Op amp. Do i need to have op amp connected to -Vcc ?

[heryrg]

Op amp

I've been out of school for some time now and forgot some of the basics. I'm using an op amp from a 2.7v battery powered application. I have a signal with low voltage ~850mV which i want to amplify 3 fold. I am using the older dual op amp ic.

i feed the input to the + terminal of op amp and from neg to output terminal i have a feedback resistor (Rf) and a resistor(R1) to ground from neg terminal. Basically a noniverting amp setup. When i was testing the op amp i could not get any gain even with higher ratio of Rf and R1.

I had the amp conected to Vcc (5volt used for testing) and to ground. Do i need to have op amp connected to -Vcc for proper operation? If yes, please answer this newb question, how do i get negative voltage from my battery?

On a sidenote, what is the easiest way to get higher voltage from low V. I was trying to use op amps since i have them.

 

[betwixt]

It sounds like your output pin is stuck either at ground or supply line because it isn't biased properly. It's difficult to tell without more information.

The first thing to try is adding a capacitor in series with the resistor going to ground. I suspect the amplifier is unable to reach mid output because both your inputs are ground referenced. The capacitor (try 10uF) will allow the '-' input to rise higher above ground potential.

If that doesn't work, also try holding the '+' input at half supply voltage by connecting 10K to ground and 10K to supply to it.

Different amplifiers work in different ways, without knowing the type it's difficult to advise.

Brian.

 

[Audioguru]

Why is there no schematic of the circuit?
Why is the 'older' opamp without a part number?

 

[stan4]

If you're using the LM324 you shouldn't have problem using it with a "single supply", I use it to receive analog input in my projects and I always use it in single supply mode.

you just have to keep your sensor signal in the positive side, remember, with negative input the opamp(output) will go to GND.

 

[heryrg]

Sorry guys for being newb-like. I was at home and didn't have the info.
The op amp is ne5532. Here is the schematic.

 

[khansaab21]

As far as schematic is concerned, there seems nothing wrong with circuit. A non inverting configuration with the op amp is made in the same way. Some op amps need higher Vcc and even some require both positive and negative supply voltages to operate properly.

Do yourself a favor friend. Get a new LM741 or OP-07. They both work fine with single supply voltages down to 3.3 V. And they are the most popular op amps around. You can get them at a very cheap rate.

But if you cant get the new op amps then try using the second op amp of the dual op amp ic chip you are using. Manier these chips get fried as they are very cheap.

 

[betwixt]

I wouldn't change a nice low noise, NE5532 for an LM741 - even if it would work at 3.3V which an LM741 will not.

Consider that the amplifier output is the difference of input voltages multiplied by it's gain. Without knowing the DC voltage at the '+' input it is impossible to say whether it would work or not.

The configuration you are using will generally work if you have both positive and negative supplies to the IC with the ground connection being the common point (0V) between them. As a single supply circuit it will struggle to work properly as it is. To further complicate it, the amplifier you are using has a pair of head-to-tail diodes between it's inputs which will make biasing it as you are even less likely to work.

It might help to think of an inverting amplifier like this:
"The amplifier will always try to maintain zero volts between its inverting and non-inverting inputs. It will do this by shifting it's output voltage sufficiently that the inverting input matches the non-inverting input despite the reduction factor of the feedback resistors."

I still say: isolate the DC path in the lower end of R1 by adding a capacitor, this will allow the DC voltage on the inverting input rise to that of the output pin. At this stage, the output pin DC voltage will follow the DC on the '+' input pin so you next want to make that about half the supply voltage so its signal can swing each way equally. Do this by adding equal value resistors from the '+' input to ground and supply, in other words make it's DC level half the supply.

Brian.

 

[heryrg]

Thanks guys.
First i'm trying to increase my voltage in my battery powered circuit(2.4v) to about 6v. Second, i have low voltage signal in the design which i compare and output accordingly, 800mv to 900mv. I want to make that 100mv difference bigger so i'll see the change easier, maybe to 2.4v and 2.7v. I thought i could use the op amp but the dam thing is not working.

Brian, is attached circuit what you meant? It acts odd, when i input 0.8v i get 1.68v on the output when input 0.9v the output is 1.64.

 

[LvW]

On pin 3 you have produced 50% of supply voltage. OK, looks not bad.
What do you think will happen, when you connect a ground referenced input voltage without a coupling cap?

 

[Audioguru]

Why don't you look on the datasheet??

Philips' datasheet for the NE5532 says its minimum supply is 6V. So throw away the 2.4V and 5V supplies.

Its inputs go from 3V above the negative supply to 3V less than the positive supply. That is why the minimum supply is 6V.

If the load is 2k ohms then its output goes from 3V above the negative supply to 3V less than the positive supply which is another reason its minimum supply is 6V.

 

[heryrg]

Can i buy a vowel??
I don't know, the input signal will have some noise ?!?!

Added after 8 minutes:

Thanks guru.
So you say if i use Vcc 6.5v with the setup of the first attached circuit, it will work??

I've tried that and it still doesn't work.

Added after 7 minutes:

https://pdf1.alldatasheet.net/datasheet-pdf/view/27248/TI/NE5532P.html
Thats the datasheet i found. It says the min voltage is 5v. Still it doesn't work with 5v supply. Do i need to supply +5v and -5v into op amp for it to work properly.

I need all this to fix a simple problem, i need up the voltage in my battery powered circuit from 2.4v to 6v. Any and all suggestions are welcome.

 

[Audioguru]

If you use a +6.5V supply then the max output voltage of the NE5532 might be as low as +3.5V.
Its minimum output voltage might be as high as +3V.
Its minimum input voltage might be as high as +3V and its max input voltage might be as low as +3.5V.
Its max output current might be as low as 10mA when shorted. An opamp might not power anything.

I quoted worst case spec's. Some NE5532 opamps might be better but not much better.

 

[betwixt]

Your schematic is now correct but I'm a little concerned about the signal you are feeding into it and as mentioned earlier, the DC effect it may be having. The two 5K resistors will set the '+' input at half potential, which is what you want but if the signal you are connecting into it also has a DC component, it will upset the level. Try feeding the input signal through another capacitor so it can't influence the DC conditions.

Note that as I pointed out before, the NE5532 has a pair of head-to-tail diodes across it's input pins so the maximum voltage you can feed into it maybe limited. It is really a device intended for amplifying very low level signals with low noise rather than a large signal. Don't forget that with restricted output voltage swing, depending on the gain you have set, even a small input might cause the output to clip badly.

You are on the right track though!

Brian.