Stability Considerations of Low-Noise Transistor

specifications, factors assigned step 5.
change R~, active which GA and Y& are device are: the functions noise of Rn, Gn and Yc parsmete
of the weighting figure, tested
Yopt = % F~in = 1 + 2R~ (Real
(Y&)
of Step
4 should
be the
specified
(y:)
+ Real
w~pt))
of the rs,
6.
If the the
results relative VSWR,
do not values
satisfy
the
desired and
STABILITY
CONSIDERATIONS WITH SIMULTANEOUS
OF
LOW-NOISE NOISE AND
TRANSISTOR POWER
AMPLIFIERS
MATCH
Les Farinon San Carlos,
Besser Electric California 94070
Abstract Microwave demonstrates transistor amplifiers may be simultaneously techniques, matched for optimum and noise stability and input/output VSWR. through This two paper amplifier
for
simultaneous
input
match
4. circuit
Rum a stability is potentially
analysis unstable,
on the resulting
circuit. circle
If the for
Feedback computed
plot the stability
as the optimum noise source impedance. In addition the gain and the stability factor of the circuit will also change. The latter change in particular will affect the design procedure, as described below. If the circuit reaches a potentially unstable state, both the load and source terminations must be selected with care to prevent oscillation. 327
and optimum
impedance appropriate
increase component
of the
configuration Smith
and determine
values
3. Optimize the above circuit and noise figure. Figure Minimum Noise Figure, and The transformation two-port SfI Optimum vs. Series noise 3 Noise Source Impedance,
optimization VSWR. gain to achieve The value during simultsneouely of GMAX (or MSG) optimization.
through
as follows: a final and low low input
GA+ R’~ (Rs (y))
Figure Noise Noise vs.
2 Figure, Impedance, Feedback
and Simultaneous
Source Parallel
elements compromise will change
source
impedances
simultaneous However, noise figure
inductive noise with
Combining albeit
feedbacks impedance in gain).
optimum
source
desired noise
direction, (this where
change at VHF higher
Z..
JX
optimum
is particularly transistors
to GMAX, This using circuit design
and noise was device. program, of the
2. forms ficant initial Select the a combination optimum toward matching input the of feedback match same optimum circuit from region noise the elements without figure. Chart. that noise causing Select transsource a ed obviously, for the practical device
suggests
also
measure?may
parameter
42’
0---
bination be used computer Although ducer Gain the The
of element to determine optimization, the power (MSG) reactive gain of the
and the circuit
Maximum to GMAX
transistor. computer aided design procedure is the following:
Map the optimum noise source impedance, the optimum 1. noise figure, and the conjugate of s1l as functions of the series and shunt feedback.
values. the
Therefore, initial values
the for
plots the
should
only
subsequent
feedback stability may
will still
typically factor, be very the
reduce close
the
transStable of
an acceptable
the case,
casual if the
observer. shunt the minimum device purposes. should be a better
For feedback noise has
exis figure no This be con-
as well
of the additions
parameter:’& 2 - j Imag
the output plane area. Compute
5. Perform noise
and select the load impedance in the stable GMAX or MSG, whichever is applicable.
and noise the variation noise
parameters. of optimum and the The reduce source
example,
2 and 3 show minimum
noise conjugate shunt show from
figure, for various at 4 GHz,
amplifier design is the significant difference between the desired source impedances for optimum noise and input Although recently device manufacturers have made match. progress in transforming the impedances closer to each other by either modifying the device geometries or diffusion process, at 4 GHz typically a 2-3 dB noise figure degradation is suffered when the input and output are simultaneously matched for minimum reflection in a 50!2 system (see Fig, 1). h such case, some form of an isolator is needed at the input to maintain a reasonable input match when the device is operating at its minimum noise figure. The cost and the associated losses of the isolator may make this approach undesirable.