12-04-2012, 04:10 PM
Transistor Oscillator and Amplifier Grids
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INTRODUCTION
Millimeter- and submillimeter-wave systems continue to
be a subject of growing interest. The applications involving
this portion of the electromagnetic spectrum cover
a broad range of scientific disciplines, varying from the
measurement of electron densities in tokamak plasmas [ 11
to studying emission spectra of distant celestial bodies [Z].
Millimeter waves correspond to the frequencies between
30 GHz and 300 GHz and the submillimeter-wave range
is regarded as the region between 300 GHz and 3 THz.
The shorter wavelengths at these frequencies allow the
use of smaller and lighter components than for microwave
QUASI-OFTICALPO WER COMBINING
An approach which overcomes the limitations of power
combiners based on scaled-down microwave systems involves
combining the output powers of many devices in free
space. Mink suggested using an array of millimeter-wave
devices placed in an optical resonator as a means of largescale
power combining [17]. While it is unlikely that solidstate
power combiners will replace high-power electron
tube sources, there is great potential for improvement in
output power and combining efficiency by using quasioptical
techniques. Because the power is combined in free
space, losses associated with waveguide walls and feed
networks are eliminated. The power can be distributed
over a larger number of devices than in a waveguide
cavity because the quasi-optical resonator can be many
wavelengths across. An external injection-locking signal
is unnecessary because synchronization of the sources is
accomplished by mutual coupling through the modes of the
resonator.
GRID OSCILLUORS
Grid oscillators are periodic arrays embedded with active
solid-state devices. The grid is placed in a Fabry-Perot
resonator to provide the feedback necessary for oscillation.
This is illustrated in Fig. 1. Two important features
distinguish grid oscillators from most quasi-optical power
combiners built from microstrip circuits. First, grid oscillators
do not necessarily have a ground plane and, as a
result, do not rely on the interaction of microstrip modes
with free-space radiation. Second, microstrip-based power
combiners tend to be a collection of individual free-running
oscillators that are weakly coupled. Thus, the operating frequency
depends primarily on the behavior of the individual
oscillators. In contrast, the elements making up an oscillator
grid are not themselves free-running oscillators. Mutual
interaction of all the devices in the grid is necessary for
oscillation to occur.
BAR-GRIDO SCILLATORS
An altemative quasi-optical grid configuration is shown
in Fig. 5. The grid consists of an array of metal bars
on which packaged devices are mounted. This structure
has been used to combine the output powers of both
transistors [32] and Gunn diodes [33]. A mirror placed
behind the grid couples the devices together and is also
used for reactive tuning. The metal bars, which are used
to provide dc bias to the devices, make an excellent heat
sink. For convenience, the devices in adjacent rows share dc
biasing. This arrangement minimizes the number of biasing
connections. It also gives the grid a symmetric structure
that can be exploited to determine the grid's embedding
impedance.
CONCLUSIONS
This paper has reviewed a variety of MESFET grids used
for the generation and amplification of microwave power.
The approach, which involves a periodic grid of transistors,
is relatively simple to implement and suitable for waferscale
integration. Bar grids, which have excellent heatsinking
capacity, offer an attractive means of combining
the output power of low-efficiency devices such as Gunn
diodes. Planar grids, although less efficient at removing
heat, are compatible with modern IC fabrication techniques.
This is a real advantage for large-scale power combining
at millimeter- and submillimeter-wave frequencies.