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# Lab 4 - Tufts University School of Engineering Department...

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Tufts University School of Engineering Department of Electrical and Computer Engineering ES3 - Introduction to Electrical Circuits Fall 2007 Lab Section: Tuesday 2:30 – 4:30 Experiment 4 Amplification, Impedance, and Frequency Response Name: xxxxx [email protected] tufts.edu Submitted to: xxxx Experiment Performed: xxxx Experiment Due: xxxxx Worked with: xxxxx

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Purpose The purpose of this lab was to create an audio amplifier that could increase the voltage of a weak signal and drive an audio speaker. An oscilloscope was then used to find the gain in voltage that an amp is able to provide. The relationship between this gain and frequency response was also studied. Finally, Thevenin equivalences were used again as a review. Introduction The main concept of this lab revolved around using an amplifier to increase the quality of sound from a speaker. In order to understand how this is possible, the way in which an amp achieves its functionality must be studied. An amp receives an input voltage that it then magnifies or “amplifies” so that the output voltage is greater. This increased voltage moving through the speaker creates a stronger force between the magnet and coil, which creates a louder, clearer sound. In order to calculate the voltage gain of the amplifier, the input and output voltage are needed. It is then just the ratio between the two, shown here: in out V v v A = (volts/volts). However, in many electrical engineering applications and settings, gain is expressed in decibels ( dB ). This is found using the following equation: = in out V v v A 10 log 20 ( dB ). Thus, zero gain would be when 0 = V A . As expected, this occurs when the output voltage and input voltage are equal, making the log equal to zero. Since an amplifier can increase the strength of a weak signal, it is ideal for connecting a microphone, pH sensor, or antenna. However, in order to provide a constant time-variable signal, a function generator was used in the lab instead. Attached to the function generator was a coupling capacitor. This capacitor is important in that it blocks unwanted DC voltages from reaching the speaker. Because DC voltages are constant, the coil and magnet will not move relative to each other, and no sound will be produced. Thus, the power from the DC voltage would be completely wasted. The blocking is done using a capacitor because its impedance becomes infinity as the radial frequency becomes zero as would be the case in a non-oscillating, constant voltage. The following equation shows this relationship: = = C j Z C ϖ ϖ ϖ 1 lim lim 0 0 . Lastly, the only remaining part of the lab was analysis of frequency response. This was done using a Bode plot. 2
Materials and Equipment LM386 audio amplifier chip 100 μF non polarized electrolytic capacitor 0.047 μF or 0.05 μF capacitor (2) 0.01 μF capacitors 10 Ω resistor 16 Ω speaker with soldered wires 9V battery with battery clip Breadboard

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