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What is an op amp? Operational Amplifier tutorial & super spy microphone circuit

What is an op amp? Operational Amplifier tutorial & super spy microphone circuit


In this tutorial I’m going to talk
about operational amplifiers, or “op amps” because they are the easiest type of
amplifier for beginners to work with. I will also show you a microphone amplifier
circuit that you can build to listen to your heartbeat or spy on people that you don’t like. So what is an amplifier? Generally speaking, an amplifier takes
a small voltage on the input and puts out a bigger voltage on the output. The “gain” of the amplifier is the amount
that you’re multiplying the voltage by. For example here is an amplifier that takes
one volt on the input and gives five volts on the output. So we say this
amplifier has gain of five. Now the diagram I’m using here with the triangle
is just a simplified symbol so let’s take a look at a more formal circuit
diagram. A bare operational amplifier will
have five important pins. Over here we have the output pin. Your output voltage would come out from
here. The + and – pins are the inverting and
non-inverting inputs of the operational amplifier. Don’t worry too much about what that
means for now just realize that these are not the power supply pins, they are your signal inputs. Now these pins are the positive and
negative voltage supplies for the operational amplifier. These voltage supplies need to be at least
a volt or two more than the output voltage you are expecting. For example let’s say your output wave is
expected to be +10V to -10V You would probably want to power your
operational amplifier with +12V and -12V. Now the reason you do this is because
unfortunately amplifiers cannot create voltages out of
nowhere. You would need a totally different circuit for that.
If you don’t give your amplifier enough voltage you will get clipping on the output voltage and
that would create many problems. When you are selecting your power supply
voltages, make sure you put some capacitors on the power supply lines to
smooth out the power. Also, if you connect two batteries in
series like this, and call the center terminal ground, you’ll get a really good positive and
negative voltage supply for your operational amplifier. Now this thing alone isn’t going to be
able to amplify anything. You are going to have to add a couple of
other components to configure your amplifier to have the right gain. I’m going to show you one of the simplest
amplifier circuits you can build: the classic non inverting amplifier. With the non inverting amplifier, you set
the gain with resistors. The gain is equal to 1 + (R2/R1) So if R1 is 2 kilohms
and R2 is 10 kilohms
you get a gain of 1 + 5 which would be 6. You usually want these resistors to be
somewhere in the kilohm range so an easy thing beginners can do is make
R1=1 kilohm, then use this equation to calculate the value of R2 to get the
gain that you want. Alright I am getting sick of theory now.
Let’s actually build something. I want to build one of those spy
listening circuits that picks up faint sounds from a microphone, amplifies them,
and lets you hear them on some iPod earphones. By doing a few measurements with my
oscilloscope, I discovered that a microphone puts out a tiny voltage of
about twenty millivolts peak to peak. By doing some more measurements I found
out that I need to give my earphones about two volts peak to peak to be able
to hear things loudly. So I need to take an input waveform
that is twenty millivolts and increase it to two volts meaning I want to design an amplifier
with a gain of one hundred. Okay here’s my final design. The microphone is just one I extracted out
of a dollar store computer microphone and I soldered some wires to it. And this part of the circuit is used to
power the microphone. This part of the circuit is called a high
pass filter, and it removes any DC voltage coming
out of the microphone to make sure that we are only amplifying a pure AC audio
signal. The operational amplifier itself is one of my
favorite microchips of all time: The LM324. And you can buy them at radio shack. The LM324 is actually four amplifiers in one, and you only have to use as many as you
want. To power the amplifier I am going to use the
same nine volt battery circuit that I showed you earlier. If you look at the resistor values I have chosen
here you can see that they will give the amplifier a gain of 101.
Close enough to 100. Finally, I added a variable resistor to
the output to give me some control over the volume. Hey wake up we’re almost done! Alright here is what it looks like built
on a breadboard and it works! If you turn up your subwoofer you will be able
to hear my heartbeat. You can also use the circuit to amplify other sounds. So far all I have talked about is changing
voltages, but it’s important to realize that just because you can have an
amplifier that can put out, for example, ten volts, and has a gain of 100, it doesn’t mean that you can actually
supply much current. For example the LM324 I used here
can only handle a few milliamps. That’s enough to power a pair of earphones
but it would be insufficient to power a proper home theater system.
For that, you will need an amplifier chip that can actually deliver several amperes of current, such as the
LM1875 made by National Semiconductor. Look at the example circuit on the LM1875
datasheet. It looks very similar to the circuit I showed you
doesn’t it? The truth is most operational amplifier chips are really
quite simple to work with and all you really need to do is pick the right operational amplifier, give it the right power supply voltages,
and set the gain using a couple of resistors. All the extra components are
for filtering and that’s going to be the subject of another tutorial. Alright I am done. Here are some more cat noises.

100 comments on “What is an op amp? Operational Amplifier tutorial & super spy microphone circuit

  1. Mr. Afro, if I understand these amp circuits corretly, it would take two chips for stereo output, right? one for each channel? thanks! your vids are a blast

  2. sorry, I think the video itself explains it. if the chip has multiple opamps, you can send both channel through the same chip

  3. I'm having a hard time finding out how to power an op amp with a single 9v, specifically for effects pedals. Any chance you might be able to shine some light?

  4. Hey, very nice tutorial :). I have just 1 question. When you count with .1uF cap. and 100kOhm rezistor, the cut-off frequency of this filter is around 15kHz. But that is high pass filter. so It should pass the frequencies 15kHz and above. Which isnt probably what we want. So did I make sonewhere mistake, or is mistake in video? (hope not :D)

    Thanks anyone for response 😉

    EDIT: I am sorry. I did a calculation mistake, so I am really sorry :). Now its clear. Nice cat, tutorial, and everything. 😉

  5. at 1:40 wouldnt that just be 0v, 9v and 18v? since theres not actually such a thing as positive voltage

  6. Hi, just want to ask where does the negative voltage (-9v) pin of the op-amp connect to? Doesn't it connect to ground?. And also would this work with just a 2AA batteries giving 3 volts instead of 9 volts?

  7. A question here, in the schematic @ 03:15, I think between R1 and ground there should be a capacitor to block GND level from negative feedback network.

  8. Hi. İt is a good idea to use 2 batteries to create positive and negative voltages. I wonder whether it is possible to the same with 2 Lab power supplies?

  9. I have seen your comment on Great Scot channel last night and that brought me to your channel, , still exploring it but so far , you are good and it worth sub. Nice job.

  10. I want to use this op amp to power something that draws 0.26A and I am not sure what output sink current and output current source mean. Can someone explain if this will work. Thanks

  11. This is why I hate watching videos from multiple sources. The one I just watched from Khan Academy explained that the gain of an opamp is typically 10^5-10^6, but you just explained that an amplifier with a gain of 5 would put out 5 volts if you put 1 volt into it. So wouldn't an opamp put out tens or hundreds of thousands of volts for every 1 volt you put in if it had that much gain? I'm confused. Also, the program I've been using, circuitmod doesn't include any power pins for opamps. I only JUST found out they have power pins. So that only further adds to my confusion. Let's hope it gets cleared up after reading more.

  12. Interested in learning about wireless power? Check out this course I teach:
    http://www.udemy.com/wireless-power-to-the-people-wireless-charging-101/?couponCode=YOUTUBE

  13. I used the microphone from the earphone and the volume is too low. Should I increase the gain by changing the resistance value of r1 and r2?

  14. when i connect a small speaker i hear lots of static. i read online that static is because of the microwave background. i think i heard the universe speaking to me.😲😲😲😲😲😲😲😲😲😲😲

  15. Hi, If i increase the amplitude of sound is there any effect on sound freq. ? If i pass the sound through 5mm ID pipe & 10mm ID pipe is there possibility of changing freq. due to change in diameter?

  16. Between you and Great Scott, I've learned more in a few days than I picked up from several semesters worth of college EE courses. Only about 40 years late.

  17. +Afrotechmods Can you please explain why a high pass filter is needed after the microphone? You said it is needed to make sure only AC voltage reaches the op-amp, but that can be done with a capacitor alone. Is there a reason to also block low frequencies?

  18. Since its 4 op amps in one, could you use the same chip build one op amp then run the amped signal into a second op amp, sot of like a pre amp and post amp?

  19. I supplied a single supply on my lm324 because i found in the data sheet that it is single supply only.. how to identify the supply required if it is dual or single using the number written in IC?

  20. i made mic amp but the sound was shitty i dont know y but i made several times but the result was same

  21. So the cat didn't like the taste of the microphone, huh? When I tore down my old Radio Shack direct coupled amp I saved the four C1051 TO-3 audio output transistors that were mounted to a very LARGE heat sink. Now to figure out what to do with them??? I'm assuming that the C1051's are 2SC1051's which are probably obsolete now with the advent of LIC's. I have another 12 volt car amplifier that used to drive a big bass speaker with 1000 watts of power but I managed to blow the audio output transistors. I guess I will be tearing down that amp as soon as I can find it in my "junk" room. I also have a model train controller that went out on me and I tore it down to find out that the voltage regulator is bad. I cannot find a proper replacement voltage regulator since the one in the controller is obsolete and I cannot find a replacement as they are no longer manufactured. It is a BD 23A. Any ideas??? This is a controller that controls DC voltage for my model trains – from 0 to about 13.6 volts DC.

  22. im new to your channel and im just wondering if you did a video about communication electronics? and by the way, the explaination is easily understood and direct to the point 👍

  23. I've been researching and learning a lot about electronics recently. And the words "you can get this easily at RadioShack" are now the bane of my existence.

  24. HELP FAST PLEASE:
    In the books they said that the inputs of an op amp are high impedance and no current go true. Therefore, how come the audio signal will pass true the opamp?

  25. Strange question. Can the LM 1872 help increase the output of my '47 Eico 147A Signal Tracer from 3 watts to 15 watts? Any advice would be greatly appreciated. Thanks! BTW GREAT VIDEO!

  26. Be very careful when testing anything involving microphones and headphones. I got nasty feedback on a microphone a while back and did some minor, fortunately, damage to my eardrum.

  27. Bro thank you very much I watched a lot and I mean too much videos but I understanded from you keep up the good work

  28. So when i want it stereo, its possible to use just one LM324? Do you have some usefull tips to do this?
    Would be very happy to hear from you 🙂 !

  29. Love your videos 😀

    I have a DJ Amp in for repair, and 2 Channels are dead.
    Could it be the Op Amps?

    Or another stage?

    They are

    NJM5532L x 2
    NJM4560L x 4

    Any advice would help please?

    I have no schematics, but have looked up datasheets for them.

  30. I know this video is 10 years old so in sense my question is a bit late.

    I am a math guy that hobbies out of my field in electronics. I have a FCC license kc2wvb and build little cw transmitters for giggles. Normally I use a simple oscillator with a single transistor amplifier that gives me perhaps 100-200 mW of power.
    I have also used canned oscillators and without amplification they put out as I remember about 20 mW.

    I realize the lm324 used in this example will not sink more than 20 mA but I was thinking that I might be able to use the lm1875 or similar to get in the neighborhood of 1 watt out of the canned oscillator. Off the cuff do you think so or am I trying to get to much amperage gain in a single step??

  31. Great video as always! And a great voice too…! Question: I built this op amp delay circuit https://youtu.be/XsBzrgHPrpQ which I found in a book. The delay works fine except when it is initially switched on the whole circuit becomes energized. You have to switch it on and and off a couple of times before it performs as intended. Is this just the bounce surge from the physical switch or is there some other reason the delay gets bypassed on the first run? I tried adding a buffer on the output which minimizes the issue, but doesn't eliminate the initial false start. Any thoughts would be helpful!

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