Page 151 - Complete Wireless Design

Amplifier Design

150 Chapter Three

the best NF. Now design the bias network to supply the chosen I , while the
C
optimal source impedance of R is matched at the transistor’s input instead of
S
the true impedance of the source. In other words, if we match the low-noise
active device to by making the LNA “see,” for instance, the antenna’s 50-
S(opt)
ohm output impedance as this value, we can make the LNA transistor
S(opt)
supply a low NF (but at an increased VSWR over a perfect power match). We
then terminate the transistor’s output by:
L S S S S(opt)
21
12

22
1 (S
11 S(opt) )
where load reflection coefficient
L
optimum source reflection coefficient for the lowest NF (as
S(opt)
found on the transistor’s data sheet).

the complex conjugate of the result
Vector quantities should be used for S.
will be close to the complex conjugate of the device’s input impedance,
S(opt)
and will, at times, be so close to S as to be almost identical. This would be
11
the optimal situation, since NF and VSWR will then be optimized. This situa-
tion is more common with GaAs FETs that are operated above 2 GHz (GaAs
FETs must be employed in the microwave regions above 2 GHz if BJTs cannot
furnish the necessary gain and noise performance).
When the S
(perfect input impedance match for power) and (perfect
11 S(opt)
NF “match”) diverge excessively from each other, system performance may
suffer, with an associated noise figure and increased mismatch loss. A way to
compromise between VSWR and NF is to employ a small amount of source
lead inductance for the low-noise FET. If source lead inductance is used under
2 GHz, where it is most needed in both FETs and BJTs, S
and will
11 S(opt)
come quite close together, thus hopefully decreasing both NF and VSWR,
while slightly decreasing gain—with minimal impact on device stability. This
increased source inductance need only be added in the form of slightly longer
source leads (before they contact the PCB’s ground plane). The length must be
found empirically, or approximated with certain RF software packages. As
stated above, stability under 2 GHz will not be dramatically affected by this
increase in source inductance, but at higher frequencies, this will not be the
case, and instability of the LNA can result. Too high a source inductance val-
ue will, nevertheless, create amplifier instability at high or low frequencies, so
the best value should be chosen carefully. Therefore, with both the BJT and
the FET, a compromise between stability, NF, and gain must be reached, since
any inductance at the emitter or source will lower gain and increase stability
at low frequencies, but will actually increase gain and decrease stability at
high frequencies in a wideband LNA.
If stability is still a concern, adding a very low value resistor (13 ohms)
between the transistor’s collector and its output matching network will
decrease gain, but force stability over a very wide band of frequencies. In fact,

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