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Elementary Amplifier Theory
10 questions By Tony R. Kuphaldt
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Question 4 of 10
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.
Reveal answerWhile a “voltage buffer” does not amplify the voltage level of a signal, it does amplify the current level of a signal.
Notes:Voltage buffers are almost ubiquitous in modern electronic circuitry, so they cannot be dismissed as useless. Discuss with your students some possible applications of voltage buffers. When would we want to amplify a signal’s current without amplifying the voltage? Do your students think there might be any application for this type of circuit in electronic test equipment (voltmeters, especially?).
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Question 5 of 10
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.
Reveal answer“Bandwidth” refers to the range of signal frequencies an amplifier can handle within specified bounds of gain, distortion, efficiency, etc.
Notes:As your students research the word “bandwidth,” they will find this term has application in many areas other than amplifiers. Discuss this term, both in the context of amplifiers and in the context of other applications.
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Question 6 of 10
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?
Reveal answerMost amplifiers’ frequency responses resemble that of low-pass filters. “Rolloff” is the term used to denote the steepness of the amplifier’s Bode plot as it attenuates the amplified signal at ever-increasing frequencies.
An “octave” denotes a doubling of signal frequency. This unit applies well to logarithmic-scale Bode plots.
Notes:Have one of your students draw a picture of a Bode plot for a (realistic) low-pass filter: that is, a non-ideal low-pass filter response. Review with your students what a log-scale plot looks like, and ask them to relate the ratio-units of “decibel” and “octave” to such a scale.
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