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Lecture 20

ESE 121 Lecture 20: Amplifiers

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Stony Brook University
Electrical Engineering
ESE 121
Jan Folkson

ESE 121 Lecture Amplifiers • The benefit of an active filter is its amplifying ability and the most basic way to think of an amplifier is an electronic device that increases the power of a signal. • There is a fundamental rule of physics called the Law of Conservation of Energy. It simply states that energy cannot be created or destroyed. It also means that you can’t construct a device that will input a small amount of energy and magically output a large amount of energy • The best that any device can get is to 100% power. That is: whatever comes in goes out. Energy can be transformed, however, and most devices transform some electric energy into heat energy reducing their efficiency below 100%. Perpetual motion machine: • Many people have tried to create a device that with more than 100% efficiency. In this case it would have to output more power than it took in. • If this were the case, it could reroute some of its output to power itself and have a surplus of power left over. This sounds awesome and would completely solve all future energy problems. • Problem is, that it would break the Law of Conservation of Energy and is unrealistic. • Through the use of an amplifier we can arrive at a situation where we have an input signal with low power and an output signal with lots of power. • Unlike the perpetual motion machine, this increase in power doesn’t come for free. • In essence we take the large amount of power in the DC external power supply or battery and transform it into a copy of the signal but with more power. This called modulation. • We’re modulating the large DC current with the small AC signal, producing the same AC signal, but with more power. • Imagine a huge pipe with lots of water flowing through it. To control the flow of water in the pipe we can turn the valve. Now imagine being able to turn the valve, not with your hand, but with a much smaller amount of water than that flowing through the pipe. • We’ve seen that amplifiers can input a small signal and output a much larger one. We need some way of describing the ratio of output to input. This is called Gain. We can have three types of gain: voltage, current, and power. • Example: We have an AC input signal measuring 2V RMS and an output measuring 30V RMS. Calculate the gain. • Or 15 Volts per Volt. For every one Volt we input, we get 15 Volts out • Example: An amplifier has a current gain of 3.5 A/A and is inputted an AC signal of 28mA RMS. Calculate the output. • Example: If multiple amplifiers are chained together in series their individual gains can just be multiplied. • Even though we refer to voltage gain as Volt per Volt, it’s really a unit-less number. Just a ratio. However there is a unit used to represent gain. It’s called the bel. • We’ve spent a lot of time discussing decibels and the bel unit was devised as a way to describe power loss in telephone wiring systems. Named after Scottish inventor, Alexander Graham Bell. • The bel was the amount of signal power loss due to resistance over a standard length of wire. The math worked out to be • Later it was decided that the unit of the bel was too big and it was decided to divide the unit by 10. The new unit was called the decibel or dB. • As we know, log units are used to describe very large ratios with small numbers, making the math considerably easier. • Our previous example done in dB: If multiple amplifiers are chained together in series their individual gains can just be multiplied. • For amplifiers connected in series, their ratio gain figures are multiplicative. Described in decibels, however, their gain figures are additive. • This all comes out of the rules of logarithms that you probably learned in trigonometry while wondering… “when am I ever going to need that?” • We’ve spent a lot of time working with dB. We’ve always expressed it as a ratio and not an absolute value. If we were to take a reference p
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