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Elementary Amplifier Theory
10 questions By Tony R. Kuphaldt
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Question 1 of 10
Fundamentally, an amplifier is a device that takes in a low-power signal and outputs a magnified (power-boosted) facsimile of the input signal. Explain how it is possible for such a device to exist. Doesn’t the Law of Energy Conservation (“Energy can neither be created nor destroyed”) preclude the existence of a power-boosting device?
Reveal answerNo physical laws are violated with an amplifier, because the added power comes from an external source: a power supply.
Follow-up question: is a step-up transformer an amplifier? Why or why not?
Notes:The amplifiers your students will be studying are electronic devices, but other types of amplifiers exist. Discuss with them some examples of common, non-electronic amplifiers.
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Question 2 of 10
An important parameter of every amplifier is gain. Explain what “gain” is, and write a simple equation defining gain in terms of signal voltage.
Reveal answer“Gain” is the degree of “magnification” that an amplifier provides its input signal. Voltage gain may be defined in two different ways:
$$A_{V(dc)} = \frac{V_{out}}{V_{in}}$$
$$A_{V(ac)} = \frac{\triangle V_{out}}{\triangle V_{in}}$$
Notes:As your students should be able to discern through context, the symbol used to represent gain in equations is the capital letter Ä”. One potential point of confusion is the difference between the two gain equations shown in the answer. Why would we have two different equations saying pretty much the same thing? If this issue comes up in discussion, you can give your students the example of an amplifier with a DC bias, where Vout = (4)(Vin) 3 volts. Here, the (AC) gain is always 4, but the DC gain varies according to how much voltage we apply to the input!
Based on this example, which gain calculation do your students think is the more practical?
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Question 3 of 10
At the heart of every amplifier is a device that uses one signal to control another. In electronics, this means a device that uses a small voltage or current signal to control a larger voltage or current.
The first electronic amplifying circuits were constructed with devices called electron tubes instead of transistors. Tubes still find specialized applications in electronics, but they have largely been replaced by transistors. Why is this? What advantages do transistors have over tubes as amplifying devices?
Reveal answerTransistors are typically much more physically rugged than electron tubes, able to withstand greater levels of vibration and stress. They are also smaller, and more energy efficient in most applications.
Notes:Electron tubes used to be the “workhorses” of the electronics world, acting as power control and amplification devices for a wide range of applications. It should be interesting to listen to your students’ feedback on this question, being that there is a lot of “tube” information on the internet. IEEE Spectrum magazine had a couple of excellent articles on electron tubes and their applications, which I would strongly encourage any interested students to read.
Question #10: “What is the overall voltage gain of two cascaded amplifiers (the output of the first amplifier going into the input of the second), each with an individual voltage gain of 3 dB?” Thought the answer to question #10, as 6 dB voltage expressed as a ratio, would be 3.98:1.
Correction: I thought a 6 dB voltage gain would be expressed in ratio as 1.99:1.
I’m also confused by the answer for the second half of question 10