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.