334 Chapter 12
cated on the graph and circled for reference. A line is
drawn through the dot to the maximum collector
current, 10 mA, and downward to intersect the VCE line
at the bottom of the graph, which, for this example, is
12 V. This line is termed the load line. The load resis-
tance RL may be computed with
(12-34)
where,
RL is the load resistance,
dVCE is the range of collector-to-emitter voltage,
dIC is the range of collector current.
In the example,
Under these conditions, the entire load line dissipates
less than the maximum value of 33 mW, with 90μA of
base current and 5 mA of collector current. The required
base current of 90μA may be obtained by means of one
of the biasing arrangements shown in Fig. 12-26.
To derive the maximum power output from the tran-
sistor, the load line may be moved to the right and the
operating point placed in the maximum dissipation
curve, as shown in Fig. 12-31C. Under these conditions,
an increase in distortion may be expected. As the oper-
ating point is now at 6.5 V and 5 mA, the dissipation is
33 mW. Drawing a line through the new operating point
and 10 mA (the maximum current), the voltage at the
lower end of the load line is 13.0 V; therefore, the load
impedance is now 1300:.
12.3 Integrated Circuits
An integrated circuit (IC) is a device consisting of
hundreds and even thousands of components in one
small enclosure, and came into being when manufac-
turers learned how to grow and package semiconductors
and resistors.
The first ICs were small scale and usually too noisy
for audio circuits; however, as time passed, the noise
was reduced, stability increased, and the operational
amplifier (op-amp) IC became an important part of the
audio circuit. With the introduction of medium-scale
integration (MSI) and large-scale integration (LSI)
circuits, power amplifiers were made on a single chip
with only capacitors, gain, and frequency compensation
components externally connected.
Typical circuit components might use up a space
4 mils × 6 mils (1 mil = 0.001 inch) for a transistor,
3 mils × 4 mils for a diode, and 2 mils × 12 mils for a
resistor. These components are packed on the surface of
the semiconductor wafer and interconnected by a metal
pattern that is evaporated into the top surface. Leads are
attached to the wafer that is then sealed and packaged in
several configurations, depending on their complexity.
ICs can be categorized by their method of fabrication
or use. The most common are monolithic or hybrid and
linear or digital. Operational amplifiers and most analog
circuits are linear while flip-flops and on–off switch
circuits are digital.
An IC is considered monolithic if it is produced on
one single chip and hybrid if it consists of more than
one monolithic chip tied together and/or includes
discrete components such as transistors, resistors, and
capacitors.
With only a few external components, ICs can
perform math functions, such as trigonometry, squaring,
square roots, logarithms and antilogarithms, integra-
tion, and differentiation. ICs are well suited to act as
voltage comparators, zero-crossing detectors, ac and dc
amplifiers, audio and video amplifiers, null detectors,
and sine-, square-, or triangular-wave generators, and all
at a fraction of the cost of discrete-device circuits.12.3.1 Monolithic Integrated CircuitsAll circuit elements, both active and passive, are formed
at the same time on a single wafer. The same circuit can
be repeated many times on a single wafer and then cut
to form individual 50 mil^2 ICs.
Bipolar transistors are often used in ICs and are
fabricated much like the discrete transistor by the planar
process. The differences are the contact-to-the-collector
region is through the top surface rather than the
substrate, requiring electrical isolation between the
substrate and the collector. The integrated transistor is
isolated from other components by a pn junction that
creates capacitance, reducing high-frequency response
and increasing leakage current, which in low-power
circuits can be significant.
Integrated diodes are produced the same way as tran-
sistors and can be regarded as transistors whose termi-
nals have been connected to give the desired
characteristics.RLdVCE
dIC-------------=RL 012–
00.01–=-------------------12
0.01=----------= 1200 :