Volume Control Lab: Amplifiers

Transcription

1 Volume Control Lab: Amplifiers Introduction In the world of electrical engineering, there are many different specialties and areas of application. Some of the areas are analog circuits, signals, and electromagnetic. In this lab we will explore the world of signals as well as circuits. This is the first lab in a three lab sequence that goes through steps for building various audio filters. This lab will give you hands on experience when it comes to understanding how circuits and signals interact with one another. We will learn how audio signals work and how audio filters are built. There will be an explanation on the theory, the circuit and questions which you will need to answer by experimenting with the circuit as well as doing some calculations. Let s get started. Lab Write-up For these filter experiments, we will use a device called an Operational-Amplifier also called an Op-amp. We will begin by showing how an op-amp works. Its main function is to amplify any signal that is inputted into it. As you can see from figure 1, there are four inputs and one output on the op-amp: Inverting input non-inverting input Positive voltage source(+vcc) Negative voltage source(-vcc) We use the voltage sources (+Vcc and -Vcc) to power on the op-amp. Items needed for this lab: It s always good practice to wear your safety goggles. Breadboard 1 - TL074CN 8-pin DIP Resistors, capacitors, and various wires Two - 9-Volt batteries connected to snap connectors A pair of powered speakers (with power adapter) Digital Multimeter +Vcc Basic op-amp connections (-) inverting input(x) (+) non inverting input(x) Output (y) -Vcc Figure 1

2 For the Op-amp, when the signal is connected to the (-) input we call it an inverting connection. The output will then have an inverted output. Example: if 5 Volts is connected to the inverting input, the output will have a negative voltage outputted such as -5Volts (if gain = 1 or unity). Figure 1 shows how op-amp connections are set. In order to power up the op-amp, we need two voltage source (+Vcc, -Vcc). In this lab, we will use two +9 Volt batteries in order to power the op-amp (pin 4 and pin 11 respectively). Connect the wire attachment to the battery as shown in figure 3.. Figure 2 Figure 3

3 Figure 4 Figure 3 and figure 4 show the connections for an op-amp in schematic view and in real life. You can find figure 3 and more information about the specification of the TL074CN op-amp circuit at the following address: Figure 5 We will use the circuit in figure 5 to create a simple op-amp circuit that takes an input and multiplies it to give a more powerful output. It will have the exact same copy of the original signal except amplified. We then use the variable resistor (RR ff ) to control the output much like the volume control on your audio receiver or stereo. In fact the volume dials on audio receivers are actually potentiometers (variable resistors). When we turn the dial, we change the resistance of the circuit, driving more current in through the op-amp and amplifying the signal.

4 Note on the speakers: For this lab, we are using powered speakers so you won t have to worry about receiver design involving impedance matching. Most speaker systems draw their power through the signal that comes in, however this makes the circuit more complicated. Powered speakers such as the ones in this lab allow you to concentrate more on the theory of signals without the complication of impedance matching. Don t worry if this section confused you, it s more of an explanation as to why your home receiver audio system doesn t use another power cable to power the speakers. How to build the circuit: 1. Use the plug shown in figure 6 to connect the audio source to the circuit board. The picture shows the connections for the plug. The number of audio inputs depends on the type of plug you have. A stereo plug will have two audio input source and one ground plug. In our case we are using a mono plug which only has one audio source cable shown in figure 6 and one ground connection. Figure 6

5 2. The next image (figure 7) shows how to connect two 9-volt batteries to power the op-amp. Connect the 9-volts to the connection cables and connect the cables to the + (pin 4) and (pin 11) volt op-amp connection as shown on the op-amp diagram in figure 2. Figure 7 3. Figure 8 shows the speaker connection for a stereo speaker. You connect the output of the circuit to the speaker to hear the effect on the music. Figure 8

6 4. Figure 9 shows how to connect the input and outputs to the op-amp. In this case we ve only connected the red plug of the speaker. If you want the audio to come out of both speakers then connect the other cable to the same node as the red cable (pin1). Figure 9 5. The main circuit to build is the volume control circuit where we connect a variable resistor, a static resistor, battery connection, as well as the audio input and output like in figure 10. Figure 10 doesn t show the input and output connection but they re the same as the one in figure 9. Figure 10

7 Tips on the circuit: 1. Make sure to connect the 9 volt batteries correctly to the op-amp. The reason for this is that you could damage the op-amp if the right connection isn t made. If you re results aren t good, then try switching to another op-amp on the same chip. 2. Connect the speaker to the output pin of the op-amp (Ex. Pin1). Connect both red and yellow cables to the output since the output will be a mono signal. Also don t forget to connect the green cable (ground) to the ground connection on your circuit. 3. Connect the circuit to an audio signal from an audio source such as a computer or mp3 player. If you don t have any music files on you, visit an online radio station or Pandora.com 4. Vary the resistance on the variable resistor by turning the dial on it left or right and observe the change in the volume coming from the speakers. 5. If it sounds like the sounds are breaking up and scratchy, try reducing the volume on your audio source (mp3 player, computer, etc). Questions: 1. Set RR ff to 10KiloΩ and calculate the amplification. Think of what amplification means and use the voltage divider rule shown below. RR Vin = 1 * Vout RR ff +RR 1 Gain = RR ff+rr 1 RR 1 2. Assume Vin = 1V and RR ff = 10KiloΩ. If the resistance of the whole circuit (no speaker) is 8 Ω and the speaker has a resistance of 8 Ω as well, figure out what the current is going through the speaker. (Hint: the circuit is reduced to two loads and a voltage source). Guess what would happen if the circuit resistance was different from the speaker resistance. 3. If we removed RR ff from the circuit, what would happen to the volume (Amplitude) of the signal? Explain why. 4. Based on what you ve learned from creating this circuit, try changing both resistance values by connecting another potentiometer to the circuit where RR 1 is and try to maximize gain. You will notice there is a certain point where the gain won t increase anymore regardless of what you do. Explain why. Conclusion: Based on this lab, we hope you ve learned one of the important uses of an op-amp when used for amplification. There are plenty of online readings if anyone is interested in learning more about opamps. Below are a couple of links to them.