situation; but with feedback, the operation is closed loop. The use of properly
applied negative feedback stabilizes the operation of the composite circuit against
changes in the amplifier, and makes the circuit very versatile.
b. Applications. Operational amplifiers have many practical uses. Figure 3-
3 shows the op ampl used as an inverting amplifier.
Other uses include
noninverting amplifiers, integrators, differentiators, oscillators, filters, and
digital circuits such as schmitt triggers, gates, and flip-flops.
Figure 3-3.
Typical operational amplifier.
3-3.
INTEGRATED CIRCUITS
An integrated circuit (IC) is a circuit made on a tiny chip of glasslike
material.
It may be a logic gate or an amplifier, or a combination of circuits
that replaces several transistors, resistors, and capacitors.
The IC chip is
sealed in an airtight package and is normally mounted on a printed circuit card.
IC's are used in electronic equipment because they reduce size, weight, and cost,
improve performance, and are more reliable than the circuitry that they replace.
To reduce cost and size, transistors, resistors, and capacitors are built into the
IC.
However, inductors, transformers, and large-value capacitors must be mounted
outside the package.
a. The Monolithic IC. The monolithic IC is the most widely used of all the
various types.
It is called monolithic because the circuit is formed within a
single semiconductor crystal. The circuit components are isolated by reverse-bias
PN junctions or by other methods.
Monolithic IC's have good-quality diodes and
transistors, but poor- to average-quality resistors and capacitors.
(1) Epitaxial growth.
The monolithic IC begins as a very thin silicon
wafer known as a substrate.
A second crystal layer is formed on the
substrate by a process known as epitaxial growth.
The crystal
structure of the growth layer must be the same as the structure of the
substrate.
Part A of figure 3-4 shows the process used to form the
epitaxial layer. The substrate containing P-type impurities is heated
in the presence of a gas containing hydrogen, and a silicon compound
containing N-type impurities.
The silicon atoms released by the heat
form a second layer in the substrate. The completed layer is shown in
B of figure 3-4. The substrate layer has positive properties and the
epitaxial layer has negative properties. A PN junction has been formed
by the layers.
309 L3
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