's negative supply pin to GND.

PS. It would be useful to include your LT-Spice .asc file in your posts when asking questions. That way other forum members can model your circuit without having to rebuild it.

Click to expand...

Havent seen an MC Escher in a long time.

 

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Tykulcsar666

New Member

• Dec 4, 2019

• #20

I've tried replacing 741's with ADA4505 and tying negative supply to ground. Output voltage seems to be 0.

 

Attachments

• project schematic 2.asc

  2.8 KB · Views: 158