10-11-2012, 04:16 PM
Distributed and Lumped Elements of Integrated Circuits
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• Passive RF and microwave integrated circuits are made up of distributed
elements, lumped elements, or combinations of both these types of
elements.
• Distributed elements consist of segments of transmission lines of the
different types that we have discussed in the preceding lectures.
• These transmission-line segments can be of various lengths, ranging
from small fractions of the guide wavelength (L) to several wavelengths.
• We consider primary lumped elements, which by definition are small in
size in comparison with the guide wavelength in any transmission line
that may be associated with them.
• Examples of lumped elements are resistors, capacitors, inductors,
discontinuities, and resonators.
• For sufficiently small dimensions, there are no significant variations in
resistance, capacitance, inductance, and so forth, except as associated
with any frequency dependence of material resistivities, permitivities,
and permeabilities.
CAPACITORS
• Capacitors are lumped circuit elements that store energy by virtue of
electric fields.
• For MICs, capacitors can be conveniently realized in several different
configurations.
• It is clear that fundamental parallel-plate capacitors consisting of a pair of
parallel planar metallic surfaces separated by a dielectric are available in
chip forms as discrete components.
Dual-Spiral “Surprise”
• The conventional spiral inductors [see Figure 3.2(d–g)] have some
disadvantages.
• The spiral inductor requires two metal layers, one for the spiral inductor
itself and another for an air bridge [Figure 3.2(g)] to provide crossover
connection between the inductor center and outer circuitry.
• In this case, the bridge crossing the turns of the spiral produces
parasitic feedback, increases insertion losses, and makes fabrication
costly.
• Technology of the air bridge structure includes additional processes, for
example, the deposition of a photoresist, which then should be
removed from between the bridge and the substrate.
• The air bridge’s physical dimensions are critical and should be
determined as a compromise between electrical and mechanical
requirements.
RESISTORS, TERMINATIONS, AND ATTENUATORS
• The desirable characteristics of film resistors are:
– Low temperature coefficient of resistance (TCR);
– Good adhesion of the resistive film to substrate and conductors;
– Minimal dimensions because transmission line effects are to be
ignored;
– Adequate dissipation capability;
– Good stable resistance value, which should not change with
time.
• Lumped-element resistors can be used in attenuators,
dividers/combiners, and biasing circuits, and they can also act as
matched terminations.
• Resistors for MIC may be realized in chip form or fabricated by
planar construction.