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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.

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