126 Chapter 4
For AC applications, an approximate counterpart to the resistor is the capacitor. Again,
a seemingly innocent two-terminal device, the capacitor appears lowly in form, but it is
critically needed, like the resistor. Consider any amplifi er circuit as an example.
Returning to the AC amplifi er example, there is always a capacitor coupling the signal in
and coupling the signal out. That’s the way to recognize an AC amplifi er by the presence
of the capacitor at the input and the output. For simple preamplifi ers, the coupling
capacitors, as they’re called, could be around 0.1 μ F in value. If we assume the signal to
be in the audio frequency range, say 10 kHz, then the capacitive reactance works out to
be 159 ohms. This is very low and practically a short circuit. As the capacitive reactance
scales inversely with the capacitance, doubling the capacitor to 0.2 μ F will reduce the
capacitive reactance by half to 79.5 ohms. In our example of the split supply with the
resistor we saw that two resistors of equal value gave us the split voltage. Generally in
an actual working circuit, you will see a capacitor placed across the lower resistor, that
is, the one connected to ground. This is typically a capacity with a large value (100 μ F),
which is really a short circuit at the audio frequencies we are working with. Another very
common way of connecting a capacitor is directly across the supply line, that is, between
the plus and the minus voltage rail. With a battery supply this is not so critical, but if
you’re using a low-voltage line adapter, using a large value smoothing capacitor (several
1000 μ F in value) will aid in producing a smoother supply source.
4.1.3 Diodes
Diodes are two-terminal devices that have a feature that is totally distinct from the
features of resistors or capacitors. They are distinctly polarity sensitive. When DC voltage
is applied to a diode, a high current will fl ow in one direction, but reversing the voltage
will, to all intents and purposes, cause no current to fl ow. Put another way, when the diode
is confi gured in what is called the forward-biased mode, the diode will conduct current.
Reverse the bias to the reverse-biased mode and no current will fl ow. This is defi ned as a
rectifying action. AC voltage, say originating from the line voltage, can be immediately
converted into a DC voltage of sorts by feeding it through a diode. The diode essentially
passes on only half of the positive and negative going waveform. Electronic circuits are
invariably powered with the positive voltage supplying the power rail ( Figure 4.6 ).
To test this out, connect up a resistor, say 100 kohms, across a 9-volt battery with a
current meter inserted between the positive battery terminal and one resistor terminal.