Oscillators are also regarded as
DC-to-RF converters. A typical oscillator
consists of an active device and a
passive frequency-determining resonant
element. The active device can
be a two-terminal device like a Gunn
or IMPATT diode or more commonly
a three-terminal device including a
junction bipolar transistor, metal
semiconductor FET or more recent
devices using newer semiconductor
materials. In order to generate a high
frequency signal an active device with
sufficient gain to compensate for
feedback loop losses is necessary. Oscillation
conditions need to be satisfied
for the circuit containing the active
device and passive element. Two
different topologies are used for this
purpose, as shown in Figure 4 in
their generalized form. A parallel
feedback oscillator is the one in
which the frequency-determining element
is used as a feedback element
between the input and output in order
to generate necessary instability
and a negative resistance oscillator is
the one in which reflection gain at a
given terminal is used to satisfy the
oscillation condition when connected
to a frequency-determining element
with the proper phase condition