Page 126 - Complete Wireless Design

Amplifier Design

Amplifier Design 125

The total mismatch loss for the entire unmatched transistor is

p
p
p
TOTAL IN OUT
3. MAG (maximum available gain) is calculated by:

MAG G
p
tu TOTAL
4. Since designing for MAG is never recommended because an amplifier can
be unstable at this high gain value, we would like to be able to compute the
MSG, or the maximum stable gain:
MSG 10 log (|S | |S |)
10 21 12
Thus, if the MAG is smaller than the MSG, then the amplifier will be uncon-
ditionally stable, unless poor circuit layout produces an external feedback
path.
Scalar approximation is a more rapid technique than the methods presented
in the prior pages, since we are employing only the magnitude of the S param-
eters, and not their phase angle. As an example, we are given a transistor with
the following S parameters:

S 0.195 167.6°
11
S 0.508 32°
22
S 0.139 61.2°
12
S 2.5 62.4°
21
Therefore S 0.195, S 0.508, S 2.5, S 0.139, and
11 22 21 12
2
G 10 log |2.5| 7.96 dB
tu 10
2

p 10 log (1 0.195 ) 0.168 dB
IN 10
2

p 10 log (1 0.508 ) 1.29 dB
OUT 10

p 0.168 dB 1.29 dB 1.46 dB
TOTAL
which demonstrates that about 1.46 dB will be gained by proper impedance
matching.
MAG 7.96 dB 1.46 dB 9.42 dB

(10.63 dB was calculated for this same transistor with the full MAG method
described earlier.)

MSG 10 log (|2.5|÷|0.139|) 12.55 dB
10
With MAG MSG by over 3 dB—even with our approximation methods—this
transistor will be very stable.

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