ADL5521ACPZ-WP AD [Analog Devices], ADL5521ACPZ-WP Datasheet - Page 37

• Manufacturer Part Number: ADL5521ACPZ-WP
• Description: 400 MHz - 4000 MHz Low Noise Amplifier
• Manufacturer: AD [Analog Devices]

Preliminary Technical Data
Tuning the LNA Input for Optimal Gain
LNAs are generally tuned for either gain or noise optimization, or
some tradeoff between the two. One figure of merit of an LNA is
how much tradeoff must be made for one of these parameters to
optimize the other. With the ADL5521 and ADL5523, S11 of 6 to
8dB at the input to the matching network can still be typically
achieved when optimizing for noise.
For optimal gain matching, the goal is to use a matching network
that converts the input impedance of the LNA to the characteristic
impedance of the system, typically 50 Ω. Correct tuning for gain
matching results in a conjugate match. That is, the impedance of
the matching network at the LNA input, looking back toward the
generator, will always be the complex conjugate of the LNA input
impedance when matched for gain.
Once the conjugate of S11 is known, a matching circuit must be
found which transforms the 50 Ω system impedance into the
conjugate S11 impedance. To do this, the designer starts at the
origin of the circle and finds components that move the 50
Ω match to S11*.
The related impedances for gain matching are shown in Figure
124. A Smith Chart representation of the conjugate match is
shown in Figure 125.
50 Ω
50 Ω
Matching
Network
Figure 124. Matching LNA Input for Gain
S11*
S11
LNA
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Tuning the LNA Input for Optimal Noise Figure
The point in the Smith Chart at which matching for optimal noise
occurs is typically referred to as Gamma Optimal, or Γ
often different than the gain matching point. Finding Γ
as obvious as the gain match. Γ
semiconductor structure and characteristics of the LNA. Typically,
the fabrication facility that produces the LNA will have this
information. Γ
testing in the lab.
Noise matching for the ADL5521 and 23 is actually very easy, as
the area of the Smith Chart where the noise figure is optimal or
near optimal is not confined to a narrow area around Γ
very advantageous as it means that component variations will play
a smaller part in board to board variation of noise figure.
The matching area for optimal noise for the ADL5523 and
ADL5521 is shown in Figure 126. Note that textbooks usually
define noise circles as a conjugate match. However, for the
purpose of this note this circle is a direct match, we will do things
slightly differently. In our case to find the correct matching circuit,
the designer must start with the S11 of the LNA, then select
components which move the S11 to within this circle.
One important aspect of the overall ADL5521 and 23 ease of
tuning is that as long as S22 is matched for a particular frequency,
this noise matching area remains very consistent in its placement
for that frequency. Said another way, if S22 is matched, we simply
have to take the measured S11 and move it into the black circle for
optimal noise matching.
Figure 125. Smith Chart Representation of Conjugate Match
OPT
can also be determined by doing source pull
S11*
S11
OPT
is a function of the
ADL5521
OPT
OPT
OPT
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