Inverting Op Amp

[Tykulcsar666]

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.

 

[Pommie]

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

Mike.

 

[Tykulcsar666]

What does it require??

 

[Pommie]

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

Mike.

 

[Tykulcsar666]

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

 

[Pommie]

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]

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]

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.

 

[Pommie]

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

Mike.

 

[audioguru]

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]

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.

 

[Tykulcsar666]

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

 

[audioguru]

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]

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.

 

[Tykulcsar666]

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

 

[Tykulcsar666]

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

 

[Pommie]

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

You could use an adapter.

Mike.

 

[ChrisP58]

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.

 

[MrAl]

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.

Havent seen an MC Escher in a long time.

 

[Tykulcsar666]

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