Question on Op Amps!

[YesJumper]

So I am reading and researching about operational amplifiers, and I saw a video from Great Scott.
He explained it very well, but there is one thing I do not understand.

Op Amps are used to amplify voltage right?
Scott connected +1V on input, and power supply pin on +12V, and on the output got +6V.
My question is, if you have 12V that you can put on power supply, why even use the amp, if you already have the amplified voltage.
I would think that I would use op amp if i only had 2 volts and wanted more, so I would input +2V, and get more out of it.

 

[Ian Rogers]

They are mainly to amplify the current.... Take a sensor input... A loadcell outputs 0 ~ 20mV... A micro ADC works with 0~5v

Well!! The adc will never read 20mV so we need to amplify, with some linearity I may add, 0~20mV to 0~5v We cannot just give the ADC 5v or the micro cannot determine the output of the cell... An instrument op-amp is used to amplify that signal to an appropriate level.. 250 times to be exact..

 

[YesJumper]

They are mainly to amplify the current.... Take a sensor input... A loadcell outputs 0 ~ 20mV... A micro ADC works with 0~5v

Well!! The adc will never read 20mV so we need to amplify, with some linearity I may add, 0~20mV to 0~5v We cannot just give the ADC 5v or the micro cannot determine the output of the cell... An instrument op-amp is used to amplify that signal to an appropriate level.. 250 times to be exact..

So what would I use to amplify voltage then?

 

[Ian Rogers]

Simple voltage can be amplified with voltage doublers / triplers or transformers… BUT!! Remember that power never changes.

say you want 50v and you only have 12v... if you need 1 A at 50v you will need 4.5 Amp at 12v...
Tell me what you want to achieve..

 

[YesJumper]

There is nothing specific I want to do, in school we've been just studying about Op Amps, and was just looking more into them, and as far as I understood they are used to amplify voltage, so I wanted to use how to use them and where I can use them.

But initially I was thinking about using Op Amp to amplify my 4.5 battery to 5...

Also can you tell me, where in theory would I use an Op Amp?

 

[Nigel Goodwin]

There is nothing specific I want to do, in school we've been just studying about Op Amps, and was just looking more into them, and as far as I understood they are used to amplify voltage, so I wanted to use how to use them and where I can use them.

But initially I was thinking about using Op Amp to amplify my 4.5 battery to 5...

Also can you tell me, where in theory would I use an Op Amp?

Your 'mistake' is in what you understand by the term 'amplify' - in reality it does no such thing.

What it does is make a larger copy of a smaller signal - think of it as an electronic Pantograph.

For example if you wanted to double the size of the letter 'A' using a Panograph, you need a piece of material twice the size of the original, and the Pantograph traces the double sized image on to the new material. https://en.wikipedia.org/wiki/Pantograph

So you haven't 'amplified' it, you started off with a large piece of material (which for an opamp would be the supply voltage to the IC), and made a larger copy.

So you can't 'amplify' your battery from 4.5V to 5V (or whatever) you need a higher voltage to start with - and opamps are about signals, not batteries.

 

[YesJumper]

Oh I understand now, I was looking at Op Amps wrong... Thank you for this explanation.
Let me just ask you now:
1. What would be an example of using an Op Amp in theory
2. How can I make 4.5V battery a 5V battery?

 

[Nigel Goodwin]

Oh I understand now, I was looking at Op Amps wrong... Thank you for this explanation.
Let me just ask you now:
1. What would be an example of using an Op Amp in theory

Any lind of preamplifier, such as a microphone preamp.

2. How can I make 4.5V battery a 5V battery?

By using a boost converter, which is a type of switch-mode power supply - bear in mind that increasing the voltage decrease the available current.

If you want 5V, start with 6V (or 9V) and use a regulator to drop it - far simpler. Again, power issues are dependent on exactly what it's been used for, there's no 'universal' solution that magically works for everything.

 

[YesJumper]

Thank you very much for your help!

Now I have a better understanding of this.

 

[KeepItSimpleStupid]

Op amps are not ideal. They basically implement (A-B)*g where g is the open-loop gain, a very big number.
They have restrictions. The output Z is not zero and the input Z is not infinate.

They have restrictions on the amount of current they can provide or how close to the power supply they can get to for an output.

They have input offsets when get translated to output offsets.

They have bandwidth restrictions.

A single supply OP amp operating on 5V will not be able to output 0V or 5V, but may be able to get close.
Watch for asymetry as in the LM324. Older OP amps that operated on +-15V were only able to output +-10V linearly.

An offset of 0.001 V and a resistance of 0.001 ohms, we know that I = V/R or 1 amp will try flow. So, eveen with the numbers being small bad things happen. The OP amp may not be capable of 1 A, so it won't work.

 

[audioguru]

I use an opamp to amplify the 0.01V from a microphone to 1V. I use a 9V or 12V battery so that when music or speech is louder than the 0.01V into the microphone then the output of the opamp can produce the increased output level.

 

[gophert]

Another cool aspect of op amps is their ability to use power supplies with ripple or noise and amplify a signal and only allowing a small fraction of that power supply noise/ripple into the output signal (called rejection ratio). Rations of 80 to 100dB are common. Power supply Noise/ripple is common if you are stepping up/down battery voltage with buck or boost regulators or using AC mains power and transformer.