In this lab, you will build a series of circuits to receive an input signal, amplify and/or selectively modify it, and then amplify it to play over a speaker.  This will give you the chance to combine most of what you've learned about analog circuits so far to make a functional device.  Let's start with a block diagram of the desired circuit.  This is a way of showing what you want different parts of a circuit to do functionally, as well as the path that signals will take. 

Each block represents a group of components that will be used together to achieve a specific task.  This circuit will take an audio signal,

After getting each of the component parts working, you can then start combining them with a better understanding of what they should be doing, which helps in tracking down any problems.

This lab will involve quite a few op-amp circuits, which is why you'll want to use the TL074CN chip, which contains four op-amps in a single chip.

A pin-out diagram for the TL074CN, pulled from page 8 of the TI datasheet.

Note that the power is connected along the middle pins instead of opposite corners

Your report will not be due until after the last day of lab

This part is the most straightforward, as you've already made a handful of amplifier circuits.  Most audio signals are a few hundred mV in amplitude, so you should check how much you'll need to amplify the signal to make the speaker play it audibly. 

Your op-amps should be sufficient for driving the speaker provided you put a resistor inline to limit the current to a few mv. You should build an op-amp follower circuit that you've constructed before and connect the speaker its the output.

Create a diagram of your circuit, either with pen and paper, a drawing program, or an online tool (see https://www.falstad.com/circuit/ or https://www.diagrams.net/) 

Describe, sketch, or include a screenshot of the output you observe.

Compare the audio from your amplifier circuit to the result you get from connecting the speaker directly to the function generator.  Are they reasonably similar?

As further testing, we'll use a more complex waveform to verify that we've got good enough fidelity.

Are they still reasonably similar?

As long as you've got the basics working, you're set for now!  Time to work on the next stage.

Now that you've got the basic set up, it's time to amplify your signal, so that you can get something louder from your speaker. The function generator (and general audio outputs) just is not made for driving devices that need a significant amount of current.

To this end, we'll modify the signal chain to include an amplification stage

It is recommended that you use an op-amp circuit for this part (e.g. either an inverting or non-inverting amplifier), but if you really felt like it you could build a transistor amplifier.

Not sure if the op-amp you have is functional at all?  Try building a follower with it (connecting $V_+$ to a signal and $V_-$ to $V_{out}$).  If the output doesn't match the input, start checking the power connections/voltages.  If the output stays at one of the rail voltages all the time it's possible that either there's a problem with your feedback loop or that the chip is dead.

In a moment we'll start doing something to alter our signal, but for now we'll prepare for that step by building a circuit to combine audio signals.

We can actually do this with a variant of an op-amp inverting amplifier circuit called a summing amplifier, shown below:

A summing amplifier circuit. Note that in the case of a single input, it is reduced to an inverting amplifier.

If there were only one input ($V_{in1}$), then the output of the circuit would be the same ($V_{out} = -1*V_{in1}\frac{R3}{R1})$. If we add another input and resistor, then the voltage across that resistor $R_2$ will still be $V_{in2}$ as the op-amp's golden rule ensures that the $V_-$ input will be held at 0V. This means that we'll end up dropping more voltage across $R_3$, and as a result we find that $V_{out} = -1*(V_{in1}\frac{R3}{R1} + V_{in2}\frac{R3}{R2})$ If all of the resistors are the same, then $V_{out} = -1*(V_{in1} + V_{in2})$. Note that this can be extended indefinitely, we can add more resistors and more inputs to just keep adding voltages.

Using your amplifier as one input and the second function generator output as the other, test your mixer circuit. Does it live up to its name?

We're going back all the way to labs 2 & 3 here, and now the task is to build a filter that will attenuate some part of your audio signal. The goal is to mix this signal back in with the original, so if you build a low-pass filter you'll end up with amplified bass frequencies. Likewise, if you make a high-pass filter, you can selectively amplify higher frequencies (such as vocals in songs). You should probably aim for your filter to operate somewhere around 1 - 4 kHz, our speakers have a crappy response outside of this range and the result might not be audible.

Using a sweep from your function generator, does your filter behave as you'd expect it to?

Listen to the output of the mixer with and without input from the filtered signal (disconnecting/connecting $R_2$ in the summing amp is a good way to achieve this. Can you hear a difference? If not, you should either amplify the filtered signal more (i.e. decrease the resistance on its input to the mixer circuit) or change the filter frequency.

Up until now, we've made do with just an op-amp for our output. It works, but it isn't ideal. So, we'll create an amplifier circuit that will increase the power delivered to our speaker.

Now, we've made a few amplifiers with Op-amps and BJTs so far, and there have always been tradeoffs. The design we'll use here is meant to deliver substantial power without heating up the transistor massively. The first stage is a comparator circuit like we built last week. However, instead of comparing the input to ground, we'll use a 100 kHz ramp here.

The result will be that the output we'll get is a series of square wave pulses, but the width of each pulse will be proportional to the amplitude of the input over a short duration. Unsurprisingly, this technique is known as Pulse Width Modulation. We're essentially converting our analog signal into a digital one, albeit a 1-bit version.

An LM311 comparator-based speaker amplifier. Note that the capacitor is polarized; the longer leg should be towards the higher voltage in the circuit. This sort of circuit is known as a Class D amplifier, if you want to know more.

The other part of the circuit uses a BJT transistor as a switch, either driving it fully on or off. When we use a large capacitor along with our speaker as a filter, we recover a fairly decent (but much louder) analog response. The signal will look horrible at this point still, since part of the filtering here is in the physical movement of the speaker. Since the material that makes up the speaker cone cannot respond with infinite speed, it acts as a form of (physical) low-pass filter. With an experiment reminiscent of the first year harmonics lab, you could figure out the resonant frequency of the speaker itself, which could (possibly) help in fine-tuning the setup.

Now, it may very well turn out that things did not work at first when you connected them.  This is okay, it happens to all of us at some point. If you need to troubleshoot, you should keep some notes on what you find and changes you make.

If you're still having trouble, here are some options you can try when troubleshooting your setup:

• Replace the audio with a pure sine wave.
• Replace the audio with a single square wave pulse.
  • This, along with appropriate triggering settings, might help you analyze what's going on at each part of the final circuit.
• Observe if the output of a stage changes as the next one is connected.
  • If this is happening, you may have a too low of an input impedance or too high of an output impedance.
  • Some impedance problems can be solved by appropriately scaling components.
  • Others can be solved by putting an op-amp follower between stages.
• Test if a stage works in isolation.
• Test if altering components has the expected effect.
  • For instance, if you change the resistors in your 555 timer circuit and its output doesn't change, there's probably a problem.
• Test if a chip functions as expected.
  • You might remove your op-amp from its place in the circuit and build a follower circuit with it on another part of the board.  If that doesn't work, then you know something's likely fishy with the chip.

The basic system, while functional, has room for improvement.  By the end of this lab, you should choose at least one meaningful way of modifying your circuit. Some examples include:

• Adding in volume control or controllable filters using potentiometers
• Adding in multiple filters to let you shape the output more
• Adding an LED indicator to indicate volume (average voltage) level
• Making the system self-contained by creating a triangle(ish) wave yourself with a 555 timer
• Add additional audio inputs, possibly with separate volume controls (Practice your DJ skills!)

As part of your report, you should create a full circuit diagram of your audio processing circuit. This means that instead of the blocks we used to describe the general behavior, you should make a schematic showing all of the parts and their connections used in your final implementation.