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RF2472PCBA-410

2.4GHz low noise amplifier with enable

RF Micro Devices 

RF2472

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 fre-

quency. The RF2472 is capable of providing outstanding linearity, but to achieve this high performance, 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 circumstances, 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 prod-

ucts 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 reac-

tive 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 fundamen-

tal 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 prob-

ably pass through self resonance below 100MHz. Beyond resonance, the reactance of the capacitor will turn inductive, but the

internal losses of the capacitor will usually prevent the component from exhibiting a large reactive impedance.

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 com-

pression 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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support, contact RFMD at (+1) 336-678-5570 or sales-support@rfmd.com.

Rev A11 DS080128

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