February 20, 1995
One Minute Papers - Questions and Answers
Is the transistor used for silence time?
During the silent passages of the music, the amplifier does not vary the amount of current passing through the speaker so that it doesn't move and doesn't produce sound. To conserve energy and to avoid heating up the speaker, a good amplifier doesn't send any current through the speaker during a quiet passage. Whether or not the amplifier actually consumes power during the quiet passage depends on the exact design of the amplifier. In the simple amplifier constructed in class, the transistor does pass some current during the quiet periods. However, that current is small and steady. Some stereo experts claim that they can hear the differences between amplifiers the do or do not consume power with their output transistors during the quiet times and claim that the power wasting amplifiers sound better.
I'm not sure that I understand the value, use of the transistor -- is it the dead time or air?
In the simple amplifier constructed in class, the transistor steals current from the speaker and tends to return the speaker cone to its resting position. As the current through the transistor decreases, the current through the speaker increases and it accelerates away from its resting position. The current shifts back and forth from the transistor to the speaker and back again with each cycle of the audio sound being reproduced. As a result, the speaker cone moves back and forth with each audio cycle. Which way the speaker cone moves (in or out) depends on the direction of current flow through it and thus the way in which the speaker is connected to the circuit. In general, you cannot hear which way the speaker goes with each cycle unless you are listening to stereo (in which case the two speakers must move in the same directions to avoid strange effects). Overall, the transistor is used all the time, but it switches the current back and forth vigorously during the loud passages and weakly during the soft passages.
How does the coil in the microphone turn sound into electric current?
The coil in a microphone is attached to a movable surface that is pushed back and forth by the sound. Near the coil is a magnet so that, as the coil moves, the magnet induces electrical currents in it. Whenever a magnet moves past a coil of wire or a coil of wire moves past a magnet, a current is induced in that coil of wire.
Once you transfer electric charge to the gate of the MOSFET transistor, you do not have to keep your finger on that gate to keep current flowing. Right?
Yes. Once you have transferred charge to the gate of the transistor, it remains there until you remove it. The current will continue to flow through the channel until the charge is taken away. Thus a tiny amount of charge can affect the flow of incredible amount of current for long periods of time. That is how computer memory works (so called "dynamic random access memory"). A tiny charge on the gate of an MOSFET transistor is all it takes to remember one bit of information.
Is the capacitor you spoke of the same thing/similar to a "Flex capacitor"? (Remember "Back to the Future")
In "Back to the Future", Dr. Brown invented the "Flux capacitor". Sadly, this device is entirely fictitious. Its name simply incorporates two common words from physics: "flux" (the amount of some physical quantity passing through a region of surface area) and "capacitor" (an electrical device that stores separated electric charge).