Amplifiers

Ever wonder how an amplifier works?
If you were to attach a mic directly to a speaker, you wouldn't want any voltage drop along the way.  My little stickman's mic can be thought of as a Thevinin Circuit, with corresponding voltage and a resistance, and the speaker can be thought of the same way, though without a V since it is a passive device.

If you attach the two devices together, it will behave like a voltage divider.  As you know from previous blogs, if you want Vout to be close to Vin, the Zsource has to be MUCH smaller than Zload.  In reality, a speaker's resistance will be extremely low, like 8Ohms, since all it is is a coil of wire next to a magnet, so it will be next to impossible to output a useful sound.  Plus, if you go to a rock concert, you want it to be Loud!!  So how do we make our signal bigger?  With an Amp!
A neat tangent, if you have a long wire running through your house, the wire will pick up an additional signal, that oscillates at 60Hz, from the electric cables in the walls!  This can be a problem, interfering with the signals you want, so the differential amplifier was created:

If you take 2 wires, following the same path so the extra signal picked up is the same, put your desired signal on one of them (aka attach it to you mic), and leave the other without a signal (aka attach to ground), a op amp (a type of differential amplifier) will subtract one from the other then increase the voltage of the signal.
Unfortunately, it will not increase your signal for free, you need to give the op amp a power source and the amp output cannot surpass that source voltage (in fact it will max out a few volts below the power source 12V --> 10.8V)

Here are two "Golden Rules" about Op Amps:
1. With negative feedback in place (attach output to negative input) the output will try to make V- = V+ 
2. Op Amps have HUGE input impedance. Therefore they draw no current.

With these rules, you can figure out how amps will behave in almost any circuit!

Here's a picture of a LM 411 Op Amp, I have labeled the pin numbers and functions:

We did tons of labs on amps, read to see what you can do with them!

Lab 8-1 Open-Loop Test Circuit:

This lab is basically just to get your op amp wired correctly...
I couldn't find a "potentiometer” to play with the voltage, so I just used a triangle wave :) same difference. It changes from +11V to -11V virtually instantaneously:








Lab 8-2: Inverting Amplifier

We used this circuit:

to make an Inverting Amplifier, and drove it with a sine wave.  I calculated a gain of 7.6, and a maximum swing of 9.84V.  I also tried to measure the impedance of the circuit, for an input impedance I got ~1K, and output impedance too small to figure out, I actually tried to use a 1ohm resistor, then read further into the lab manual and realized they actually warn you not to do that, whoops!

Here's the signal I got: 

 (blue is IN yellow OUT)

As you can  see, there is 

a phase switch between in and out

 That's why it's an "inverting" amplifier!

V out = - R2 / R1  * V in

Lab 8-3: Non-inverting Amplifier
You can probably guess what this one will look like.  Here's the wiring:

It looks pretty similar, but notice the voltage is applied to the positive instead of the negative pin. I found a gain of 11 this time, but the resistance was a little wonky.  The Rin was huge, my 1MegaOhm resistor didn't even put a dent in the output! (no surprise, because of golden rule #2)  so I measured Cin.  I did this by finding the f3dB frequency, 20kHz.  RC was 8x10^-6.  As you can see in the graph, the IN and OUT are in phase with one another

Vout  = (1 + R2 / R1)  *  V in 

Lab 8-4: The Op Amp Follower

This one is a very simple idea, If you remember golden rule #1, the amp will try and make the - input the same as the +, so if you think about it, Vout will be exactly the same as Vin.  I also attempted to measure Zin, by adding a 1M resistor, which approximately cut the output signal by half, so Zin ~1M.


Lab 8-5:  Op Amp Current Source

In this experiment, we used a cute little blue box that was a variable resistor, if you twist the dial in the back, you can change the resistance across the box. The op amp (black box) is trying to keep the Voltage at pin 2 equal to the voltage at pin 3 (which is at .75V currently), but if we up the resistance of the blue box, then the op amp can't out put enough power to keep the pin at 0.75 V.  You can see it waiver with an oscilloscope or an ammeter. The current should be ~4mA, but after the output voltage peaks if you still increase R, you can see I drop.

Note:  The pretty looking diagrams are from "The Art of Electronics Student Manual" by Thomas Hayes