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Inverting Op Amp

Tykulcsar666

New Member
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

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

Mike.
 

Pommie

Well-Known Member
Most Helpful Member
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
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

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audioguru

Well-Known Member
Most Helpful Member
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
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.
 

audioguru

Well-Known Member
Most Helpful Member
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
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.
 

ChrisP58

Well-Known Member
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

Well-Known Member
Most Helpful Member
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.
 

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