334 Week 9: Alternating Current Circuits
Figure 136: A full-wave rectifier made out of four diodes. The “headphones” are the resistance in
the center of the diamond of diodes. Verify that the current always passes through this resistor from
left to right, regardless of whether the voltage difference top to bottom is positive or negative.
flipped (but not smeared) signal is shown below in figure 137. Compare it to the original signal and
you can see that as long as the headphones are massive enough to be unable to respond to the very
high frequency rippleanyway, you’ll be able to hear the music, voices, or whatever that was encoded
on the carrier to a high degree of accuracy.
(^00246810)
1
2
3
4
t
Rectified AM carrier
Figure 137: The AM encoded signal after it has been received by a tuned, band-pass filter and
full-wave rectified. Note that theaverageoutput voltage will very closely track the original signal.
This section should provide you with more than enough information tounderstand and even build
a crystal radio of your own. Note well: thisgeneral processof encoding and decoding information
on to/off of carrier signals is one of thefundamental basesof modern civilization. High pass, low
pass, and band pass/reject circuits are ubiquitous. Even if you yourself never actuallybuildan
electronic circuit, knowing a bit about how they work and in particularknowing what things such as
“impedance matching” are and why they matter can really improve your understanding and ability
to work with electronic devices in many laboratory environments.
In this chapter we have already remarked on the content of the next one. We have learned all of
Maxwell’s Equations already, but one of them isbroken; in particular, it doesn’t take into account
the fact thatcharge is conservedand that there is a certainambiguityin the particular open surface
Sone can choose that is bounded by any given (specified) closed curveC. We need to fix this,