Audio Engineering

118 Chapter 4

with the voltage source. Where else do we fi nd the innocent resistor lurking? Operational
amplifi ers, or op-amps, have a devastating amount of power packed into a tiny eight-
pin dual-in-line (DIL) package. Gain setting, the most common feature for an op-amp,
is determined by two resistors. Regardless of the sophistication and variety of op-amps
(and there are many), they all have to depend on the lowly resistor to function. A resistor
is like the mortar holding the bricks together that ultimately form a house. Mortar’s not
much to look at or get excited about, but where would bricks be without it?

Split bias voltages are found everywhere in op-amp circuits running off a single battery.
The positive noninverting pin must be biased in order to halve the supply voltage. Two
resistors of equal value placed across the supply voltage and ground nicely provide the
required split voltage.

In a slightly different form, but nevertheless still a resistor, there is the potentiometer,
which is nothing more than a variable resistor. Figure 4.1 shows the two basic resistor
types. All radio receivers, stereo amplifi ers, cassette recorders, and other such devices
have volume controls for obvious reasons. Resistors come in a variety of practically
infi nite values, from the typically used values of a few ohms to a few megohms. The
LED example uses a current-limiting resistor that can vary from a few hundred ohms to
a few thousand ohms depending on the supply voltage and the LED brightness required.
Gain-setting resistors can range anywhere between a few kohms to a Mohm. Resistors
for the split bias supply typically are 100 kohms in value. Resistors are usually associated
with DC circuits, as we’ve seen, and provide a number of useful functions, but most
commonly they control current.

Other than limiting current, one of the next most common functions of the resistor is to
act as a potential divider circuit. In the simplest case, two equal resistors are placed across

Fixed resistor Potentiometer

Color band 3

Color band 2

Color band 1

Figure 4.1 : Fixed resistor and potentiometer.