# Elementary Amplifier Theory

## Discrete Semiconductor Devices and Circuits

• #### Question 1

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?

• #### Question 2

An important parameter of every amplifier is gain. Explain what “gain” is, and write a simple equation defining gain in terms of signal voltage.

• #### Question 3

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 Tubes still find specialized applications in electronics, but they have largely been replaced by transistors.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?

• #### Question 4

A very common type of amplifier used in electronic circuits is the voltage buffer, sometimes called a voltage follower. There are two simple forms of this circuit, one using a single transistor and the other using an integrated circuit called an operational amplifier: The voltage gain of each of these devices is unity (AV = 1). My question to you is this: what possible use is an amplifier that doesn’t even amplify the voltage of its input signal? If the output voltage is the same magnitude as the input voltage, then is this circuit really amplifying anything at all? A straight piece of wire outputs the same voltage that it receives in!

Explain the practical purpose for these very popular amplifier circuit configurations.

• #### Question 5

An important operational parameter of an amplifier is its bandwidth. Describe what “bandwidth” means in the general sense, and give an example of an amplifier application where bandwidth is important.

• #### Question 6

Most of the simple amplifiers you will be initially studying tend to lose gain as the frequency of the amplified signal increases. This loss of gain is sometimes quantified in terms of rolloff, usually expressed in units of decibels per octave (dB/octave).

What, exactly, is “rolloff?” What is an “octave,” in the context of the units of measurement used to specify rolloff? If we were to plot the response of a typical amplifier in the form of a Bode plot, what type of filter circuit characteristic (band-pass, band-stop, etc.) would it best resemble?

• #### Question 7

A Class-A transistor amplifier uses a single transistor to generate an output signal to a load. The amplifier shown here happens to be of the “common collector” topology, one of three configurations common to single-transistor circuits: An analogue for this electronic circuit is this water-pressure control, consisting of a variable valve passing water through an orifice (a restriction), then on to a drain: The “input” to this amplifier is the positioning of the valve control handle. The “output” of this amplifier is water pressure measured at the end of the horizontal “output” pipe.

Explain how either of these “circuits” meets the criteria of being an amplifier. In other words, explain how power is boosted from input to output in both these systems. Also, describe how efficient each of these amplifiers is, “efficiency” being a measure of how much current (or water) goes to the load device, as compared to how much just goes straight through the controlling element and back to ground (the drain).

• #### Question 8

A class-B transistor amplifier (sometimes called a push-pull amplifier) uses a pair of transistors to generate an output signal to a load. The circuit shown here has been simplified for the sake of illustrating the basic concept: An analogue for this electronic circuit is this water-pressure control, consisting of two variable valves. One valve connects the output pipe to a supply of pressurized water, and the other connects the output pipe to a source of vacuum (suction): The “input” to this amplifier is the positioning of the valve control handle. The “output” of this amplifier is water pressure measured at the end of the horizontal “output” pipe. Valve action is synchronized such that only one valve is open at any given time, just as no more than one transistor will be “on” at any given time in the class-B electronic circuit.

Explain how either of these “circuits” meets the criteria of being an amplifier. In other words, explain how power is boosted from input to output in both these systems. Also, describe how efficient each of these amplifiers is, “efficiency” being a measure of how much current (or water) goes to the load device, as compared to how much just goes straight from one supply “rail” to the other (from pressure to vacuum).

• #### Question 9

An amplifier has a voltage gain of 5 and a current gain of 75, both figures being ratios. Calculate the following gains:

Power gain (as a ratio)
Power gain (dB)
Voltage gain (dB)
Current gain (dB)
• #### 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? Express the overall voltage gain in decibels, and also as a ratio.

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