I have an op amp that does not tolerate signals below ground

[David lopez]

I have a lm224 operational amplifier that does not tolerate signals below ground. What operational amplifier tolerates signals below ground?

 

[sophiecentaur]

Without seeing your circuit diagram I can suggest that you either have no negative power rail or that it is not connected. An Op Amp is usually operated between + and - supplies and Earth / Ground reference will lie somewhere near the middle (but the two supply volts do not need to be exactly symmetrical about your ground). This power supply arrangement will allow + and - values for inputs and outputs.
You may be victim to the problem of the mathematical model of an Op Amp making assumptions when you try to translate it into the practical world. Look at the Spec Sheet of the device and look at the diagrams of typical circuits.

PS If you want to operate with only one Power Supply then you can create a reference voltage at 'half rail volts' and operate everything relative to that voltage (a local zero Volts). Real Ground volts would correspond to V_ee in your circuit and V_ss to your power supply volts. See this link. This can give you brain ache initially but it's not too hard to cope with in the end. Stick to using two (bipolar) power supplies if you can.

 

[berkeman]

Without seeing your circuit diagram

https://www.physicsforums.com/threa...illator-but-it-wont-work.973306/#post-6192194

 

[berkeman]

What operational amplifier tolerates signals below ground?

As you have been instructed in your other thread(s), use split power supplies for the opamp. No opamp is going to work with signals below ground when you power it with a single positive supply.

There are often alternatives where you can shift all of the signals up so they are between ground and the single positive rail, but it's best for you to learn to use split supplies first, IMO.

 

[DaveE]

Often "below ground" signals are dealt with by adding in enough positive bias to pull the signal up above ground. So, for example, if your input is -1V to -2V you can add in a resistor network that adds in +3V, then the op-amp can do what you want in the new range of +2V to +1V.
Similarly, when people say that you need a negative power supply, they may not be considering that you can bias your signals up to within the input range of the opamp. It was common many years ago to use two power supplies, but now it is more common to use these techniques to reduce costs by eliminating the extra power supply requirement. You can search for "virtual ground" in circuits to learn more, but the concept is much more general than that, you can invent your own too.

 

[DaveE]

One part you can look into is the old LM10. It is a single supply op-amp plus a low voltage reference that was designed for this sort of problem. Since they have been around forever, they should be cheap and easy to find. Of course there are many newer parts that you may like better than the LM324 too.

 

[David lopez]

Sorry. I am a beginner. This all very new to me. Any websites for beginners that will
Teach me how to do this?

 

[Baluncore]

If you read the data sheet for the LM224/LM324 you will find some interesting specified limits.
1. For the op-amp to function the supply voltage must be above 3V and less than 30V.
2. The inputs must never go more than 0.3V below the negative supply, or damage may occur.
3. The inputs work for voltages between the negative supply and 1.5V below the positive supply.
4. The output cannot be expected to rise above 1.5V below the positive supply.
5. The output will probably not fall below 20mV above the negative supply.
6. The output cannot sink more than 10mA, or source more than 20mA.
7. It will be difficult to turn on an LED with such a low voltage and current.

On a 3.6V supply, that does not give you much voltage headroom.
All your signals must be between 20mv and ( 3.6 - 1.5 ) = 2.1V.

If you make a common reference voltage at about 1.1 volts, it will then give you a swing of 1V above or 1V below that common reference voltage.

 

[sophiecentaur]

Sorry. I am a beginner. This all very new to me. Any websites for beginners that will
Teach me how to do this?

Take a step backwards here.
If you are at a really basic level then you really need to do your own searching for information (Q and A is unlikely to work). There are so many sites - some good and some bad - with information and you need to look at a lot of them (perhaps a dozen) and find one the makes sense to you and with a style you like. Do not stick to just one because every one of them could have some nonsense on it which will need checking against others. Some serious effort may be needed for this so don't get too impatient. (Later, you will read what you wrote in this thread and be amazed at just how much you didn't know. )

If you want to progress as fast as possible with this subject (don't we all?) then you need to start with something really simple. The easiest introduction into electronics these days can be logic circuits because the signals are just On or Off (1 or 0 etc) and you can buy integrated circuits with multiple gates in one package and you can connect them in various ways (use the examples on the sites you use because most combinations do nothing useful - monkeys and typewriters). Inputs can be just a set of cheap switches and testing can be done with an LED and resistor, Power supply is just +5V and 0V (for most systems) and all the 'fiddly stuff' is more or less taken care of.
The initial digital approach gives you limited experience but it can be a quick way in. There are many websites with examples of "simple logic circuits" etc. and you can get used to just connecting things up and getting an instant result.
For an Analogue way in, then you should start with simple resistance (passive) circuits and become familiar with how a "potential divider" circuit behaves. But you cannot do without a basic DMM for measuring volts with. Similar searching will get you plenty of information. "Simple Analog Circuits"

 

[sophiecentaur]

You have split this into two threads, which is bad practice and information can be lost.

 

[Tom.G]

Any websites for beginners that will teach me how to do this?

Try this Google search: (over 9 800 000 hits)
https://www.google.com/search?&q=basic+op-amp+circuits
Keep up the curiosity and trying things, that's how we learn. My first attempt at design was when I was about 13 or 14 years old. There was a circuit for a two transistor amplifier that I built. It worked. Then I modified it to a three transistor. It didn't. In fact the transistor I added got too hot to touch. Ouch! That's when I decided I wanted to find out "Why."

Hang in there David, the initial start is the hardest.

Cheers,
Tom

 

[jim hardy]

The 124/224/324 opamps work fine with a dual supply.

Connect a pair of 6 volt or 9 volt batteries in series
and declare their junction "ground".
Connect +6 (or +9) to amp's V+
and -6 (or -9) to V- to amp's V-
and their junction to circuit common.

 

[sophiecentaur]

Connect a pair of 6 volt or 9 volt batteries in series
and declare their junction "ground".

Clearly the best solution for a beginner. Firstly it allows them to get over a significant hurdle about Potential Difference and what it means to the designer and secondly, they can continue with the project, ignoring the offset problem and using a 'regular Ground' connection.

 

[jim hardy]

If you are at a really basic level then you really need to do your own searching for information (Q and A is unlikely to work). There are so many sites - some good and some bad - with information and you need to look at a lot of them (perhaps a dozen) and find one the makes sense to you and with a style you like.

Indeed .

@David lopez

When starting out it is useful to study circuits that have already been designed and figure out why they work.
In my day National's AN31 was a handy reference for "How do i do something" type questions.
The guys at National didn't show power supply connections to their circuits because they assumed anybody reading would be experienced enough to know about single vs dual supplies, common mode voltage, etc.

To my surprise the National engineers wrote an article saying they got a lot of questions from people who built a circuit but did not power it at all because the schematic didn't show any power connections.
Wow !

I notice AN31 is still around but it's now a TI document (TI bought National some years back).
The questions must have persisted because they now show power supply connections in their sample circuits.
Here's a link - have fun !
http://www.ti.com/lit/an/snla140c/snla140c.pdf
Wow i see it was updated just this March ! I was reading it forty plus years ago !
Must be a good teaching tool to have survived all thee years.

a few pages in is 124 series with split supply.

Practice writing transfer function equations for the circuits.
Write KVL for both inputs, set them equal and solve for Vout/Vin.

Soon enough they'll become intuitive.

old jim