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Practical circuit applications
using that strange diode:
the varactor
by BIL L M ENG EL
The varactor is a simple two-terminal
device extending dependable operation in
the VHF. UHF, as well as microwave frequencies
by utilizing the voltage-variable
capacitance of a po junction. The varactor
provides a wa y of tuning circuits, multiplying
and dividing frequencies, controlling
frequencies, and performing other functions.
A varactor. which is a special-purpose
junction diode. has been designed to
make its junction capacitance useful; it is
beca use of this propert y of a varactor that
capacitance. which is an unavoidable nuisan
ce in conventional diodes. is purposely
cult ivat ed into the varacto r. The basic configuration
for the varactor is show n in the
illustration belo w.
~N O D[
~ A TH O D [
The operating portion of a varacto r is in
the reg ion where a conventional diode
would be conside red to be cut off - principally
in the region between forward conduction
and reverse breakdown. In most
cases, the varaetor is reverse-biased since
in th is state it draws a minimum of current,
making it essent ially voltage-operated. The
behavior of the pn junction of the var actor
at different applied bias potentials is as
follows:
ZERO BIAS - At zero bias, the contact
potential is determined by the semiconductor.
T here is no change in capa citance
and no current flowin g at this time .
FORWARD BIAS~ When for ward-biased.
high forward cl;lrrent flows as the external
voltage applied is in series with the
contact potential. T he contact potential
decreases thus increasing the capacitance.
REVERSE BIAS - When reve rse-biased.
the external voltage appl ied is in parallel
with the contact 'potential. The contact
pot ential increases, extremely low reverse
cur rent flows, and the capacity decreases.
The propert y 'of being able to vary the
capacitance by ch anging the applied voltage
ena bles the varactor to do the work of a
con ventional variable capacitor many times
its size. The capacitance of a varacto r
varies inversely as the reverse voltage, and
directl y as the for ward voltage. It may also
be noted that the capacitance of a varactor
also varies nonlinearly. Varactors also have
a Q approaching that of air trimmer capacitor
s, so the y could be used in such locations
as rf front ends and high-effic ienc y
multipliers as well as other normally sensitive
circuits.
T he varactor diode by itself is unique in
frequency multiplying and dividing. F irst,
the rf signal itself is the only power required
to operate the varactor. Secondly,
16 73 MA GAZINE