RF2472G datasheet(4/10 Pages) RFMD

4 / 10 page

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.

RF2472G bảng dữ liệu(PDF) 4 Page - RF Micro Devices