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Low-Noise, High-Linearity
Broadband Amplifier
Table 1. Shunt-Resistor Noise-Figure
Values
RF Input Power Control Using the
Operational Amplifier
In a cable system, the power level at the LNA input is
R SHUNT ( ? )
450
250
125
? S11 ? (LNA) (dB)
-6
-8
-10
NOISE FIGURE (dB)
5 to 5.5
5.5 to 6
6 to 6.5
typically restricted to a maximum value to maintain lin-
earity. This is accomplished by connecting a variable
attenuator at the input of the LNA and varying the atten-
uation with the operational amplifier output. The opera-
tional amplifier receives a DC control input that is
proportional to LNA output power. See Typical
Applications Information
Bias Current
The resistor, R BIAS , connected between BIAS and GND
controls the LNA current. To make the current insensi-
tive to temperature fluctuations, select a 1%, low tem-
perature coefficient resistor for R BIAS . The current
drawn by the LNA is calculated using the following for-
mula:
I BIAS ≈ 0.58V / (R BIAS + DC resistance of L BIAS )
It is important to include the inductor resistance in the
above equation as it is typically 1 ? to 2 ? . The
MAX3524 EV kit uses a nominal inductor with DC resis-
tance of 1.4 ? . Higher values of R BIAS may be used to
reduce supply current predominantly at the expense of
linearity. Circuit board layout and source impedance
may require the value of I BIAS to be optimized for best
Application Circuit .
Layout Issues
A properly designed PC board is essential to any
RF/microwave circuit. Use short interconnect and con-
trolled impedance lines on all high-frequency inputs
and outputs. Use low inductance connections to
ground on all GND nodes and place decoupling
capacitors close to all V CC connections. The EP is the
ground for the MAX3524 and must be soldered to
ground for proper operation.
Pin Configuration
TOP VIEW
linearity.
V CC 1
10 RFOUT+
Input and Output
The LNA input is single-ended. The RF input signal is
coupled to RFIN through a DC blocking capacitor. The
LNA outputs drive a differential load, such as a mixer,
through DC blocking capacitors. The equivalent input
LNA impedance is 330 ? resistive in parallel with 1.8pf,
as shown in Figure 2. The approximate equivalent dif-
ferential output impedance of the LNA is 60 ? . To
RFIN
RFGND
OPOUT
OPIN-
2
3
4
5
MAX3524
μ MAX
9
8
7
6
V CC
REFOUT-
OPIN+
BIAS
achieve S11 less than -6dB, an insertion loss of greater
than 1dB must exist between the cable input and
MAX3524. This loss typically comes from a diplexer
and PIN attenuator in a cable modem application. A
shunt resistor may be added at the input of the LNA to
improve the return loss (S11). Typically the return loss
of the system is 2dB better, as explained above. The
S11 and noise-figure values for different shunt resistors
are given in Table 1.
TRANSISTOR COUNT: 550
Chip Information
6
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