4-510
RF2472
Rev A10 040927
Theory of Operation
The RF2472 is a low-noise amplifier with internal bias circuitry. It is DC-coupled on the input and output; therefore, it can
be used to arbitrarily low frequency. It has useful gain to above 6GHz. Its design is optimized for use at 2.4GHz.
Because of the high-frequency gain, the designer must take care to ensure that the device will remain stable outside the
desired operating frequency. The RF2472 is capable of providing outstanding linearity, but to achieve this high perfor-
mance, the circuit designer must pay attention to the terminations that are presented to low-frequency intermodulation
products.
Stability
The RF2472 must be stabilized for frequencies outside of the desired operating range. Ground connections should be
kept as short as possible. Wherever practical, ground should be provided by a via hole directly to a continuous ground
layer. Highly reflective terminations to the RF input and output pins should be avoided whenever possible. In most cir-
cumstances, a resistor in parallel with an inductor in the bias line on pin 5 will improve the stability of the circuit. See the
application schematics for examples. The 10nH inductor in the bias line is part of an output impedance matching circuit.
At higher frequencies, the impedance of the matching circuit, alone, would become highly inductive. The large reactive
termination of the output port could cause the circuit to oscillate at a high frequency. The resistance in parallel with the
inductor adds a real part to the high-frequency termination that will have a stabilizing effect on the circuit.
Linearity
The 22nF bypass and coupling capacitors in the application schematics may seem excessively large for circuits intended
to operate at 1.9GHz and 2.4GHz. These large capacitors provide a low impedance path to ground for second-order
mixing products that leads to improved third-order intermodulation performance. The effect is most easily seen for the
input coupling capacitor. A 100pF capacitor would provide low enough impedance to couple a 2.4GHz signal into the
input pin of the RF2472. However, low-frequency intermodulation products caused by second-order nonlinearities would
be presented with a large reactive impedance at the input pin. Relatively large voltages for these low-frequency products
would be allowed to mix with the fundamental signals at the input pin, resulting in relatively large, in-band, third-order
products.
With a large coupling capacitor, the low-frequency products would be presented with a low impedance, via the input
source impedance, resulting in a lower voltage at the input pin. These products, in turn, would mix at a lower level with
the fundamental signals to produce lower in-band, third-order products.
Some designers may be concerned about the self-resonant frequency of large coupling capacitors. A 22nF capacitor will
probably pass through self resonance below 100MHz. Beyond resonance, the reactance of the capacitor will turn induc-
tive, but the internal losses of the capacitor will usually prevent the component from exhibiting a large reactive imped-
ance.
Third-Order Intercept versus 1-dB Compression Point
For many devices, the third-order intercept point is approximately 10dB higher than the 1-dB compression point. This
rule of thumb does not apply for the RF2472. It is normal to find that the third-order intercept point is 20 dB higher than
the 1-dB compression point. This behavior is common for SiGe devices. The reason for the difference is that the 10dB
rule is based on a simple third-order polynomial model for device nonlinearities. For SiGe devices this simple model is
not a good fit.
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